You may order additional copies of this publication by sending 

your mailing information in label form through one of the following 

media. Please specify the publication title and series number.

Publishing Services

Telephone (970) 498-1392

FAX (970) 498-1122

E-mail rschneider@fs.fed.us

Web site http://www.fs.fed.us/rm/publications

Mailing address Publications Distribution

Rocky Mountain Research Station

240 West Prospect Road

Fort Collins, CO 80526

Rocky Mountain Region, Forest Health Protection. 2010. Field guide to 

diseases & insects of the Rocky Mountain Region. Gen. Tech. Rep. 

RMRS-GTR-241 Fort Collins, CO: U.S. Department of Agriculture, 

Forest Service, Rocky Mountain Research Station. 336 p.

Abstract

This field guide is a forest management tool for field identification of 

biotic and abiotic agents that damage native trees in Colorado, Kansas, 

Nebraska, South Dakota, and Wyoming, which constitute the USDA 

Forest Service’s Rocky Mountain Region. The guide focuses only on tree 

diseases and forest insects that have significant economic, ecological, and/

or aesthetic impacts; this guide does not necessarily cover all possible 

damage agents. Management suggestions are provided where available. 

The field guide is divided into two sections: one describes both diseases 

and damage caused by animals and abiotic factors, and the other describes 

insects. Agents are presented by the type and/or location of the injury on 

the tree. Written descriptions, color photographs, a general index, an index 

by host tree species, descriptive tables, and line drawings are all provided 

to assist users in identifying damaging agents.

Authors

Kurt K. Allen

James T. Blodgett

Kelly S. Burns

Robert J. Cain

Sheryl L. Costello

Tom J. Eager

Jeri Lyn Harris

Brian E. Howell

Roy A. Mask

Willis C. Schaupp, Jr.

Jeffrey J. Witcosky

James J. Worrall
Acknowledgments

Great appreciation is extended to the entomologists and 

pathologists who have studied and published on the often 

complicated life cycles, subtle taxonomic variations, and effective 

management strategies of the diverse organisms presented in this 

field guide. References are cited at the end of this guide.

The following individuals have all provided invaluable 

observations through personal communication and published 

materials: Whitney Cranshaw and William Jacobi with Colorado 

State University; David Leatherman and Ingrid Aguayo, formerly 

with Colorado State Forest Service; Mark Harrell with Nebraska 

Forest Service; John Ball with South Dakota State University; 

Robert Mathiasen from Northern Arizona University; Les Koch 

with the Wyoming State Forestry Division; Jose Negron with 

the USDA Forest Service, Rocky Mountain Research Station 

(RMRS); and John Schmid, formerly with RMRS. Field guides 

produced in the Pacific Northwest, Northern and Intermountain, 

and Southwestern Regions of the USDA Forest Service provided 

insights for layout and design.

Many of the photographs presented here were taken by Federal, 

State, and university pathologists and entomologists from the 

Rocky Mountain Region or adjacent USDA Forest Service 

regions and were made available by the University of Georgia 

through www.bugwood.org. Thanks to MaryLou Fairweather 

with USDA Forest Service, Southwestern Region, for providing 

scanned photographs from the Field Guide to Insects and 

Diseases of Arizona and New Mexico.

Special thanks also to Jose Negron and Bill Jacobi for their 

reviews and comments, to Justin Backsen and Diane Hildebrand 

for their help during manuscript preparation, and to Lindy 

Myers, Lane Eskew, and Connie Lemos for editing, layout, and 

publication of the field guide.

ii
Contents

About This Field Guide............................................................................1

Role of Diseases and Insects in Forest Ecosystems .................................2

Diagnosing Tree Problems.......................................................................3

Diseases

Dwarf and True Mistletoes

Introduction to Dwarf Mistletoes........................................................6

Douglas-Fir Dwarf Mistletoe............................................................10

Limber Pine Dwarf Mistletoe ...........................................................12

Lodgepole Pine Dwarf Mistletoe......................................................13

Pinyon Dwarf Mistletoe ....................................................................15

Southwestern Dwarf Mistletoe..........................................................16

Juniper Mistletoe................................................................................18

Root Diseases and Stem Decays

Introduction to Decay Diseases.........................................................19

Annosus Root Disease .......................................................................25

Armillaria Root Disease.....................................................................28

Black Stain Root Disease...................................................................31

Coniophora Root and Butt Rot.........................................................35

Root Diseases with White Pocket Rots.............................................38

Schweinitzii Root and Butt Rot ........................................................40

White Mottled Rot............................................................................42

Aspen Trunk Rot ...............................................................................44

Brown Crumbly Rot..........................................................................46

Gray-Brown Saprot ...........................................................................48

Red Ray Rot.......................................................................................49

Red Ring Rot.....................................................................................51

Rust-Red Stringy Rot and Red Heart Rot........................................53

Stem Decays of Hardwoods in the Plains.........................................56

Cankers

Introduction to Cankers and Overview of Aspen Cankers...............60

Black Canker......................................................................................64

Black Knot of Cherry and Plum........................................................66

Botryodiplodia Canker.......................................................................67

Cryptosphaeria Canker......................................................................69

Cytospora Canker of Aspen...............................................................71

Cytospora Canker of Conifers...........................................................73

Hypoxylon Canker.............................................................................75

Sooty-Bark Canker............................................................................77

iii
Wilts

Introduction to Wilt Diseases............................................................79

Dutch Elm Disease............................................................................81

Oak Wilt............................................................................................84

Pine Wilt............................................................................................87

Verticillium Wilt................................................................................89

Rusts

Introduction to Rust Diseases............................................................91

Broom Rusts of Spruce and Fir.........................................................94

Comandra and Stalactiform Blister Rusts.........................................97

Gymnosporangium Rusts................................................................100

Western Gall Rust ...........................................................................103

White Pine Blister Rust...................................................................105

Foliage Diseases

Introduction to Foliage Diseases......................................................107

Anthracnose .....................................................................................110

Brown Felt Blight ............................................................................113

Cercospora Blight of Junipers..........................................................114

Davisomycella and Lophodermella Needle Casts...........................115

Elytroderma Needle Cast ................................................................118

Ink Spot ...........................................................................................120

Marssonina Leaf Blight...................................................................121

Melampsora Rusts ...........................................................................124

Red Band and Brown Spot Needle Blights.....................................125

Septoria Leaf Spot and Canker .......................................................128

Shoot Blights

Pine Shoot Blight and Canker.........................................................129

Shepherd’s Crook.............................................................................132

Abiotic Injuries and Miscellaneous

Burls, Galls, and Tumors..................................................................134

Animal Damage...............................................................................135

Abiotic Foliage Damage ..................................................................138

Abiotic Stem Damage .....................................................................145

Wetwood..........................................................................................149

Insects

Bark Beetles

Introduction to Bark Beetles............................................................154

Ash Bark Beetles..............................................................................156

Aspen bark beetles ...........................................................................157

iv
Blue Spruce Engraver ......................................................................159

Cedar Bark Beetles...........................................................................161

Douglas-Fir Beetle...........................................................................162

Douglas-Fir Pole and Engraver Beetles...........................................163

Elm Bark Beetles .............................................................................165

Fir Engraver.....................................................................................167

Limber Pine Engraver .....................................................................169

Lodgepole Pine Beetle.....................................................................170

Mountain Pine Beetle......................................................................172

Pine Ips Species (Engraver Beetles).................................................174

Pinyon Ips........................................................................................177

Pinyon Twig Beetles.........................................................................179

Red Turpentine Beetle .....................................................................181

Roundheaded Pine Beetle................................................................183

Spruce Beetle ...................................................................................184

Spruce Engraver Beetles..................................................................187

Twig Beetles.....................................................................................188

Western Balsam Bark Beetle............................................................190

Western Pine Beetle.........................................................................193

Defoliators

Introduction to Defoliating Insects .................................................195

Aspen Leafminer .............................................................................197

Bagworm..........................................................................................199

Cottonwood Leaf Beetle..................................................................201

Douglas-Fir Tussock Moth .............................................................202

Elm Leaf Beetle ...............................................................................204

Fall Webworm..................................................................................207

Forest Tent Caterpillar.....................................................................209

Large Aspen Tortrix.........................................................................212

Needleminers...................................................................................214

Oak Leafroller..................................................................................216

Pandora Moth..................................................................................218

Pine Butterfly...................................................................................220

Pine Sawflies....................................................................................222

Tiger Moth ......................................................................................225

Western Pine Budworm...................................................................227

Western Pine Tussock Moth ...........................................................229

Western Spruce Budworm...............................................................231

Western Tent Caterpillar .................................................................233

Wood Borers

Introduction to Wood Borers...........................................................236

Agrilus quercicola ...............................................................................239

v
Ambrosia Beetles .............................................................................240

Bronze Poplar Borer.........................................................................242

Carpenter Ants ................................................................................244

Carpenterworm................................................................................245

Cottonwood Borer...........................................................................247

Elm Borer.........................................................................................248

Flatheaded Wood Borers (Metallic Wood Borers)..........................250

Juniper Borers ..................................................................................252

Lilac (Ash) Borer .............................................................................253

Poplar Borer.....................................................................................255

Roundheaded Wood Borers (Longhorned Beetles) ........................256

Twolined Chestnut Borer ................................................................258

Wood Wasps (Horntails).................................................................260

Sap-Sucking Insects, Gall Formers, and Mites

Introduction to Sap-Sucking Insects, Gall Formers, and Mites......262

Cooley Spruce Gall Adelgid............................................................265

European Elm Scale ........................................................................266

Gall (Eriophyid) Mites....................................................................267

Giant Conifer Aphids......................................................................269

Oystershell Scale..............................................................................270

Petiolegall Aphids............................................................................272

Pine Needle Scale ............................................................................274

Pinyon Needle Scale ........................................................................276

Spider Mites.....................................................................................278

Bud and Shoot Insects

Introduction to Bud, Shoot, Branch, and Terminal Insects.............280

Pine Tip Moth.................................................................................281

Pitch Moths.....................................................................................282

Pitch Nodule Moths........................................................................283

White Pine Weevil and Lodgepole Pine Terminal Weevil .............285

Glossary................................................................................................287

References............................................................................................297

Host-Pest Index...................................................................................312

General Index ......................................................................................322

Instructions for Submitting Insect and Disease Specimens 

for Identification..............................................................................335

Technical Assistance Sources .............................................................336

vi


USDA Forest Service RMRS-GTR-241. 2010. 1

About This Field Guide

This field guide details the most commonly encountered diseases and insects of forest trees in the Rocky Mountain Region. Descriptions of diseases, 

insects, and physical injuries focus on the most diagnostic features of each. Color 

photographs, line drawings, and tables are used to illustrate and emphasize characteristics described in the text. Diseases and insects in plains hardwood trees are 

not covered in depth. Ornamental trees are sometimes affected by the diseases 

or insects included in this guide but may not be specifically mentioned as hosts.

This guide presents diseases and then insects. Entries are arranged according to the part of trees typically damaged by the agent described. The disease 

section describes dwarf and true mistletoes, decays (including root diseases and 

stem decays), cankers, wilts, rusts, foliage diseases, shoot blights, and abiotic 

injuries and miscellaneous diseases. The insect section describes bark beetles, 

defoliators, wood borers, sap-sucking insects, gall formers, mites, and bud and 

shoot insects.

At the end of the guide is a subject index, a host-pest index to damaging 

agents by tree species and part of the tree affected, and a glossary of terms. The 

host-pest index provides a rapid means of assessing the number and variety of 

agents described for each tree species.

This field guide applies to the USDA Forest Service’s Rocky Mountain 

Region, which includes Colorado, Wyoming, South Dakota, Kansas, and 

Nebraska (fig. 1). Additional hosts, diseases, and insect pests may be encountered outside this Region that are not included here, and a few of the diseases and 

insects included in this guide may not be seen in other areas.

Figure 1. Rocky Mountain Region area covered by this Field Guide. Image: Sheryl 

Costello, USDA Forest Service.
2 USDA Forest Service RMRS-GTR-241. 2010.

Plant pathologists and entomologists are available to assist resource managers 

with identification of insects and pathogens encountered in the forests. A list of 

offices that provide this assistance is included at the back of this field guide along 

with instructions for specimen collection and shipping.

Role of Diseases and Insects in Forest 

Ecosystems

Most forest insects and diseases are native to and important players in the 

forest ecosystems of the Rocky Mountain Region. Some insects and diseases 

reduce tree growth, cause mortality, reduce timber productivity, create hazardous 

situations in developed sites, and change wildlife habitat, fire hazard conditions, 

and overall watershed quality. In terms of mortality and growth loss, the impacts of diseases and insects on forests are far greater than those of fire or any 

other disturbance. Along with fire, insects and diseases are the major disturbance 

agents for changing forest age, density, composition, and structure on a stand 

or landscape level. They provide food and habitat for other wildlife, increase 

structural diversity of forests, and are important forest recyclers in the Rocky 

Mountains.

Typically, forest insects are at low or endemic levels. From time to time, 

significant outbreaks cause rapid changes across the forest. For example, bark 

beetles can build to high populations relatively fast and can cause widespread 

tree mortality, significantly altering forest conditions. Diseases are more likely 

to increase gradually or remain at similar levels over time, depending on forest 

conditions. Dwarf mistletoes and root diseases are ever-present in the forest and 

are some of the main contributors to growth loss, reduced vigor, and mortality in 

their hosts. Insects and diseases often interact, such as when root diseases predispose trees to bark beetle attack.

At times, natural or human-made events or conditions can cause insects or 

diseases to become more active. This can occur from the high-elevation subalpine forests to the urban forests that line the streets of our cities and towns. 

Fire, wind, and drought can weaken host trees and create favorable environments for insects and diseases to attack. Altered age, density, composition, and 

structure can also lead to greater susceptibility and increased disease and insect activity. For example, through past harvesting practices and fire exclusion, 

many ponderosa pine stands in southern Colorado now have dense understories 

of shade-tolerant white fir, with consequent increased activity of root diseases to 

which it is susceptible.

Forest insect and disease management often involves changing the conditions 

that have created the pest situation. Reducing stand density in conifer stands 

can lower bark beetle impact. Removing mistletoe-infected overstory trees from 

a regenerating pine stand or converting to even-aged management where that 

is consistent with the natural disturbance regime can protect regeneration and 
USDA Forest Service RMRS-GTR-241. 2010. 3

reduce mistletoe impacts in future stands. Increasing the diversity of cultivars that 

are planted along our city streets and windbreaks can also reduce pest impacts.

The main goal of most forest managers is to maintain a healthy forest that 

is capable of producing a variety of resources. Knowledge of the role that forest insects and diseases play is critical to achieving the desired conditions from 

the forest. This manual provides users with a tool to begin identifying and understanding forest insects and diseases that may be detrimental to management 

goals. Please refer to your State Department of Agriculture or your local pest 

control specialist for the rules, regulations, and applicable allowances concerning specific control measures.

Diagnosing Tree Problems

Several steps must be taken in order to effectively diagnose tree problems. 

The following are general guidelines:

•Properly identify the tree. It is important to know exactly what species yo

are looking at. It is also important to know what the leaves, bark, trunk, and 

roots should look like under “normal” conditions.

•Carefully observe the symptoms expressed. Look for patterns on the tree an

in the surrounding vegetation.

○ Check for host specificity. Biotic agents tend to affect one species, are 

clumpy in distribution, show progressive symptoms, and usually impact 

specific plant parts. Abiotic agents tend to affect many species relatively 

uniformly.

○ Carefully examine the types of symptoms and the part of the plant 

impacted.

○ Typical symptoms include: underdevelopment of tissues or organs 

(stunting and malformed leaves); overdevelopment of tissues or organs 

(galls, brooms, and stress cones); necrosis (death) of plant parts (wilting, 

dieback, and leaf spots); and alteration of normal appearance (chewing 

and chlorosis [pale coloration in leaves and flowers]).

○ Examine how the symptoms are distributed. If the entire tree is impacted, 

there is likely something wrong with the roots or stem, or there may be 

an environmental cause. Single or randomly scattered affected branches 

are often associated with insects or diseases.

•Determine the history of the tree and the site. Has the root system been disturbed? Have chemicals been used? Has there been any harvesting? Othe

site factors, such as changing water relations, extreme temperatures, or 

wind, may affect tree vigor.

•Look for signs of biotic agents such as fungal fruiting bodies, parasiti

plants, larvae, or adult insects.

•Identify agents. Laboratory tests may be necessary in some circumstance
4 USDA Forest Service RMRS-GTR-241. 2010.

Sources of Additional Information

Additional information on insects and diseases can be found online at:

•  USDA Forest Service, Rocky Mountain Region: http://www.fs.usda.gov/

goto/r2/fh;

•  USDA Forest Service, Northern and Intermountain Region: http://www.

fs.fed.us/r1-r4/spf/fhp/mgt_guide/; 

•  USDA Forest Service, Southwestern Region: http://www.fs.fed.us/r3/resources/health/field-guide/index.shtml;

•  Colorado State University Extension: http://www.ext.colostate.edu/pubs/

pubs.html#insects; and

•  State Forestry Agencies and County Extension Offices.

Updated Forest Insect and Disease Leaflets can be found online at:

•  USDA Forest Service, Forest Health Protection: http://www.fs.fed.us/r6/nr/

fid/wo-fidls/index.shtml.

Images and publications can also be found online at:

•  University of Georgia and USDA Forest Service online database: http://

www.bugwood.org/.
USDA Forest Service RMRS-GTR-241. 2010. 5
6 USDA Forest Service RMRS-GTR-241. 2010.

Table 1. Dwarf mistletoes and their hosts in the Rocky Mountain Region.

Dwarf mistletoe (DM) Primary hosta Other hostsa

Lodgepole pine DM Lodgepole pine Secondary: ponderosa pine

Arceuthobium americanum Occasional: whitebark and limber pines

Rare: Engelmann and blue spruce, bristlecone pine

Limber pine DM Limber pine, Rare: ponderosa pine, lodgepole pine

A. cyanocarpum Whitebark pine,

Bristlecone pine

Pinyon DM Pinyon pine None

A. divaricatum

Douglas-fir DM Douglas-fir Rare: subalpine fir, Englemann and blue spruce

A. douglasii

Southwestern DM Ponderosa pine Occasional: bristlecone pine, lodgepole pine

A. vaginatum subsp. Rare: limber and southwestern white pine, blue spruce

cryptopodium

a Hosts are in the following categories:

 Primary: more than 90% infection when close to heavily infected trees.

 Secondary: frequently infected (50-90% infection) when close to heavily infected principal hosts.

 Occasional: occasionally infected (5-50% infection) when close to heavily infected principal hosts.

 Rare: rarely infected (≤5% infection), even when close to heavily infected principal hosts.

Introduction to Dwarf Mistletoes

Parasitic vascular plants with conifer hosts

Pathogens—Dwarf mistletoes (Arceuthobium spp.) are parasitic plants of conifers that obtain almost all of their needs, including water, mineral, and carbon 

nutrients, from their hosts. Fulfilling these resource requirements stresses infected trees, causing reductions in growth, cone, and seed production and, with high 

infection levels, mortality. Dwarf mistletoes are some of the most common and 

easily identified disease agents where they occur in the coniferous forests of the 

Rocky Mountain Region.

Hosts—Five species of dwarf mistletoes occur in this Region, each with a specific set of susceptible hosts (table 1).

Signs and Symptoms—Dwarf mistletoes produce aerial shoots on branches or 

stems of infected trees (fig. 2). Shoots are nearly leafless and vary in color; yellow, brown, purple, or green shoots are common. Plants size also varies within 

and among species. Douglas-fir dwarf mistletoe shoots are often shorter than the 

host’s leaves, while southwestern dwarf mistletoe shoots are typically 3-6 inches 

(7-15 cm) long. When shoots are shed, characteristic basal cups remain (fig. 3). 

Infection with dwarf mistletoe also causes characteristic deformities in the host. 

Witches’ brooms are areas of profuse, dense branching often induced by dwarf 

mistletoe infection (fig. 4). Branch swellings are often found in the immediate 

vicinity of local infections (fig. 2). Cankers (areas of dead cambium) are often 

associated with older infections. Dieback of the host’s foliage from the top-down 

and eventual mortality is often observed in trees that have been infected for many 

D W A R F A N D T R U E M I S T L E T O E S 
USDA Forest Service RMRS-GTR-241. 2010. 7

years (fig. 5), depending on the species of dwarf mistletoe and host, the level of 

infection, and site factors. These symptoms on the hosts are often associated with 

mistletoe infection but may also be caused by other agents. Plants or basal cups 

should be present for positive identification of dwarf mistletoe infection.

Disease Cycle—Dwarf mistletoes have separate male and female plants 

(figs. 6-7). Seeds are produced annually on mature female plants. These are explosively released and typically fly less than 33 ft (10 m). Upon germination, the 

dwarf mistletoe plant produces a specialized root-like structure that contacts the 

phloem and xylem of the host, from which the parasite obtains nutrients and water. Aerial shoots appear 3-5 or more years after infection; the time period before 

shoots are visible is known as the latent period (fig. 8).

Dwarf mistletoes spread both within and between tree crowns. As a result 

of the explosive seed-dispersal mechanism, infections tend to build up initially 

in the lower portion of the crown and spread gradually upward. Lateral spread 

Figure 2. Aerial shoots of American dwarf mistletoe plant on lodgepole 

pine. Note the swelling in the branch associated with the aerial shoots. 

Photo: Brian Howell, USDA Forest Service.

Figure 3. After aerial shoots are 

shed and basal cups remain as 

signs of dwarf mistletoe infection. 

Photo: Kelly Burns, USDA Forest 

Service.

Figure 4. Large witches’ brooms formed on 

a ponderosa pine that is heavily infected 

with southwestern dwarf mistletoe. The top 

of the tree is dying back. Photo: Bob Cain, 

USDA Forest Service.

Figure 5. 

Lodgepole pine 

killed as a result 

of lodgepole pine 

mistletoe infection. 

Note the typical 

witches’ brooms. 

Photo: Jim 

Worrall, USDA 

Forest Service.

D W A R F A N D T R U E M I S T L E T O E S
8 USDA Forest Service RMRS-GTR-241. 2010.

D W A R F A N D T R U E M I S T L E T O E S 

of dwarf mistletoe through single-storied stands averages about 1.5 ft (0.5 m) 

per year. Spread is most rapid from infected overstory to adjacent regeneration. 

Long-distance seed dispersal by birds is not common but can introduce dwarf 

mistletoe to new areas.

Impact—As parasites, dwarf mistletoes cause significant changes in physiological processes and structural characteristics of infected trees, resulting in changes 

Figure 8. Generalized life cycle 

of dwarf mistletoes, as exemplified by lodgepole pine dwarf 

mistletoe on lodgepole pine (from 

Hawksworth and Dooling 1984).

Figure 6. Male flowers on shoots of southwestern dwarf 

mistletoe. Photo: Brian Howell, USDA Forest Service.

Figure 7. Immature female berries on shoots 

of southwestern dwarf mistletoe. Photo: Brian 

Howell, USDA Forest Service.
USDA Forest Service RMRS-GTR-241. 2010. 9

in the structure and function of forest communities. Tree growth and vigor usually decline when more than half of the crown is parasitized. Most trees survive 

infection for decades, but small trees tend to decline and die more quickly than 

large ones. Tree mortality in areas with extensive infection is often three to four 

times higher than in uninfected areas. Bark beetles frequently attack heavily infected trees, especially during drought.

Extensive dwarf mistletoe infection greatly reduces forest productivity. 

However, infection has some benefits for wildlife. Large witches’ brooms provide nesting and seclusion habitat for birds and small mammals. Snags created 

by dwarf mistletoe infection offer habitat for cavity-nesting birds. A few species 

are known to feed on shoots of dwarf mistletoes and the sugar-rich phloem found 

in and around infection sites (fig. 9).

Management—The first step when making management decisions in stands 

infected with dwarf mistletoe is to quantify the incidence and severity of infection. Although many systems have been used to rate levels of infection by dwarf 

mistletoe, one is now used almost universally: Hawksworth’s 6-class dwarf mistletoe rating (DMR) system (fig. 10). Many disease parameters and management 

recommendations are 

provided in terms of 

DMR because this system has been used for 

many years. A tree’s 

DMR ranges from 0 

(uninfected) to 6 (over 

half the branches infected throughout the 

crown). Rate each third 

of the crown on a scale 

from 0 to 2, then sum the 

thirds for the tree rating 

(fig. 10). Binoculars 

should be used to enhance detection.

Figure 9. Evidence of squirrel or porcupine feeding on sugar-rich phloem found near southwestern dwarf 

mistletoe infections. Photo: Brian Howell, USDA Forest Service.

Figure 10. The 6-class Hawksworth dwarf mistletoe rating system (from 

Hawksworth and Wiens 1996).

D W A R F A N D T R U E M I S T L E T O E S
10 USDA Forest Service RMRS-GTR-241. 2010.

D W A R F A N D T R U E M I S T L E T O E S 

Silvicultural control of dwarf mistletoes can be effective and should be considered for use in a variety of stand conditions and dwarf mistletoe infection levels. 

Because dwarf mistletoes require a living susceptible host, silvicultural options 

include pruning, harvesting, and favoring non-host species. Due to the explosive 

seed dispersal mechanism, implementing buffer strips around infection centers 

or around sanitized patches can also be effective. A thorough discussion of management options based on stand conditions and objectives is outside the scope of 

this manual but can be found on the Region 2 Forest Health Management website at: http://www.fs.fed.us/r2/fhm/bugcrud/DM_MgmtGuide_R2.pdf.

References: 66, 70, 158

Douglas-Fir Dwarf Mistletoe

Large brooms, small aerial shoots

Pathogen—Douglas-fir dwarf mistletoe (Arceuthobium douglasii) causes large 

brooms but has very small shoots. Aerial shoots are olive green, average 4/5 inch 

(20 mm) long (maximum 3 inches or 8 cm) and 1/25-1/16 inch (1-1.5 mm) in diameter, and have fan-shaped branching (fig. 11).

Douglas-fir dwarf mistletoe occurs throughout the range of Douglas-fir in the 

central and southern mountains of Colorado. It is absent from northern Colorado 

(except for the extreme northwest) and from the portion of Wyoming in the 

Rocky Mountain Region (fig. 12).

Hosts—Douglas-fir dwarf mistletoe primarily infects its namesake, 

although several true firs and 

spruces are occasional or rare 

hosts.

Figure 11. Douglas-fir dwarf mistletoe parasitizing Douglas-fir. Note that the mistletoe 

plants are growing with the branch (systemic infection). Photo: Robert Mathiasen, 

Northern Arizona University.

Figure 12. Distribution of Douglas-fir dwarf mistletoe in the 

Rocky Mountain Region (from Hawksworth and Wiens 1996).
USDA Forest Service RMRS-GTR-241. 2010. 11

Signs and Symptoms—Douglas-fir dwarf mistletoe has the smallest shoots 

(fig. 13) of all mistletoes in the Region, but it can form the largest witches’ brooms 

(fig. 14). Douglas-fir infections grow along with the infected host branches (systemic infection). Mistletoe shoots may be spread along young host branches 

or be aggregated near the annual bud scars. Because shoots are so small, they 

are normally detectable only in branches close to the ground. Witches’ brooms, 

which are used for detection and rating, become noticeable about 10 years after 

infection and develop best in direct sunlight. Brooms occur mostly in the lower 

half of tree crowns. They can weigh hundreds of pounds, can break off of the 

tree, and are considered hazards in developed sites.

Impact—Dwarf mistletoe is the most detrimental disease of Douglas-fir. Damages 

typically associated with dwarf mistletoe infection are: growth reduction, spike 

tops (fig. 15), reduced cone and seed production, and mortality. Infections have 

reportedly increased in abundance since the late 1800s. In northern Idaho and 

western Montana, Douglas-fir stands have become more widespread due to fire 

suppression, a history of selective harvesting that removed pines and encouraged 

Figure 13. Douglasfir dwarf mistletoe 

plants, which are 

approximately the 

same length as 

the host’s leaves. 

Photo: USDA Forest 

Service.

Figure 14. Large 

brooms are typical 

in Douglas-fir heavily infected with 

Douglas-fir dwarf 

mistletoe. Photo: 

USDA Forest 

Service.

Figure 15. Trees 

heavily infected 

with Douglas-fir 

dwarf mistletoe 

typically die from the 

top-down, causing 

dead (spike) tops. 

Photo: USDA Forest 

Service.

D W A R F A N D T R U E M I S T L E T O E S
12 USDA Forest Service RMRS-GTR-241. 2010.

D W A R F A N D T R U E M I S T L E T O E S 

shade-tolerant species, and white pine blister rust, which largely eliminated 

western white pine.

Data on growth effects from western Montana indicate that light, medium, 

and severe infections caused decreases in basal area growth rate of 14, 41, and 

69%, respectively. Effects on height growth were similar. Horizontal spread in 

single-storied stands is estimated at 1.5-2 ft (45-61 cm) per year. Upward spread 

in crowns is about 4-6 inches (10-15 cm) per year.

Please see the Introduction to Dwarf Mistletoes entry for disease cycle and management information.

References: 59, 70

Limber Pine Dwarf Mistletoe

Infects five-needle pines

Pathogen—Aerial shoots of limber pine dwarf mistletoe (Arceuthobium cyanocarpum) are yellow-green, 1 1/4-2 3/4 inches (3-7 cm) long, and up to 1/13 inch 

(2 mm) diameter. Branching is fan-shaped and shoots are densely clustered 

(fig. 16). Limber pine dwarf mistletoe generally occurs in five-needle pine 

(predominantly limber pine) stands along the Continental Divide in the Rocky 

Mountains but also occurs in other mountain ranges (fig. 17).

Hosts—Almost all of the five-needle pines in the Rocky Mountain Region, including limber, whitebark, and Rocky Mountain bristlecone pine, are primary 

hosts of limber pine dwarf mistletoe. The only endemic white pine that is not a 

host in nature is southwestern white pine, although it has been infected in greenhouse trials.

Signs and Symptoms—Signs of infection 

include aerial shoots and basal cups left 

Figure 16. Limber pine dwarf mistletoe parasitizing 

limber pine. Photo: Brian Howell, USDA Forest 

Service.

Figure 17. Distribution of limber pine dwarf 

mistletoe in the Rocky Mountain Region (from 

Hawksworth and Wiens 1996).
USDA Forest Service RMRS-GTR-241. 2010. 13

after shoots have fallen off branches. Limber 

pine dwarf mistletoe causes small, tightly 

clustered witches’ brooms (fig. 18). Other 

symptoms of infection include swelling of 

branches at infection sites, dieback, and 

eventual mortality of heavily infected trees.

Impact—Limber pine dwarf mistletoe causes 

extensive mortality of limber pine in many 

parts of the Rocky Mountains and can also 

cause mortality in other hosts when infection 

levels are high. It is the most important native disease of high-elevation white pines in 

the West; only white pine blister rust is more 

damaging. Lateral spread in single-storied 

stands is estimated to be 1.5-2 ft (45-61 cm) 

per year.

Please see the Introduction to Dwarf Mistletoes entry for disease cycle and management information.

References: 70, 171

Lodgepole Pine Dwarf Mistletoe

Common cause of brooming in lodgepole pine

Pathogen—Lodgepole pine dwarf mistletoe (Arceuthobium americanum) is the 

most widely distributed, one of the most damaging, and one of the best studied 

dwarf mistletoes in North America. Aerial shoots are yellowish to olive green, 

2-3 1/2 inches (5-9 cm) long (maximum 12 inches [30 cm]) and up to 1/25-1/8 inch 

(1-3 mm) diameter (figs. 19-20). 

The distribution generally follows 

that of its principal host, lodgepole 

pine, in the Rocky Mountain Region 

(fig. 21).

Hosts—Lodgepole pine dwarf mistletoe infects primarily its namesake, 

but, as noted in table 1, ponderosa 

pine is considered a secondary host 

of this species. However, lodgepole 

pine dwarf mistletoe can sustain itself and even be aggressive in pure 

stands of Rocky Mountain ponderosa pine in northern Colorado 

and southern Wyoming sometimes 

a mile or more away from infected 

Figure 18. Tightly clustered brooms are a 

symptom of limber pine dwarf mistletoe. 

Photo: Brian Howell, USDA Forest Service.

Figure 19. Flowering male lodgepole pine dwarf 

mistletoe plant parasitizing lodgepole pine. Photo: Brian 

Howell, USDA Forest Service.

D W A R F A N D T R U E M I S T L E T O E S
14 USDA Forest Service RMRS-GTR-241. 2010.

D W A R F A N D T R U E M I S T L E T O E S 

lodgepole pine. This infection generally occurs in areas outside the range of ponderosa pine’s usual parasite, southwestern dwarf mistletoe.

Signs and Symptoms—Signs of infection are shoots and basal cups (fig. 20) 

found at infection sites. Symptoms 

include witches’ brooms, swelling 

of infected branches, and dieback. 

Lodgepole pine dwarf mistletoe infections grow systemically with the 

branches they infect, sometimes 

causing large witches’ brooms with 

elongated, loosely hanging branches.

Impact—Heavily infected trees 

experience reduced diameter and 

growth, reduced cone production, 

and eventual mortality (fig. 22).

Spread rate in even-aged stands 

can be about 1.7 ft (50 cm) per 

year in open stands and 1.2 ft 

(36 cm) per year in dense stands. 

Intensification (increase in number 

of infections over time) occurs most 

quickly in stands 15-60 years old in 

Colorado. During that time, DMR 

increased one class in 14 years (see 

“Management” in the Introduction to 

Dwarf Mistletoes entry). A feature of 

Figure 22. Dead and dying lodgepole pine heavily 

infected by lodgepole pine dwarf mistletoe. Photo: Brian 

Howell, USDA Forest Service.

Figure 20. Female lodgepole pine dwarf mistletoe plant 

with immature fruit parasitizing lodgepole pine. Note the 

basal cups left behind where old shoots have fallen off. 

Photo: Brian Howell, USDA Forest Service. Figure 21. Distribution of lodgepole pine dwarf 

mistletoe in the Rocky Mountain Region (from 

Hawksworth and Wiens 1996).
USDA Forest Service RMRS-GTR-241. 2010. 15

this species that is potentially useful in management is that the upper elevational 

limit is usually about 600-650 ft (185-200 m) below the upper elevational limit 

of lodgepole pine for a given latitude. Experiments have shown that the mistletoe 

can survive at higher elevations but it cannot reproduce because the fruit is killed 

by early autumn frosts before it can mature.

Please see the Introduction to Dwarf Mistletoes entry for disease cycle and management information.

References: 68, 70

Pinyon Dwarf Mistletoe

Disease of pinyon on Colorado’s Western Slope

Pathogen—Aerial shoots of pinyon dwarf mistletoe (Arceuthobium divaricatum) are olive green to brown, about 3-5 inches (8-13 cm) long, and up to 

1/6 inch (4 mm) diameter. Shoots often have a long, thin, and spreading appearance (fig. 23). Branching is fan-shaped.

Pinyon dwarf mistletoe is found throughout the pinyon range in the western 

quarter of Colorado but is absent in pinyon stands east of the Continental Divide 

(fig. 24).

Hosts—Pinyon dwarf mistletoe infects only pinyons.

Signs and Symptoms—Signs of infection include aerial shoots and basal cups. 

Symptoms include witches’ brooms, swelling of infected branches, and dieback. 

This dwarf mistletoe may not result in well-developed witches’ brooms, but 

those that do develop are usually small.

Figure 23. Female 

pinyon dwarf 

mistletoe plant 

parasitizing piñon 

pine. Note the 

olive-green color 

of shoots. Photo: 

Robert Mathiasen, 

Northern Arizona 

University.

Figure 24. Distribution of pinyon dwarf mistletoe 

in the Rocky Mountain Region (from Hawksworth 

and Wiens 1996).

D W A R F A N D T R U E M I S T L E T O E S
16 USDA Forest Service RMRS-GTR-241. 2010.

D W A R F A N D T R U E M I S T L E T O E S 

Impact—Pinyon dwarf mistletoe is considered less lethal than other dwarf mistletoes of the region. However, growth loss and mortality can be significant when 

infection is severe (DMR 5 or 6).

Other dwarf mistletoes greatly reduce seed production of their hosts, but such 

effects on pinyon are unknown. Reduced seed production could be particularly 

important in pinyon because the nuts are collected for food by humans and wildlife, and they are necessary for species reproduction.

Please see the Introduction to Dwarf Mistletoes entry for disease cycle and management information.

References: 70, 118

Southwestern Dwarf Mistletoe

Infects ponderosa pine in Colorado

Pathogen—Aerial shoots of southwestern dwarf mistletoe (Arceuthobium vaginatum ssp. cryptopodum) vary in color from orange to reddish brown to almost 

black. Shoots are the largest of dwarf mistletoes in this Region and are approximately 4 inches (10 cm) long (maximum 11 inches or 27 cm) with a basal diameter of 1/13-3/8 inch (2-10 mm) (fig. 25). This species is unusual among dwarf 

mistletoes in temperate regions in that seed germination occurs immediately 

after dispersal in the fall rather than in the following year. Within the Rocky 

Mountain Region, southwestern dwarf mistletoe is found in southern Colorado 

on the Western Slope extending into northern Colorado on the Front Range 

(fig. 26). No dwarf mistletoe occurs in the Black Hills National Forest, where 

ponderosa pine is most productive in the Region.

Hosts—Southwestern dwarf mistletoe primarily infects the Rocky Mountain variety of ponderosa pine in the Four Corners states (Colorado, Utah, Arizona, and 

Figure 25. Male southwestern dwarf mistletoe plant parasitizing 

Rocky Mountain ponderosa pine. Photo: Brian Howell, USDA 

Forest Service.

Figure 26. Distribution of southwestern 

dwarf mistletoe in the Rocky Mountain 

Region (from Hawksworth and Wiens 1996).
USDA Forest Service RMRS-GTR-241. 2010. 17

New Mexico) with a small distribution 

in west Texas. Occasionally, southwestern dwarf mistletoe will infect bristlecone pine and lodgepole pine. It rarely 

infects limber pine, southwestern white 

pine, and blue spruce.

Signs and Symptoms—Signs of infection include aerial shoots and basal 

cups, and symptoms include witches’ 

brooms, swelling of infected branches, 

and dieback (fig. 27).

Impact—Damage is usually greater along the Front Range than in southwestern Colorado. Witches’ broom development can be weak, but large and robust 

brooms with thick, distorted branches are common in older infectons. Mortality 

from southwestern dwarf mistletoe was quantified in a 32-year study at Grand 

Canyon National Park. Ninety percent of uninfected or lightly infected (DMR 

0-1 at the start) trees survived the entire study period. Of heavily infected trees 

(DMR 6), only 5% over 9 inches (23 cm) diameter at breast height (DBH) survived, and none survived in the 4-9 inches (10-23 cm) size class. Intermediate 

infection levels were associated with intermediate mortality levels. Infection intensified during the study, so much so that most trees that died were in DMR 

class 6 by the time of death. Based on the data, the authors estimated the halflife of trees (time in which half the trees are expected to die) by DMR class, as 

described in table 2.

Estimates of spread rate in single-storied stands vary. Recent estimates of 

2-3 ft (61-91 cm) per year indicate that southwestern dwarf mistletoe has one 

of the faster spread rates. Earlier estimates were about 1.3 ft (0.4 m) per year in 

open stands and 0.9 ft (0.3 m) per year in dense stands. Spread from overstory to 

understory is faster in ponderosa than in lodgepole pine.

Figure 27. Heavily infected southwestern ponderosa 

pine with characteristic broomed branches and top 

dieback. Photo: Brian Howell, USDA Forest Service.

Table 2. Expected half-life (time, in years, for half 

the trees to die) of ponderosa pine infected with 

southwestern dwarf mistletoe at Grand Canyon 

National Park (ref. 67).

4-9 inches >9 inches

Initial DMR DBH DBH

0-1 NDa NDa

2-3 30 57

4-5 17 25

6 7 10

Total 14

a

 No decrease in longevity detected; half-life too long 

to estimate.

D W A R F A N D T R U E M I S T L E T O E S
18 USDA Forest Service RMRS-GTR-241. 2010.

D W A R F A N D T R U E M I S T L E T O E S 

Please see the Introduction to Dwarf Mistletoes entry for disease cycle and management information.

References: 12, 65, 67, 70, 115

Juniper Mistletoe

Minor effects on junipers

Pathogen—Juniper mistletoe (Phoradendron 

juniperinum) is the only member of the true mistletoes that occurs within the Rocky Mountain 

Region (fig. 28).

Hosts—Within the Rocky Mountain Region, 

juniper mistletoe is found in the pinyon-juniper 

woodlands of southwestern Colorado (fig. 29) 

and can infect all of the juniper species that occur there.

Signs and Symptoms—Juniper mistletoe 

plants are generally densely branched in a spherical pattern and are green to yellow-green (fig. 

30). Unlike most true mistletoes that have obvious leaves, juniper mistletoe leaves are greatly 

reduced, making the plants look similar to, but somewhat larger than, dwarf 

mistletoes. However, no dwarf mistletoes infect junipers in the Rocky Mountain 

Region.

Disease Cycle—Juniper mistletoe plants are either male or female. The female’s 

berries are spread by birds that feed on them. As a result, this mistletoe is often 

found where birds prefer to perch—on the tops of taller trees (fig. 28), near water 

sources, etc. When the 

seeds germinate, they 

penetrate the branch 

of the host tree. In the 

branch, the mistletoe 

forms a root-like structure that is used to gather water and minerals. 

The plant then produces aerial shoots that 

produce food through 

photosynthesis.

Impacts—Impacts associated with juniper 

mistletoe are generally Figure 29. Distribution of juniper mistletoe in the Rocky Mountain Region 

(from Hawksworth and Scharpf 1981).

Figure 28. Juniper mistletoe plants 

on one-seed juniper in Mesa Verde 

National Park. Photo: USDA Forest 

Service.
USDA Forest Service RMRS-GTR-241. 2010. 19

minor. While the 

true mistletoe plants 

do receive some 

small proportion 

of their carbon nutrition from their 

hosts, they are considered only “water 

and mineral” parasites. Unlike dwarf 

mistletoes, true 

mistletoes produce 

most of their own 

food through photosynthesis. However, 

during periods of 

drought stress, when 

the host trees have shut down their transpiration to conserve water, juniper mistletoe plants continue to transpire, causing further drought stress on the host.

Management—Juniper mistletoe is generally not considered worth managing 

as the impacts are minor. If management is desired, infected branches can be 

effectively pruned because juniper mistletoe requires a living host to survive. If 

mistletoe shoots are removed, they will resprout.

References: 53, 69

Introduction to Decay Diseases

Wood decays of roots and stems

Many fungi decay wood of live trees in the Rocky Mountain Region. The 

diseases are often called heart rots, but frequently the decay is not restricted to 

heartwood. They are generally divided into stem decays (trunk rots) and root and 

butt rots.

Decay Types—Each fungus causes one of two types of decay: white rot or 

brown rot. These are easily distinguished (table 3, fig. 31) and are features used 

to diagnose the disease.

Stem Decay Disease Cycle—Stem-decay fungi release wind-disseminated 

spores. Some infect through wounds, but other more specialized pathogens do 

not require obvious wounds. They can infect small twig or branch stubs, and then 

either grow into the stem and inner wood or become dormant and wait until the 

tree grows around them and they become embedded in heartwood. When sufficient food is available through decay, a fruiting body (conk) is produced. Spores 

from the conk repeat the cycle.

Figure 30. Closeup of juniper mistletoe on juniper branch. Photo: Robert 

Mathiasen, Northern Arizona University.

R O O T D I S E A S E S A N D S T E M D E C AY S
20 USDA Forest Service RMRS-GTR-241. 2010.

Root Disease Cycle—Root and butt rots often have a 

more complex disease cycle. As with stem decays, windblown spores can be the initial inoculum. Spores may 

infect wounds in the butt or root crown, or they may percolate through the soil to infect roots. In general, these 

pathogens kill and decay roots, decay inner wood in the 

butt, and often kill sapwood and cambium in the root 

crown.

An important difference from stem decays is that most, 

but not all, of these diseases can also spread locally. The 

pathogen grows from infected roots to roots of neighboring 

healthy trees at contacts and grafts or, in one case, through 

soil. Root disease centers result in the stands characterized 

by older mortality in the middle and more recent mortality, 

symptomatic live trees, and apparently healthy trees toward 

the outside. The pathogen may survive for many years in 

dead root systems, infecting future tree generations. In 

many cases, this local spread is much more common than 

spores initiating new infections.

Impact—Impacts of decay diseases vary greatly, depending on the disease type and the specific disease. They 

may include: loss of fiber to decay (cull), growth loss, 

direct mortality, predisposition to bark beetles, uprooting 

or snapping of live trees, and provision of wildlife habitat 

(figs. 32-36).

These diseases may affect various values, including 

timber, wildlife, aesthetics, and recreation. In developed 

sites, the potential for failure of live trees can significantly threaten safety and property.

Table 3. Features of white and brown rot.

Chemistry Color Texture Other

White rot All wood components Generally white, Varies: spongy, Texture types vary

removed, either but can be stringy; some among white rot fungi;

simultaneously yellowish to types are called some produce zone

or lignin preferentially, reddish brown laminated, pitted, lines in wood; there

in early stages or pocket rot may be mats or

 based on texture/ rhizomorphs; pocket

 appearance rots may have black 

flecks in pockets

Brown rot Cellulose and Brown, often with When advanced, Decay is fairly uniform;

hemicellulose chains a sheen on split wood shrinks with some fungi produce

broken early, then surfaces early on cubical checks and white mats or felts or

removed; lignin can be crumbled wispy fine cords in

remains to a powder checks, causes rapid 

 strength loss

Figure 31. Dead, decaying 

stem with white rot in the 

outer wood and brown rot 

in the inner wood caused 

by two different fungi. The 

white rot is stringy-fibrous; 

the brown rot breaks easily 

across the grain, has no 

fibrous strength, and is 

crumbly when dry. Photo: 

Jim Worrall, USDA Forest 

Service.

R O O T D I S E A S E S A N D S T E M D E C AY S 
USDA Forest Service RMRS-GTR-241. 2010. 21

Figure 32. Irregular snap of 

live spruce due to butt rot. 

Photo: Jim Worrall, USDA 

Forest Service.

Figure 33. Uprooting of blue spruce 

brought up large root plate. This is typical 

of windthrow, but some roots are rotted by 

Armillaria ostoyae. Photo: Roy Mask, USDA 

Forest Service.

Figure 34. Aspen stem with 

aspen trunk rot (note the 

conks) has begun to fail. 

Photo: Jim Worrall, USDA 

Forest Service.

Figure 35. Typical failure associated with mottled root rot 

of aspen: roots are stubbed near the root collar and are 

almost all rotted. Also note the conk above the root crown. 

Photo: Jim Worrall, USDA Forest Service.

Figure 36. Snapping of spruce with white 

pocket rot; only a few centimeters of 

sound wood remain. Photo: Jim Worrall, 

USDA Forest Service.

R O O T D I S E A S E S A N D S T E M D E C AY S
22 USDA Forest Service RMRS-GTR-241. 2010.

R O O T D I S E A S E S A N D S T E M D E C AY S 

Three important factors that contribute to the abundance and severity of these 

diseases are stand age, stand composition, and a history of wounding. Age, in 

particular, is closely correlated with amount of stem decay. Species composition 

is a major influence because, though some pathogens are fairly host-specific, in 

general there is a range of susceptibility of tree species to opportunistic, woundinfecting decay fungi. Ponderosa pine may represent the most resistant extreme, 

while true firs and aspen are among the Region’s most susceptible species. A history of wounding is also associated with these opportunistic decay fungi in that 

there is generally a correlation between the size of wounds and their likelihood 

of infection.

Wood-decay diseases in this Region are listed in tables 4 and 5. Note that one 

root disease, black stain root disease, is not listed; it is the only major root disease 

in the Region that does not involve wood decay. These lists are not exhaustive, 

but include most decays that have been documented in the Rocky Mountain 

Region.

References: 170, 192, 194
USDA Forest Service RMRS-GTR-241. 2010. 23

Table 4. Root and butt rots of Region 2. Host abbreviations: DF = Douglas-fir, ES = Engelmann spruce, F = true firs, P = pines, QA = aspen, S = spruces, SAF = subalpine fir, WF = white fir. Uncommon hosts are in parentheses.

Common

Pathogen Disease hosts Decay IndicatorsArmillaria solidipes and other Armillaria root disease All White rot; stringy-spongy, wet, zone lines Basal resinosis, crown fading, mushrooms, fansspecies (honey mushroom)Coniophora puteana ES, SAF Brown rot; thin, pale, brown cords in checks Butt may show checks or collapse, indicating partial 

failure; fruiting inconspicuous after tree dies

Flammulina populicola QA White rot; yellow, stringyFomitopsis pinicola Brown crumbly rot S Brown rot; thick fungal mats ConkGanoderma applanatum White mottled rot QA White rot with mottled white/light tan areas; Conks usually present(artist’s conk) infrequent zone linesHeterobasidion annosum Annosus root rot P White rot; may appear laminated, stringy, Disease center, conks

or with pits/pockets

Heterobasidion parviporum Annosus root rot WF (SAF, S) White rot; may appear laminated, stringy, Disease center, conks

or with pits/pockets

Lentinellus montanus SAF, (ES) White rot None; fruits during snowmelt on downed logsOnnia tomentosa/leporina Red root rot (tomentosus S (other conifers) Reddish stain becoming white pocket rot Basal resinosis, conks

and circinatus root rots)

Phaeolus schweinitzii Schweinitzii butt rot DF (other conifers) Brown rot Possible ephemeral fruiting on or around tree(cow-pie fungus)Phellopilus nigrolimitatus Big white pocket rot S (other conifers) White pocket rot with large pockets Usually nonePholiota alnicola SAF, ES White rot; stringy MushroomsPholiota squarrosa SAF, QA, (ES) White rot; in aspen a gray-brown stain Mushrooms(scaly Pholiota) becoming soft and light tan, then stringyPleurotus populinus QA White rot MushroomsSistotrema raduloides QA White rot NoneVesiculomyces citrinus SAF, ES White rot, yellowish, stringy None; usually fruits on downed trees

R O O T D I S E A S E S A N D S T E M D E C AY S
24 USDA Forest Service RMRS-GTR-241. 2010.

R O O T D I S E A S E S A N D S T E M D E C AY S 

Table 5. Stem decays of Region 2. Host abbreviations: DF = Douglas-fir, ES = Engelmann spruce, F = true firs, LPP = lodgepole pine, PP = ponderosa pine, QA = aspen, S = spruces, SAF = subalpine fir, WF = white fir. Uncommon hosts are in parentheses. Pathogen Disease Common hosts Decay Indicators Amylostereum chaillettii SAF, S Whiter rot; stringy None (fruiting inconspicuous, ephemeral, and uncommon) Antrodia serialis SAF Brown rot None (usually fruits after tree dies) Cryptosphaeria lignyota Cryptosphaeria canker QA White mottled rot Canker Dichomitus squalens red ray rot PP White pocket rot, but sometimes Dead, often fallen branches with conks difficult to recognize as such; decay often in radial, star-like pattern Echinodontium tinctorium rust-red stringy rot WF (other conifers) Conks (Indian paint fungus) Fomitiporia hartigii SAF White rot Conks may appear on undersides and base of branches Laurilia sulcata ES White pocket rot; yellowish, may be wet and spongy Peniophora polygonia QA White rot; yellow-brown, stringy Phellinidium ferrugineofuscum ES White rot; laminated, may have Conks are rare small pits with black flecks and white transverse streaks Phellinus tremulae aspen trunk rot QA White rot; firm to spongy, Conks, bird cavities (white trunk rot) yellowish tan in some areas Porodaedalea pini red ring rot ES, LPP, DF, SAF White pocket rot; sometimes with Conks, punk knots abundant zone lines; decay may progress into roots Stereum sanguinolentum red heart rot SAF, ES White rot; initial red stain, becomes Usually none, fruits on slash and logs (bleeding Stereum) light brown, dry, friable, with white fungal sheets when advanced Veluticeps abietina/fimbriata S, F Brown pocket rot Conks
USDA Forest Service RMRS-GTR-241. 2010. 25

Figure 37. The pore surface of 

Heterobasidion parviporum from a white fir 

stump. Photo: Jim Worrall, USDA Forest 

Service.

Annosus Root Disease

Infects fresh stumps and spreads root-to-root

Pathogen—Recent studies show that annosus root disease is caused by two 

closely related fungi, both formerly known as Heterobasidion annosum. The 

two pathogens are H. annosum (in the strict sense) and H. parviporum. These 

are North American variants of species in Europe and Asia. The North American 

variants may be recognized and named as separate species in the future.

Hosts—Heterobasidion annosum is a pine specialist. In this Region, it has been 

found to cause disease only on pines and eastern redcedar and only in the Bessey 

District of the Nebraska National Forest. However, it occurs in Arizona, New 

Mexico, Idaho, and the Midwest, so it may occur undetected elsewhere in the 

Region.

Heterobasidion parviporum favors spruce and fir species but has been found 

only in mixed conifer forests within the range of white fir in southern Colorado. 

It is common on white fir and occurs occasionally on associated subalpine fir, 

Douglas-fir, and blue and Engelmann spruce. It has not been found in our sprucefir forests outside the range of white fir in this Region.

Signs and Symptoms—In some cases, resin flow may be evident near the root 

collar as the tree defends itself against attack. Diseased pines may eventually 

show crown thinning and yellowing. In pines, the disease is most active in the 

sapwood, killing tissues as it progress. In other hosts, the fungus grows first in 

inner wood once it reaches the root collar, so butt rot is a more prominent feature 

of the disease.

Decay may be preceded by a pink to dull violet stain of the wood. Later, 

small, poorly defined pockets or pits are often evident. Small black flecks can 

often be found in well-developed pockets, and wood may separate along the annual rings (laminated rot). Finally, the pockets are lost as the entire mass of wood 

becomes spongy or stringy.

Conks are frequently found in white fir 

disease centers but usually in protected, moist 

microsites such as under litter, inside hollow 

stumps, and even down in hollow root channels. Perennial and tough, they can be up to 

a foot wide. Depending on where they form, 

they may have an irregular brown cap or 

bracket or be completely flat on the substrate. 

The pore surface is whitish with small pores; 

the flesh is creamy tan (figs. 37-38). In some 

cases, especially with H. annosum, only tiny 

“popcorn” conks may be found. Fresh conks 

have a strong mushroom aroma.

R O O T D I S E A S E S A N D S T E M D E C AY S
26 USDA Forest Service RMRS-GTR-241. 2010.

R O O T D I S E A S E S A N D S T E M D E C AY S 

Disease Cycle—The disease 

cycle in pines begins with 

freshly cut stumps. Windblown spores infect stumps of 

live trees within a few weeks 

of cutting. The fungus grows 

down into the stump roots. 

Where there are root contacts, 

the fungus may grow across 

and infect neighboring trees, 

eventually creating a root disease center. Centers typically 

have a stump in the middle, 

old dead and downed trees 

nearby, recent mortality farther out, and live trees that may have crown symptoms on the outside. The fungus fruits on stumps and infected trees, produces 

spores, and completes the cycle. The fungus may survive many years in dead 

root systems and can infect successive tree generations.

The disease cycle may work similarly in true firs, but evidence suggests that 

stump infection may not be the only way for new disease centers to be initiated. 

New infections may occur through basal scars and even through direct infection 

of roots by spores in the soil.

Impact—In pines, H. annosum impact in this Region is geographically restricted 

and is not a significant concern, although the pathogen could conceivably move 

into and cause substantial damage to important pine forests elsewhere in the 

Region. In white fir, disease centers and mortality are common and the impact is 

substantial. Ecologically, the abundance of white fir and this disease in formerly 

pine-dominated forests is due to fire 

exclusion and early harvest of seral 

species. Thus, restoration to a more 

open, pine-dominated forest maintained by fire would greatly reduce the 

disease’s impact.

In pines, the sapwood and cambium 

are often killed before extensive decay 

occurs, and trees tend to die standing. In firs and spruces, especially in 

larger trees, extensive root and butt rot 

often occur to the point that live trees 

may fail mechanically before dying 

(figs. 39-41). However, the fir engraver, Scolytus ventralis, is attracted to 

diseased firs and may kill them before 

direct mortality or failures occur.

Figure 38. Closer view of the pore surface of Heterobasidion 

parviporum from a white fir stump. Photo: Jim Worrall, USDA 

Forest Service.

Figure 39. Typical pattern of decay from 

Heterobasidion parviporum in a white fir stump in 

Colorado. Photo: Jim Worrall, USDA Forest Service.
USDA Forest Service RMRS-GTR-241. 2010. 27

Figure 43.

Commercial borax 

product application 

on a stump. Photo: 

Pete Angwin, 

USDA Forest 

Service.

Figure 42. A commercial borax 

product. Photo: Pete Angwin, 

USDA Forest Service.

Management—Management of annosus root disease is based on two approaches: using resistant species and preventing primary infection. (See comments on white fir ecology under “Impact.”)

•Manipulating species composition. Recent advances in understanding th

pathogen species and their host specialization provide greater opportunity 

for management through species composition. Where pines or eastern redcedar are infected, other species may be planted or favored, and should 

generally be resistant. Where white fir is infected, species other than true 

firs and spruces will likely be successful.

•Chemical protection of stump tops. When applied shortly after cutting, bora

powder (available commercially as Sporax or Tim-bor) effectively prevents 

establishment of H. annosum and H. parviporum in stump tops (figs. 42-

43). This prevents establishment of new disease centers but will neither 

eradicate existing infections nor prevent wound infection on residual trees.

Figure 40. Butt-rotted, live white fir after failing due to 

Heterobasidion parviporum in Colorado. Photo: Jim 

Worrall, USDA Forest Service.

Figure 41. Decayed hollow in white fir stump with 

residual branch traces, caused by Heterobasidion 

parviporum in Colorado. Photo: Jim Worrall, USDA 

Forest Service.

R O O T D I S E A S E S A N D S T E M D E C AY S
28 USDA Forest Service RMRS-GTR-241. 2010.

R O O T D I S E A S E S A N D S T E M D E C AY S 

•Biological control of stump tops. Several benign fungi are aggressive stum

colonizers and can colonize the wood before the pathogen, preventing the 

pathogen from effective establishment. They are applied as stump top treatments, like borax. The most widely tested fungus is Phlebiopsis gigantea.

References: 90, 158, 168, 193

Armillaria Root Disease

Most important root disease in the Rocky Mountain Region

Pathogen—Armillaria root disease is caused by many species of Armillaria, 

a genus of mushroom-forming fungi. Armillaria solidipes (= A. ostoyae) is the 

most common species in the Rocky Mountain Region, but there are indications 

that additional species are present in Wyoming.

Hosts—Armillaria root disease has a very wide host range and has been found 

on almost all common tree species and in all major forest types in the Rocky 

Mountain Region. It is also very widespread, probably occurring in all forests of 

the Region. It seems to be most abundant in spruce-fir and mixed conifer forests.

Signs and Symptoms—Signs are physical evidence of the pathogen and are 

diagnostic (unique to the disease) 

when reliably identified:

•Mushrooms. Mushrooms are th

most conspicuous sign of the 

pathogen (fig. 44). However, 

they are only abundant in certain years and only last for a few 

weeks, usually in late summer to 

fall. The other signs are almost 

always present with the disease.

•Rhizomorphs. These brown t

black, root-like fungal organs 

are several millimeters in diameter, branched, and composed of 

fungal hyphae aggregated within 

a protective sheath. They grow 

in soil or along roots and infect 

when they encounter susceptible hosts.

•Mycelial fans. These whitish, fan-like layers of fungal tissue grow betwee

bark and wood as the fungus invades roots and lower stems (figs. 45-46).

Symptoms may not appear until very late in disease development and are not 

diagnostic when they do appear. Symptoms may include:

•Resinosis. Resin exudation near the soil line (above or below) is a goo

symptom (figs. 46-47). It occurs mostly in the resinous conifers but not in 

all cases. It is evidence of the host attempting to defend itself.

Figure 44. Mushrooms of Armillaria ostoyae can be 

abundant for short periods in late summer to fall during 

wet years. Mushrooms are usually in clusters around 

the base of a tree. Caps are honey-brown with small 

scales; gills are white and attached to the stem, which 

is white to scurfy brown at the base with a poorlyformed ring. Photo: William Jacobi, Colorado State 

University.
USDA Forest Service RMRS-GTR-241. 2010. 29

Figure 47. Resinosis is a symptom often caused 

by Armillaria root disease near the soil line. 

Photo: Jim Worrall, USDA Forest Service.

Figure 48. Armillaria decay with zone lines. 

Photo: Jim Worrall, USDA Forest Service.

•Wood decay. Openings or uprooting of the tree may reveal decay in the woo

of roots or lower stem. Otherwise, decay is hidden and difficult to detect. 

The decay is a white rot and often contains zone lines (fig. 48).

•Crown symptoms. These include slower terminal growth, thin or yellow foliage, branch dieback, and stress crop of cones (particularly in Douglas-fir

Disease Cycle—The pathogen survives many years in dead roots, perpetuating the disease on a site. It has two main ways of spreading. One is local 

spread: the fungus grows across root contacts and grafts between infected 

and healthy roots or, in some cases, via rhizomorphs that grow through soil. 

Figure 45. Mycelial fans under the bark are the 

characteristic sign of Armillaria root disease. Photo: 

USDA Forest Service training slide set.

Figure 46. Symptoms and signs of Armillaria root 

disease: mycelial fan barely visible in the bark on 

the left (near fingers); resinosis apparent in upper 

right; decay in a root near the soil line. Photo: Jim 

Worrall, USDA Forest Service.

R O O T D I S E A S E S A N D S T E M D E C AY S
30 USDA Forest Service RMRS-GTR-241. 2010.

R O O T D I S E A S E S A N D S T E M D E C AY S 

This can result in gradually expanding 

disease centers (fig. 49). However, many 

of the Region’s subalpine forests are completely colonized, so infection is spatially 

random rather than occurring in discrete 

centers. The fungus also spreads is by producing mushrooms with wind-dispersed 

spores. Spores are rarely successful, and 

it is thought that this mode of spread is 

uncommon, especially in the arid West. 

Small, stressed, or highly susceptible trees 

may succumb quickly. Older, vigorous, or 

somewhat resistant trees may survive for 

many years with the disease progressing 

slowly or remaining isolated in initially 

infected roots.

Impact—Armillaria root disease causes 

mortality, growth reduction, cull, and predisposition to other lethal agents. Root 

diseases lead to stand heterogeneity, creating openings and patches of coarse woody 

debris, uneven age classes, and 

altered composition. Mortality is 

often through mechanical failure 

of live trees, which can be dangerous in developed sites (figs. 

50-51). Impacts are not quantified 

in the Rocky Mountain Region 

(though annual volume losses in 

subalpine fir alone in Colorado 

were estimated at 10,500 m3

[370,800 ft3]), but because the 

disease is widespread in most 

forest types, impacts are 

undoubtedly substantial. 

In lodgepole pine, mortality is common up to about 

age 20, when the mortality rate greatly decreases 

(fig. 52). In most other species, impacts seem to be 

more severe in mature trees. 

Subalpine fir is often killed 

quickly, but Engelmann 

spruce often survives until 

Figure 49. Armillaria root disease center with 

standing mortality. Photo: Jim Worrall, USDA 

Forest Service.

Figure 51. Aftermath of an Armillaria root disease center in a 

campground. Photo: Pete Angwin, USDA Forest Service.

Figure 50. Vehicle crushed by a tree infected with Armillaria 

root disease that failed while green. Photo: Jim Worrall, 

USDA Forest Service.
USDA Forest Service RMRS-GTR-241. 2010. 31

mechanical failure. In the Front Range 

and Black Hills, 62% and 75% of ponderosa pine attacked by mountain pine beetle 

were also infected by Armillaria root disease. Similar interactions probably occur 

in spruce-fir forests.

Management—Management of Armillaria 

root disease is difficult. The following approaches used in other areas are impractical 

in this Region’s forests but can offer general 

guidance:

•Manage for resistant species. Almos

all tree species in the Rocky Mountain 

Region are susceptible, and few are 

suitable for a given site. However, 

lodgepole pine becomes resistant at about 20 years and aspen is usually 

little affected, so some heavily impacted stands can be converted by favoring resistant species in thinning or during regeneration.

•Tailor management in disease centers. In subalpine forests in this Region

the disease is typically random rather than aggregated in discrete disease 

centers. However, where centers are recognized, management can be tailored to them.

•Stump extraction. In the Pacific Northwest, long-term experiments hav

demonstrated that removing stumps during regeneration can significantly 

reduce root disease in the future stand. This is sometimes used operationally 

there, but has not been attempted in the Rocky Mountain Region.

•Biological. Research suggests that competitive fungi might be manipulate

to reduce the food available to the pathogen and perhaps even replace it. 

However, these approaches are not operational at this time.

References: 94, 154, 199, 200

Black Stain Root Disease

Mortality centers in pinyon

Pathogen—Black stain root disease is caused by the fungus Leptographium wageneri var. wageneri. This variety infects only pinyons, including two-needle as 

well as singleleaf pinyon in other Regions.

Hosts—The disease causes expanding patches of mortality in many pinyon 

stands of the western slope of the southern and middle Rocky Mountains as far 

north as Idaho (figs. 53-54). It has not been found east of the Continental Divide. 

Two other varieties do not occur in this Region: L. wageneri var. ponderosum infects lodgepole, Jeffrey, and ponderosa pine; and L. wageneri var. pseudotsugae

Figure 52. Armillaria in young lodgepole pine. 

Photo: Pete Angwin, USDA Forest Service.

R O O T D I S E A S E S A N D S T E M D E C AY S
32 USDA Forest Service RMRS-GTR-241. 2010.

R O O T D I S E A S E S A N D S T E M D E C AY S 

infects Douglas-fir. Both those varieties occur primarily in the northern Rocky 

Mountains, British Columbia, and Pacific coast states.

Signs and Symptoms—In advanced disease, foliage is sparse and sometimes 

chlorotic (yellow). A mortality center is often evident, with old snags near the 

Figure 53. Dead and dying pinyon near the edge of a mortality center caused by black stain root 

disease. The smaller trees in the foreground may not be affected yet because their smaller root 

systems are not yet contacting infected roots. Photo: Jim Worrall, USDA Forest Service.

Figure 54. Distribution of black stain root disease in Colorado (from Landis and Helburg 1976).
USDA Forest Service RMRS-GTR-241. 2010. 33

center, recent mortality farther out, and symptomatic, live trees at the edge 

(fig. 53).

An intense black stain can be found in wood of the roots, root collar, and 

often the lower stem. Unlike the pattern of blue stain, which progresses radially 

along the rays to the inner sapwood, black stain progresses longitudinally and 

somewhat tangentially. Longitudinally, it forms long streaks following the wood 

grain (fig. 55). In cross section, it appears as arcs following short segments of 

annual rings (fig. 56).

Disease Cycle—Unlike blue-stain fungi, which colonize rays, this pathogen 

colonizes the tracheids (water-conducting and structural elements) of the sapwood and causes a wilt disease. It aggressively invades living wood of the roots, 

root collar, and lower stem until trees are killed. It does not decay wood.

Where roots grow closely together, the fungus can grow from tree to tree. In 

this way, mortality centers expand. Rate of radial expansion of disease centers in 

pinyon is about 3.3-6.6 ft (1-2 m) per year, but they do not expand indefinitely. It 

is not clear why expansion eventually ceases.

Other varieties of the pathogen are transmitted long distances by insect vectors, primarily root weevils and bark beetles. The vectors are attracted to dying 

roots, so the disease is associated with stand disturbance, especially thinning and 

road construction. As the insects burrow into the soil looking for those roots, 

they tend to graze on intermingled roots of other trees, thereby inoculating the 

pathogen. A vector has not been identified in pinyon, but it is strongly suspected 

that there is one and that the relationship 

to stand disturbance is similar to that of 

the disease caused by the other varieties.

Fruiting of the fungus is almost microscopic and difficult to observe in the 

field. Minute black stalks are formed in 

Figure 55. Black stain in the lower stem of 

diseased pinyon. Stem with the wood exposed in 

tangential view. Photo: Jim Worrall, USDA Forest 

Service.

Figure 56. Black stain in the lower stem of diseased 

pinyon. Stem with the wood exposed in transverse view. 

Photo: Jim Worrall, USDA Forest Service.

R O O T D I S E A S E S A N D S T E M D E C AY S
34 USDA Forest Service RMRS-GTR-241. 2010.

R O O T D I S E A S E S A N D S T E M D E C AY S 

cavities under the bark, such as insect galleries. Sticky masses of spores are produced on the stalks. Insects that contact the fruiting structures are readily coated 

with spores.

Impact—In Colorado, disease frequency is clearly associated with soil depth, 

precipitation, site quality, and tree size and density. Stands with large or dense 

trees may facilitate root-to-root spread of the pathogen because of more frequent 

root contacts. However, rate of disease expansion was not related to density in 

one study. Because the pathogen is restricted to pinyon, mixed stands inhibit 

tree-to-tree spread of the disease.

In the Rocky Mountains, the disease is most active in southwestern Colorado, 

especially in the Four Corners area, but it occurs all along the Western Slope (fig. 

54). Over the long term, black stain is an important disturbance agent in regulating structure and composition of pinyon-juniper stands. It creates structural 

heterogeneity in the forest, with openings that provide habitat diversity for other 

plants and animals. In this sense, the disease promotes old-growth characteristics 

of pinyon-juniper stands.

The pathogen dies quickly after host tissue dies and after a stand replacing 

fire. Disease incidence then increases with time since regeneration. Just as fire 

makes the disease less likely, the disease makes fire more likely. One of the 

major impacts of the disease is an increase in dead fuels. Thus, a fire-disease 

feedback loop can be envisioned that contributes to disease reduction and forest regeneration. The pinyon engraver commonly attacks trees with black stain. 

Under endemic beetle populations, black-stain centers are probably a major 

source of beetles.

Management—No control approach has been effective in pinyon. Cutting and 

burning killed or symptomatic trees does nothing to stop the disease, but because pinyon engraver often invades diseased trees before death, sanitation may 

be effective in preventing beetle outbreaks. Any replanting should be with species other than pinyon. Trenching around a disease center to sever all media 

through which the pathogen may grow has been attempted but was unsuccessful, probably because the pathogen is often in at least two trees beyond the last 

symptomatic one. Another approach is removing all pinyon from in and around 

disease centers, waiting four years, and replanting pinyon. This could be effective because the pathogen does not survive long in roots once they are dead. If 

implemented, at least three healthy-looking trees beyond the center should be 

removed.

Any discretionary pinyon cutting would best be done in the fall or winter. 

Vectors of the other forms of black stain root disease, which are attracted to fresh 

cuts, are active in spring and early summer; presumably, the same is true of the 

pinyon form.

References: 73, 92, 101, 110, 188
USDA Forest Service RMRS-GTR-241. 2010. 35

Figure 57. Engelmann spruce tree that failed 

due to Coniophora root and butt rot. Photo: Jim 

Worrall, USDA Forest Service.

Figure 58. Gray-brown mycelium associated with brown cubical rot. Photo: Jim 

Worrall, USDA Forest Service.

Coniophora Root and Butt Rot

Brown rot with gray-brown cords and mycelium

Pathogen—Several species of Coniophora may cause this disease in the Rocky 

Mountains, but they are difficult to distinguish, and there is no evidence that they 

differ in pathology or ecology. The most common is C. puteana, and in a broad 

sense this name can be used for all of them.

Hosts—Engelmann spruce and subalpine fir are probably the most common 

hosts, but the disease has been found in lodgepole pine, and likely all of the 

Region’s conifers can be infected.

Signs and Symptoms—The fungus causes 

a brown cubical rot in the roots and base of 

the stem. Frequently, there are no external 

indicators, but there may be cavities near 

the root collar that reveal the decay. When 

decay is advanced and little sound wood remains, the root collar may crack open due to 

wind. Mechanical failure of live trees may 

eventually occur at the roots or lower stem 

(fig. 57).

When the decay becomes exposed, characteristic mycelium and mycelial cords 

are the most frequent diagnostic signs 

(figs. 58-60). Some pieces of decayed wood 

may have a thin, velvety, gray-brown mycelium on the surface. More commonly, 

gray-brown to grayish white mycelial cords 

are present in the decayed wood and under 

the bark of dead roots. These are similar 

to but less well-defined and less organized 

than the black rhizomorphs of Armillaria

species.

The fruiting 

body is rarely 

seen and normally appears 

after the tree 

is down. It is a 

thin layer with 

an irregular 

surface (no 

R O O T D I S E A S E S A N D S T E M D E C AY S
36 USDA Forest Service RMRS-GTR-241. 2010.

R O O T D I S E A S E S A N D S T E M D E C AY S 

pores), brown in the center and whitish at the margin, and a few inches in diameter 

(figs. 61-62).

Other diseases causing brown root and butt rot can be distinguished from this 

one by carefully inspecting the decayed wood. Phaeolus schweinitzii, the cowpie 

fungus, has no cords associated with the decay. Fomitopsis pinicola, the red-belt 

fungus, may occasionally infect the butts of conifers. It has no cords but has distinct, thick, creamy white mycelial mats that form in splits of the decayed wood.

Disease Cycle—The disease cycle has not been well studied. Basal wounds and 

fire scars are thought to be infection courts, at least in some cases. Infection centers have not been observed, and there is apparently no evidence for root-to-root 

spread from an infected tree to a neighbor.

Impact—The incidence and impact of Coniophora root and butt rot have not 

been quantified in the Rocky Mountain Region. In pristine Norway spruce forests of northern Finland and northwestern Russia, Coniophora root and butt 

rot is the most important mortality agent, causing death by mechanical failure. 

Figure 59.

Gray-brown mycelium and mycelial 

cords. Cords are 

often finer and 

more delicate than 

these. Photo: Jim 

Worrall, USDA 

Forest Service.

Figure 60. Mycelial cords. Cords are often finer and more delicate than these. Photo: Jim 

Worrall, USDA Forest Service.
USDA Forest Service RMRS-GTR-241. 2010. 37

Figure 61. Young fruiting body. 

Photo: Jim Worrall, USDA Forest 

Service.

Figure 62. Young fruiting body. Note the fine cords associated with the fruiting body. Photo: Jim Worrall, 

USDA Forest Service.

In northwestern Ontario, it is one of the three most important of 21 decay fungi 

that contribute to black spruce mortality. In a large study of white spruce, black 

spruce, and balsam fir in the same area, Coniophora root and butt rot was the 

fourth most frequent of 30 identified root diseases.

From inoculations of white spruce, C. puteana infected more trees and grew 

faster than Onnia tomentosa. During the 3 years after inoculation, C. puteana

infected more trees and grew faster in those that died than in those that remained 

alive. This suggests that host vigor plays a role in suppressing infections by this 

fungus.

Management—Because it is difficult to detect in standing trees, management 

of this disease is not often a consideration. Where it is detected in developed 

sites or in timber stand improvement projects, infected trees should be removed. 

Otherwise, it is not considered a threatening disease but a native part of ecosystem function.

References: 111, 182, 183, 184

R O O T D I S E A S E S A N D S T E M D E C AY S
38 USDA Forest Service RMRS-GTR-241. 2010.

R O O T D I S E A S E S A N D S T E M D E C AY S 

Root Diseases with White Pocket Rots

Big white pocket rot and red root rot

Pathogen—Several pathogens cause a white pocket rot in the roots and butts 

of conifers in the Rocky Mountain Region. However, discussed in this entry 

are Phellopilus (Phellinus) nigrolimitatus, which causes a root disease called 

big white pocket rot and two species, Onnia (Inonotus) tomentosa and leporina

(formerly misnamed O. circinata), which cause a disease known as red root rot, 

or tomentosus and circinatus root rot, respectively. The following two pathogens 

(described separately in this guide) also cause white pocket rots and may be 

confused with the fungi described in this section: Porodaedalea (Phellinus) pini

often grows down into roots and causes red ring rot in conifer stems; annosus 

root rot may also appear as a white pocket rot.

Hosts—Big white pocket rot and red root rot can infect most conifers, but they 

infect mostly spruce species in this Region.

Signs and Symptoms—Big white pocket rot (caused by Phellopilus nigrolimitatus) usually has no external indications. The fruiting body (conk) is uncommon 

and usually forms after the tree is dead. Conks are perennial, are flat on the bark, 

and often have a small shelf or cap at the upper end that is somewhat soft and 

spongy. The pore surface is cinnamon-colored and smooth with very small pores. 

The internal flesh is brown and typically has one or more black lines.

Red root rot (caused by Onnia species) usually has no external symptoms, but 

there may be some basal resinosis. Fruiting is uncommon but may be abundant 

in certain years. Fruiting bodies arise from buried roots with a short, more or less 

central stem or appear directly on the tree, usually at the root collar (figs. 63-64). 

Conks are annual and soft to leathery. The cap is circular with a sunken center, 

up to 4 inches (11 cm) in diameter, yellowish brown, and velvety. The lower, 

pore surface is pale brown but darkens with age. The two species of Onnia are 

virtually indistinguishable in the field, but conks of O. leporina are more likely 

to be on the base of the tree and in fewer numbers than those of O. tomentosa.

These diseases all cause a white pocket rot. Big white pocket rot has ellipsoidal pockets up to about 3/8 by 1 inch (10 by 25 mm), rectangular and white, 

separated by pale brown, firm wood (figs. 65-66). Pockets in the other decays 

Figure 63. Onnia tomentosa, showing caps from above. Photo: Jim Worrall, USDA Forest Service.
USDA Forest Service RMRS-GTR-241. 2010. 39

are about 1/13 by 3/8 inch (2 by 10 mm) 

(fig. 67). Early decay caused by the 

Onnia species is indicated by a reddish 

stain of the wood (fig. 68). Roots in the 

soil may have small patches of golden 

brown fungal tissue on the surface 

(fig. 64). Zone lines could be present in 

decays by any of these fungi.

Disease Cycle—Initial infection is by 

spores from conks. The infection site is 

unknown but could be roots in the soil or 

basal stem wounds. Decay progresses for 

Figure 65. Big white pocket rot. Pockets are up to 3/8 inch (1 cm) wide and many have white cellulose 

remains. Photo: Jim Worrall, USDA Forest Service.

Figure 66. Big white pocket rot. Decayed wood has 

weathered, leaving relatively sound wood between 

the pockets. Photo: Jim Worrall, USDA Forest 

Service.

Figure 68. Red root rot caused by Onnia

sp. Stain preceding decay in the butt of 

a sapling in British Columbia. Photo: Jim 

Worrall, USDA Forest Service.

Figure 67. Red root rot caused by Onnia sp. 

Advanced decay. Photo: Borys M. Tkacz, USDA 

Forest Service, Bugwood.org.

Figure 64. Onnia tomentosa, showing caps from 

the sides and small patches of fungal mycelium on 

roots. Photo: Jim Worrall, USDA Forest Service.

R O O T D I S E A S E S A N D S T E M D E C AY S
40 USDA Forest Service RMRS-GTR-241. 2010.

R O O T D I S E A S E S A N D S T E M D E C AY S 

many years before fruiting occurs. In the case of big white pocket rot, fruiting does 

not occur until after the tree is dead and down. With red root rot, there may be direct 

growth of the fungus tree-to-tree between contacting roots, leading to mortality centers, but that has not been shown with big white pocket rot.

Impact—Big white pocket rot is associated with old-growth stands and largediameter trees. It causes extensive wood decay of the roots and butt and can 

grow far up the stem, leading to mechanical failure of live trees. In Scandinavia, 

Phellopilus nigrolimitatus is a red-listed (protected) species. Studies have linked 

it to old-growth conditions and suggest that forest management reduces its population. This may be a consideration if management is considered in old-growth 

stands.

Red root rot is a damaging disease of spruces and other hosts elsewhere, causing large mortality centers. Although infected trees can be found in the Rocky 

Mountain Region and fruiting is sometimes abundant, major damage and mortality centers have not been documented thus far.

Management—Big white pocket rot can lead to tree failure in developed sites 

with large trees. Because of the difficulty of detection and damage that can be 

done by large trees, sounding and increment coring should be done carefully to 

detect the disease. For timber management in old-growth stands, consider the 

general abundance of the fungus, its presence in the area, and its potential sensitivity to management. However, there are no data indicating how common or 

rare the fungus is in this Region. Red root rot is normally not damaging enough 

to consider during timber management, but as with all root rots, it can be important and contribute to hazard when it occurs in developed sites.

References: 57, 158, 169

Schweinitzii Root and Butt Rot

Red-brown cubical root and butt rot of conifers

Pathogen—Schweinitzii root and butt rot is caused by Phaeolus schweinitzii, 

also known as the velvet-top or cowpie fungus.

Hosts—All conifers are probably susceptible to the disease, but the most common host in the Rocky Mountain Region is Douglas-fir. Infrequent hosts include 

lodgepole pine and Engelmann spruce.

Signs and Symptoms—Trees infected with P. schweinitzii rarely display outward symptoms unless they are in the advanced stages of the disease, so diagnosis 

often occurs after the tree loses structural support and topples or is windthrown. 

Possible symptoms may include thinning crowns, poor shoot growth, and/or 

branch dieback. Wood decay may be visible in openings on the stem or nearby 

stumps (figs. 69-70). Incipient decay is yellow to red and dry. Advanced decay 

is red-brown and cubical and sometimes has thin, resinous felts present in the 

cracks.
USDA Forest Service RMRS-GTR-241. 2010. 41

Figure 71. Pore surface of young Phaeolus 

schweinitzii conk. Photo: Jim Worrall, USDA 

Forest Service.

Figure 72. Typical Phaeolus 

schweinitzii conk with brown, 

velvety surface; yellowish margin; and greenish undersurface. 

Photo: Jim Worrall, USDA Forest 

Service.

Figure 73. Old 

Phaeolus schweinitzii conk that 

is entirely brown, 

dry, and brittle. 

Photo: USDA 

Forest Service, 

Bugwood.org.

Occasionally, fruiting bodies can be seen 

on the ground emerging from diseased roots of 

stumps or living trees (figs. 71-73). Infrequently, 

they emerge directly from the tree’s base or 

stump. Fruiting bodies are annual, spongy, 

and mushroom-like with large, irregular pores 

on the undersurface. Caps are red-brown and 

velvety, margins are yellowish brown, and undersides are green when fresh, becoming brown 

with age. As they dry, they become entirely 

brown and brittle and resemble cow pies. Caps 

are usually 5-10 inches (13-25 cm) in diameter with short stems.

Figure 69. Phaeolus schweinitzii decay in a Douglas-fir stem. 

Photo: Jim Worrall, USDA Forest Service.

Figure 70. Brown cubical rot typical of 

Phaeolus schweinitzii. Photo: Joseph 

O’Brien, USDA Forest Service, Bugwood.org.

R O O T D I S E A S E S A N D S T E M D E C AY S
42 USDA Forest Service RMRS-GTR-241. 2010.

R O O T D I S E A S E S A N D S T E M D E C AY S 

Disease Cycle—Spread of P. schweinitzii occurs primarily by means of winddispersed spores produced in conks. Root-to-root infection may occur, but it appears to be very uncommon. Therefore, diseased trees are dispersed in stands 

rather than in discrete disease centers. The fungus gains entry through basal 

wounds, particularly fire scars or damaged roots. Conks are produced annually 

from decaying wood. The fungus can persist for many years in stumps and dead 

trees.

Impact—Schweinitzii root and butt rot is a major disease of mature Douglas-fir. 

Decay is generally confined to the heartwood and is found in the roots and lower 

10 ft (3 m) of the stem. Wood loses its structural integrity rapidly as decay progresses, and susceptibility to breakage and windthrow increases. Infected trees 

may become more susceptible to Douglas-fir beetle or Armillaria root disease.

Management—Butt rot can be detected by sounding the lower stem with an ax 

and coring wood. Management strategies include avoiding wounding, removing infected trees, and harvesting on shorter rotations. Remove trees showing 

evidence of schweinitzii root and butt rot in recreation areas and other developed 

sites.

References: 10, 159

White Mottled Rot

Root rot that topples live aspen

Pathogen—White mottled rot is caused by the fungus Ganoderma applanatum. 

The fruiting body of the fungus is known commonly as “artist’s conk” for reasons explained below.

Hosts—Although G. applanatum occurs on many tree species (often on dead 

trees) across the northern hemisphere, it occurs primarily on living aspen in the 

Rocky Mountains. It can persist for some time after the tree dies and may also 

occur on cottonwoods.

Signs and Symptoms—The fruiting body is a conk (shelf fungus) that occurs 

at the base of infected aspens, usually within about 12 inches (30 cm) of the 

soil line (figs. 74-75). It is initially a white bulge, and the margin remains white 

when actively growing. It becomes more or less flat and may grow up to about 

12 inches (30 cm) wide and project out the same distance. The upper surface 

is irregular-shaped and brown to gray; the lower surface is pure white when 

fresh with fine pores. Although not every infected tree has conks, most advanced 

infections produce conks before the tree falls over or dies. Conks are the only 

useful indicator of infection in live, standing trees.

Where the fresh white underside of a conk is bruised, as with a finger or stick, 

it immediately and permanently becomes dark brown. Artists may use this reaction to draw pictures, leading to the common name of the fungus, “artist’s conk” 
USDA Forest Service RMRS-GTR-241. 2010. 43

Figure 76. Artwork drawn using only a stylus and the fresh, sensitive 

pore surface that turns dark brown where bruised. Photo: Jim Worrall, 

USDA Forest Service.

Figure 77. Typical failure of live aspen due to 

white mottled root rot. Roots usually break near 

the root collar and no significant root plate or 

ball is lifted from the soil. Photo: Jim Worrall, 

USDA Forest Service.

(fig. 76). If the conk is then left to dry, 

the contrast remains and the surface is 

no longer sensitive.

The fungus causes 

extensive decay of the 

roots and butt of the tree 

(fig. 77). Decayed wood is 

mottled with alternating, 

small, white and light tan 

areas. The wood eventually becomes spongy, and 

black zone lines may develop in it.

Disease Cycle—Infection 

may occur from airborne spores or, in some 

cases, from contacts with infected roots of 

neighboring trees. Little is known about 

how or where spores cause infection, but it 

may occur at small wounds in the roots or 

root collar. Root-to-root spread, leading to 

disease centers or groups of infected trees, 

seems to occur primarily on the best aspen 

sites with deep soil.

After the tree is infected for several 

years, the pathogen has gathered sufficient 

resources to reproduce. Conks are produced 

near the base of the tree. Microscopic, airborne spores are released from the pores on 

the underside of conks.

Conks can produce huge numbers of 

spores. In only 4 by 4 inches (10 by 10 cm) 

Figure 74. Conk of the pathogen Ganoderma 

applanatum. The margin is white and blunt and the 

underside is white when actively growing. Photo: 

Jim Worrall, USDA Forest Service.

Figure 75. Another conk of the pathogen 

Ganoderma applanatum. The margin is white and 

blunt and the underside is white when actively 

growing. Photo: Mary Lou Fairweather, USDA Forest 

Service.

R O O T D I S E A S E S A N D S T E M D E C AY S
44 USDA Forest Service RMRS-GTR-241. 2010.

R O O T D I S E A S E S A N D S T E M D E C AY S 

of conk surface, it has been estimated that 4.65 billion spores can be produced 

and released into the air in a 24-hour period.

Impact—White mottled rot is by far the most important root disease of aspen in 

the West. Infected trees usually fall over while alive, often with healthy-looking 

crowns. The disease is typically associated with about 90% of windthrow of live 

aspen trees. It can be particularly important in developed sites as it contributes 

to hazard trees. Small trees or trees on poor sites with dry, shallow soils may be 

killed before decay leads to windthrow.

The disease seems to be most common and damaging on moist sites with 

good aspen growth. It is not known if this is because of favorable physiology of 

vigorous trees or favorable soil characteristics of good aspen sites. In particular, 

root-to-root spread, leading to disease centers, occurs primarily on good aspen 

sites.

Management—Practical methods for preventing or reducing the incidence of 

white mottled root rot are not available. In developed sites with aspen, hazard 

tree inspectors should be trained to recognize conks. In the long run, Regional 

policy strongly encourages shifting vegetation away from aspen in developed 

sites.

Because the disease kills and decays roots, there is concern that it may affect the success of suckering after cutting infested stands. However, studies are 

needed to determine whether this occurs and how large the effect is.

References: 79, 143

Aspen Trunk Rot

Common conk on aspen

Pathogen—Aspen trunk rot (also called white trunk rot) is a stem decay (heart 

rot) of living aspen. It is caused by the fungus Phellinus tremulae.

Hosts—This pathogen occurs only on living aspen.

Signs and Symptoms—The fruiting body is a conk (shelf fungus). The top of 

the conk generally slopes down and the bottom slopes up, so it is roughly triangular in profile (fig. 78). It is hard and woody, black and cracked on top, and purplish brown with tiny pores on the bottom. Inside is a granular core at the point 

of attachment with a hard flesh layer above and a tube layer below. The conks are 

perennial and may live up to about 20 years. A new tube layer is produced each 

year at the bottom, but the layers are indistinct and difficult to count.

Decayed wood is firm to spongy, fibrous, yellowish tan, and often has a sweet, 

wintergreen odor. Decay columns rarely become hollow. Diffuse zone lines are 

scattered through the decayed wood (figs. 79-80), particularly in earlier stages 

near the edge of the decay column. The decay is a white rot. Because cellulose is 

only degraded as it is used by the fungus (unlike brown rot), decayed wood has 

some intact cellulose until very late stages and remains fibrous. Therefore, it can 
USDA Forest Service RMRS-GTR-241. 2010. 45

Figure 80. Cross section from a different tree that shows the 

bulge in the column where a conk was produced. Photo: Jim 

Worrall, USDA Forest Service.

be used in limited proportions in products 

such as waferboard.

Disease Cycle—The pathogen infects 

branch stubs or small dead branches, eventually growing into the inner wood. It may infect wounds but does not require substantial 

wounds. It does not colonize dead trees and 

dies soon after the host dies. After decaying for perhaps 5 years or more, it begins to 

grow out to the surface along branch traces 

to produce conks. Microscopic spores produced in conks are airborne and travel long 

distances. Spores can cause infection if they 

land on a suitable point.

Impact—Aspen trunk rot is the most common stem decay of aspen in North America 

(but in a study in Colorado, incidence of 

Peniophora polygonia was slightly higher). 

More importantly, it decays the greatest volume of wood. Infected trees lose an average 

of 70% of wood volume in cull. Stand age 

and site quality can be important. On good 

sites with deep soils and adequate moisture, 

incidence of decaying trees increased linearly with age from near 0% at 40 years to 91% 

at 160 years. Cull increased to roughly 50% at 160 years. On poor sites with dry, 

shallow soils, decay was higher, and less dependent on age. The pathogen may 

kill slow-growing trees directly by growing into and killing older sapwood. Tree 

breakage is another impact of particular concern in developed sites.

Figure 78. Fruiting body (conk) of Phellinus 

tremulae, the cause of white trunk rot of aspen. 

There are often multiple conks on one tree. Also, 

note the cavity excavated by a woodpecker. 

Such cavities are almost invariably excavated 

in aspens with white trunk rot, sometimes below 

the conk so that the cavity has an awning. 

Photo: Jim Worrall, USDA Forest Service.

Figure 79. Longitudinal section showing the 

extensive column of decay and zone lines. 

Photo: Jim Worrall, USDA Forest Service.

R O O T D I S E A S E S A N D S T E M D E C AY S
46 USDA Forest Service RMRS-GTR-241. 2010.

R O O T D I S E A S E S A N D S T E M D E C AY S 

Phellinus tremulae provides important habitat for cavity-nesting birds 

(fig. 78), although in the long term it can also contribute to deterioration of aspen stands and loss of aspen cover type. Red-naped sapsucker, Williamson’s 

sapsucker, downy woodpecker, and hairy woodpecker nest primarily in aspen in 

many areas. Nests are almost always excavated in trees with decay. Aspen may 

be important for nesting because it has stem decay much more frequently than 

other common tree species of the Rocky Mountains.

Management—Compared to other decays, conks are reliably produced and are 

useful indicators for detecting and estimating decay: 75-85% of trees with cull 

due to aspen trunk rot have conks. Perhaps one additional tree may be infected, 

then, for every three or four trees with conks. Average cull for trees with conks 

is 82%, but only 40% for infected trees without conks. On a linear basis, decay 

generally extends 8-12 ft (2.4-3.7 m) in each direction from conks, and cull increases with number of conks.

Harvesting stands before decay becomes severe is probably the most effective 

management approach in stands managed for timber. Clonal variation in susceptibility to decay has been demonstrated. Clones differed not only in percent 

decay but also in position of rot columns and type of rot. Focusing management 

on clones with low levels of decay should result in future stands with the same 

resistance.

Partial cutting in aspen stands is strongly discouraged. Stands often deteriorate rapidly within 5 years after partial cutting. Wounding and subsequent 

canker infections, sunscald, and boring insects weaken and kill residual trees. 

Clearcutting is most likely to result in prolific sprouting of aspen. It interrupts 

succession to conifers and consequent loss of aspen cover type.

References: 4, 36, 75, 79, 82, 179, 181

Brown Crumbly Rot

Common saprot of conifers

Pathogen—Brown crumbly rot is caused by Fomitopsis pinicola (= Fomes pinicola). The fungus is a very common decayer of conifers and is known as the redbelt fungus.

Hosts—Fomitopsis pinicola is found on dead conifers and aspen. Occasionally, 

it may be found on dead parts of living trees.

Signs and Symptoms—The red-belt fungus forms perennial conks that are 

corky and shelf-like. Tops of conks are leathery to woody and range in color 

from cream, gray-brown, reddish brown, and dark brown to almost black. They 

often have a reddish orange growing margin (red belt) and a whitish to creamcolored pore surface (figs. 81-82). Conks vary in size and can be slightly over a 

foot (30 cm) in diameter.

Young conks are amorphous masses of white or cream-colored tissue 

(fig. 83). Later, the fruiting body develops into the typical perennial shelf conk 
USDA Forest Service RMRS-GTR-241. 2010. 47

from 3-18 inches (7.6-45.7 cm) wide with a light 

lower surface of minute pores filled with reproductive spores. A reddish orange band typically 

forms just inside the white to cream-colored 

edge of the conk’s upper surface—the point 

where a new pore layer is produced annually.

Initially, the wood becomes yellowish to 

pale brown and later develops into the crumbly 

brown cubical rot (fig. 84). In later stages, the 

wood develops cubical cracks, in which white 

sheets of mycelium can develop.

Disease Cycle—Spores are disseminated by 

wind. The spores typically germinate on and 

colonize dead trees, but the airborne spores can 

infect and colonize living trees through 

wounds and broken tops.

Impact—Brown crumbly rot is one of the 

most common and important wood decays 

in North America. It performs an important ecological role in the degradation of 

woody material necessary for nutrient recycling in a forest. It is almost exclusively 

a saprobe, rotting dead trees and stumps. 

Both sapwood and heartwood are readily decayed. Rarely, this fungus causes a 

heartrot of living conifers where a large 

wound has allowed this very weak pathogen to enter the heartwood. Ponderosa 

pines killed by fire or beetles are often 

invaded by F. pinicola and decay rapidly, 

reducing salvage potential. Figure 84. Brown cubical rot with white mycelial 

mats, caused by Fomitopsis pinicola. Photo: 

James T. Blodgett, USDA Forest Service.

Figure 81. Red belt conks often have a distinctive red 

band along the perimeter when mature. Photo: Susan 

K. Hagle, USDA Forest Service, Bugwood.org. Figure 82. The distinctive red band might also 

be absent. Photo: James T. Blodgett, USDA Forest 

Service.

Figure 83. The distinctive band is usually absent on immature conks. Photo: 

John W. Schwandt, USDA Forest Service, 

Bugwood.org.

R O O T D I S E A S E S A N D S T E M D E C AY S
48 USDA Forest Service RMRS-GTR-241. 2010.

R O O T D I S E A S E S A N D S T E M D E C AY S 

Management—Injuries caused by falling trees or mechanized equipment during 

thinning can offer the fungus an opportunity to become established in standing 

trees. Therefore, injury prevention can reduce volume loss.

Fomitopsis pinicola is an important component of the coniferous forest ecosystem because it decays dead trees and logging slash, leaving a lignin-rich 

residue that is a major organic component of the upper soil layers. This residue enhances water-holding and cation-exchange capacities of soil. The residue 

is also very resistant to further decomposition and, therefore, enhances carbon 

sequestration by forests. Decayed snags and fallen trees are habitat for many 

amphibians, birds, and mammals.

References: 1, 57, 72, 125, 145

Gray-Brown Saprot

The pouch fungus

Pathogen—Gray-brown saprot is caused by Cryptoporus volvatus (= Polyporus 

volvatus), the pouch fungus.

Hosts—Gray-brown saprot is common on most recently killed conifers except 

cedar and five-needle pines. Conks are frequent on dead ponderosa pine.

Signs and Symptoms—Fruiting bodies of the pouch fungus are often scattered 

on the bark surface of trees killed by bark beetles and fire (fig. 85). Rounded, 

white, or tan conks that are about 1 inch (2.5 cm) in diameter form on the outer 

bark (figs. 85-86). This fungus causes a grayish to gray-brown-colored white rot 

of the sapwood (fig. 87).

Disease Cycle—Fruiting occurs within 1-3 years after a tree dies. Conks often 

emerge through holes in the bark produced by bark beetles or woodborers. Conks 

are leathery and scaled at first with an air space and a pink pore layer inside. At 

maturity, the conks break open on the underside, releasing spores. Spores are disseminated by wind, and there is evidence that bark beetles may acquire spores as 

they fly due to electrostatic forces. New conks may be produced on a dead tree 

each year for up to 3 years. Conks live only one summer and deteriorate on the 

tree within 2 years.

Impact—The pouch fungus is a common saprotting organism of recently killed 

trees. It can cause extensive sapwood decay, and it rapidly colonizes the outer 

sapwood (fig. 87).

Management—Prompt salvage is the only means of damage control. Timber 

volume loss caused by pouch fungus decay can be substantial. The rapid decay 

of sapwood necessitates timely harvest of bark beetle and/or fire-killed trees. 

Occasionally, pouch conks appear on trees with green foliage following a lightintensity fire or bark beetle strip attack. Usually, these trees die by the following 

year and should be marked for salvage removal if consistent with the project 

guidelines.

References: 1, 57, 145, 175
USDA Forest Service RMRS-GTR-241. 2010. 49

Red Ray Rot

Difficult to detect in living trees

Pathogen—The fungus Dichomitus squalens (= Polyporus anceps) causes red 

ray rot, sometimes known as red rot.

Hosts—The hosts include ponderosa and pinyon pine. This decay is common on 

ponderosa pine in the Black Hills.

Figure 87. Saprot 

caused by pouch 

fungus in a tree 

attacked by 

Douglas-fir beetle. 

Photo: Russ Hogan, 

Bugwood.org.

Figure 85. Numerous conks of the pouch fungus on boles. 

Photo: Sandy Kegley, USDA Forest Service, Bugwood.org.

Figure 86. Close-up 

view of the pouch 

fungus. Photos: 

Susan K. Hagle, 

USDA Forest Service, 

Bugwood.org.

R O O T D I S E A S E S A N D S T E M D E C AY S
50 USDA Forest Service RMRS-GTR-241. 2010.

R O O T D I S E A S E S A N D S T E M D E C AY S 

Signs and Symptoms—Dichomitus 

squalens rarely produces a flat, annual fruiting body on the underside 

of dead branches or stems with intact 

bark (fig. 88). The pore surface is 

white when fresh and ages to yellow. 

The indicators of decay are difficult to 

identify on standing trees and include 

decayed branch stubs and fruiting on 

downed branches. The fungus causes 

a white pocket rot. Like other wood 

decays, it has two distinct stages: incipient and advanced.

The incipient stage is characterized 

by a reddish brown discoloration of the 

affected wood, with no obvious changes 

in structure or strength. The advanced 

stage includes a radial pattern of stain 

and decay (fig. 89). This is characterized by small, often poorly defined, 

white pockets in the discolored wood, 

accompanied by progressive changes 

in structure and reduction in strength. 

As decay progresses, the pockets become more and more numerous until 

they merge and give the affected wood 

the appearance of a fibrous white mass 

(fig. 90). Eventually, the white, lint-like 

material disappears, leaving the bleached, grayish brown, decayed wood in either a stringy or a somewhat amorphous condition.

Both stages of red ray rot are usually visible in a board sawed from a decayed log. 

At the point where rot started in the trunk heartwood, advanced decay often forms a 

cavity. Extending in both 

directions from this point 

are more or less continuous columns of advanced 

decay, bordered by incipient decay.

Disease Cycle—The 

spores are dispersed by 

wind, land in cracked 

bark crevices of dead 

Figure 90. Decay caused by Dichomitus squalens in a branch. Photo: 

Jim Worrall, USDA Forest Service.

Figure 88. Conks of Dichomitus squalens underneath 

a dead ponderosa pine branch. Photo: Southwestern 

Region, USDA Forest Service.

Figure 89. Decay caused by Dichomitus squalens

in old growth ponderosa pine. Photo: Jim Worrall, 

USDA Forest Service.
USDA Forest Service RMRS-GTR-241. 2010. 51

branches, and germinate to colonize the area between the bark and wood and 

eventually the heartwood. The fungus fruits abundantly on the lower side of decaying dead material in close contact with the ground. The flat, white fruiting 

bodies appear about 4 years after infection and then develop annually during the 

rainy season for about 6 years.

Impact—Dichomitus squalens is the most common decayer of ponderosa pine 

in the Black Hills. It has been found in Colorado, but seems to be relatively rare 

there. It is a decayer of slash (a saprobe) as well as a decayer of live trees. It 

causes a significant amount of cull in live trees greater than 150 years old. Red 

ray rot may provide habitat for cavity nesting birds and other wildlife.

Management—Timber volume loss caused by red ray rot can be considerable. 

Reducing tree wounds might reduce the probability of infection, but this fungus 

can enter through dead branches. Trees infected by this fungus should be marked 

for removal to improve the residual stand and to release healthy trees, if consistent with the project guidelines.

References: 56, 57, 159

Red Ring Rot

White pocket rot of conifers

Pathogen—Red ring rot is a wood-decay disease of the inner wood of stems of 

living conifers. It is caused by the fungus Porodaedalea (Phellinus) pini.

Hosts—Most conifers in the Rocky Mountain Region can be infected. Engelmann 

spruce, lodgepole pine, subalpine fir, and Douglas fir are commonly infected in 

some areas of the Region.

Signs and Symptoms—The 

fruiting body is a conk (shelf 

fungus), but it occurs less commonly on infected trees than 

Phellinus tremulae does on aspen. It usually occurs at branch 

stubs or knots (fig. 91). The 

upper surface is reddish brown 

to blackish, concentrically furrowed, and somewhat hairy 

near the margin. The lower surface is usually sloped, yellowish 

brown, and covered with circular to irregular-shaped pores. 

The overall texture is tough and 

corky. Figure 91. Conk of Porodaedalea (Phellinus) pini on 

Engelmann spruce beneath a branch stub. Many infected trees 

do not have conks. Photo: Jim Worrall, USDA Forest Service.

R O O T D I S E A S E S A N D S T E M D E C AY S
52 USDA Forest Service RMRS-GTR-241. 2010.

R O O T D I S E A S E S A N D S T E M D E C AY S 

In some hosts, punk knots, 

which are also definite indicators 

of the disease, occur more commonly than conks. Punk knots 

are swollen or sometimes sunken 

knots that are resinous and do not 

callus over normally (figs. 92-93). 

They are filled with fungal tissue. 

To investigate a suspicious knot, 

shave it with a hatchet or stout 

knife to see if it has the characteristic reddish brown fungal tissue 

in it, perhaps soaked partly with 

resin.

Decay caused by Porodaedalea 

pini is also fairly unique and diagnostic. In early stages, infected 

rings may become reddish to purple in tangential arcs, giving the 

disease its name. Later, a white pocket rot develops (figs. 94-95). The defect is sometimes called white speck. Pockets are mostly hollow but are delignified and contain 

white residual cellulose. There may also be black specks in the pockets; these are 

chemical by-products of delignification. Borders between the pockets may be relatively undecayed. In some cases, the wood may also have distinct black zone lines 

throughout (fig. 95); this decay may be caused by a closely related species that has 

Figure 93. Front view of punk knot of Porodaedalea

(Phellinus) pini after shaving, revealing resin and 

reddish, golden brown fungal tissue that has replaced 

the branch trace. Photo: Jim Worrall, USDA Forest 

Service.

Figure 94. Typical white pocket rot of 

Porodaedalea (Phellinus) pini with white 

pockets bordered by reddish, relatively 

undecayed wood. Photo: Jim Worrall, USDA 

Forest Service.

Figure 92. Punk knots and decay caused by Porodaedalea

(Phellinus) pini. A slightly swollen, resinous knot in oblique 

view. Photo: Jim Worrall, USDA Forest Service.
USDA Forest Service RMRS-GTR-241. 2010. 53

yet to be described. Phellopilus 

nigrolimitatus causes a similar 

decay that may be confused with 

red ring rot, but the pockets tend 

to be larger.

Disease Cycle—Spores produced in conks are disseminated 

by wind. Large wounds are apparently not the usual site of infection as is true of some fungi. 

Spores that land on a suitable 

small wound or twig stub may 

infect and grow into the inner 

wood. When decay is sufficient to provide enough resources, a new conk may be 

produced. Time from infection to conk production may be 10-20 years or more.

Impact—Red ring rot is the most common decay in Engelmann spruce and 

causes the largest decay columns. It is also important in lodgepole pine and is 

the second most important decay in subalpine fir. In a study of those three species, Porodaedalea pini caused 64% of all defect, including non-decay defects. 

It causes extensive cull, especially in old stands. Decay may extend 4 ft (1.2 m) 

above and 5 ft (1.5 m) below conks or punk knots. Decay tends to occur in the 

lower stem and may even develop into the large roots. It is not restricted to the 

heartwood and may develop outward into the sapwood. It sometimes leads to 

mechanical failure of live trees, causing hazard in recreation sites.

Decayed trees may provide nesting, denning, or hiding habitat for animals. 

Although most cavity nesting by birds in this Region is apparently in aspen, various animals may take advantage of advanced decay to excavate the inner wood. 

Old dead and downed trees may still be useful for such purposes. Dead trees usually decay from the outside-in and often do not provide such habitat.

Management—Where emphasis is on timber management, trees with indicators 

should be removed during any entry. Indicators and the amount of decay and cull 

associated with them were studied in refs. 80 and 86. Besides conks and punk 

knots, indicators include forks and dead rust brooms. If decay is frequent, consider reducing rotation age to minimize losses. Prevent injuries to trees during 

logging to prevent new infections of decay fungi.

References: 80, 86, 194

Rust-Red Stringy Rot and Red Heart Rot

Indian paint fungus and bleeding Stereum in firs

Pathogen—Rust-red stringy rot is caused by Echinodontium tinctorium. It is 

one of the few pathogens that has a common name—Indian paint fungus. Red 

Figure 95. Very advanced white pocket rot of Porodaedalea

(Phellinus) pini with black zone lines. Photo: Jim Worrall, 

USDA Forest Service.

R O O T D I S E A S E S A N D S T E M D E C AY S
54 USDA Forest Service RMRS-GTR-241. 2010.

R O O T D I S E A S E S A N D S T E M D E C AY S 

heart rot is caused by Stereum (Haematostereum) sanguinolentum, also known 

as the bleeding Stereum.

Hosts—Indian paint fungus attacks firs, hemlocks, and less commonly other 

species in western North America, but it is primarily found on white fir in the 

Rocky Mountain Region. Bleeding Stereum occurs primarily on subalpine fir 

and Engelmann spruce in this Region, though it can also attack other conifers.

Signs and Symptoms—Indian paint fungus produces conks frequently at 

branch stubs, at wounds, and even inside hollow stems. The conk is hard and 

woody, hoof-shaped, and perennial (figs. 96-98). The upper surface is blackish 

and rough with crevices. The lower surface is grey-brown with hard, blunt, thick 

teeth. The inside is brilliant brick- or rust-red, but it fades over time after exposure. This tissue was ground into a powder and used as paint by some Native 

Americans, giving the fungus its name. The only other indicator of this disease is 

punk knots known as “rusty knots.” Although there is no swelling as with punk 

knots caused by Porodaedalea pini, the interior of the knot shows the rust-red 

color characteristic of the decay.

Bleeding Stereum fruits frequently on 

logs and slash, but infrequently on live 

trees. Conks are small, thin, inconspicuous, 

and leathery (fig. 99). Portions are appressed to the bark, often with a projecting 

cap. The lower surface has neither pores 

Figure 96. Indian paint fungus; upper view of conk. 

Note that the conk was removed with the branch 

stub at which it fruited. Photo: Jim Worrall, USDA 

Forest Service.

Figure 97. Indian paint fungus; lower view of conk. 

Photo: Jim Worrall, USDA Forest Service.

Figure 99. Small, leathery fruiting bodies of the bleeding Stereum. Photo: Jim Worrall, USDA Forest Service.

Figure 98. Indian paint fungus; inner view of 

conk. Photo: Jim Worrall, USDA Forest Service.
USDA Forest Service RMRS-GTR-241. 2010. 55

Figure 101. Log with decay and stain of red heart rot. 

Photo: Rocky Mountain Region, USDA Forest Service.

Figure 102. Balsam fir stem with advanced red 

heart rot in cross section. Photo: Jim Worrall, 

USDA Forest Service.

nor teeth; it is fairly smooth. The upper surface 

is slightly hairy with concentric zones of different shades of brown, orange, and grey; the 

lower surface is also shades of light brown with 

a white margin. If the conk is wounded when 

fresh and moist, it bleeds red liquid, giving the 

fungus its name. There are no other external indicators, except wounds that may be points of 

infection.

Rust-red stringy rot begins as golden tan 

patches in the wood. Fine radial tunnels may develop, presumably following the rays. Rust-red 

streaks and patches begin to appear; the wood 

becomes noticeably soft and eventually has 

a tendency to separate along the annual rings 

(laminated rot). Finally, it becomes stringy and 

darker brown with rusty patches (fig. 100).

Wood in the early stage of red heart rot is 

water-soaked and reddish brown (figs. 101-

102). Thin, white fungal tissue may appear, and 

as decay advances, the wood becomes drier, 

soft, and finely stringy.

Disease Cycle—Spores of Indian paint fungus 

do not require wounds to infect a tree. Small wounds or tiny, shade-killed twigs 

may be infected. The fungus becomes dormant after infection. Eventually, after 

it is incorporated into heartwood, the fungus may resume growth and initiate 

decay. Conks form years later and spores produced on the teeth are wind-blown 

to start the cycle over.

Red heart rot is frequently associated with conspicuous trunk wounds 

Figure 100. Longitudinal section through 

stem, branch, and conk of Indian paint 

fungus, showing discolored, decayed 

wood. Photo: Kelly Burns, USDA Forest 

Service.

R O O T D I S E A S E S A N D S T E M D E C AY S
56 USDA Forest Service RMRS-GTR-241. 2010.

R O O T D I S E A S E S A N D S T E M D E C AY S 

and broken tops, so presumably those are common sites of infection. Fruiting 

is uncommon on infected trees but common on logs and slash. Apparently, the 

spores come from those substrates, and parasitism of live trees may be a reproductive “dead end” for the fungus.

Impact—Rust-red stringy rot can form huge, long decay columns in old white 

fir. There may be less than 1 inch (2.5 cm) of sound shell left on a 30-inch 

(76-cm) DBH tree. Decay extends 10-16 ft (3.0 to 4.9 m) beyond a conk, so a 

single conk indicates extensive decay. In addition to substantial cull, the disease 

poses significant hazards in developed sites. Hazard tree inspectors dealing with 

white fir should carry binoculars to look for conks, carefully sound the trees, and 

core if needed.

Red heart rot is the most important stem decayer in subalpine fir and the second most important in Engelmann spruce.

Management—Rust-red stringy rot can be detected by conks, rusty knots, sounding, and coring. In timber management, remove infected trees. Cull factors are 

available based on indicators for cruising. In developed sites, hazard should be 

mitigated immediately, either by closing affected sites or by removing the tree. 

Infection sites are not conspicuous wounds, so there is no practical way to prevent infections.

Red heart rot is more difficult to detect, as the only indicators are usually 

potential sites of infection. Decay columns are usually not hollow so sounding 

may be less definitive. Infected trees should be preferentially removed where 

appropriate. Avoid wounding trees because red heart rot is likely to infect such 

wounds in spruce-fir stands.

References: 43, 81, 86

Stem Decays of Hardwoods in the Plains

Numerous decay fungi, numerous hosts

Pathogen—Many fungi decay wood in the roots, butts, and stems of hardwoods in the Great Plains of South Dakota, Nebraska, Kansas, and Colorado. 

Three stem-decay fungi are presented here: Phellinus igniarius, Fomitiporia

(Phellinus) punctata, and Perenniporia fraxinophila (table 6).

Hosts—Phellinus igniarius has a wide host range, infecting species in over 20 

genera of hardwoods. It is common in birch, but has also been found in ash, black 

walnut, poplars, buckthorn, and willows.

Fomitiporia punctata also has a wide host range. In a survey of North Dakota 

windbreaks, plantings, and natural stands, it was found on live willow, ash, 

Prunus, Rhamnus, Caragana, and Syringa.

Perenniporia fraxinophila infects primarily ash species, with a few records 

on other hardwood genera and even junipers.
USDA Forest Service RMRS-GTR-241. 2010. 57 Table 6. Descriptions of conks of three stem-decay fungi. Decay fungus Shape Surface features Other features Phellinus igniarius Projects up to 4 3/4 inches (12 cm); Upper surface gray to black, hairless, Conk is hard and woody. Interior is dark reddish brown; upper surface curved down; rarely becoming deeply cracked and crusty. tubes with bits of white tissue. almost flat. Lower surface is Lower surface pale to dark; cinnamon generally flat or angled to purplish brown. Pores on lower slightly upward. surface are fine. Fomitiporia punctata Spreads flat on the surface, but Margin is at first yellowish brown, Interior is dark reddish brown. often with a cap on the upper side. becoming black and cracked. Pore surface is yellowish to grayish brown. Pores are very fine. Perenniporia fraxinophila Varies from projecting up to Upper surface may be reddish brown Interior is buff to pale yellowish brown, corky. 2 3/4 inches (7 cm) with a distinct initially, becoming cracked, crusty, cap to completely flat on and grayish black. Pore surface is the surface. ivory to buff; larger pores than the other species.

R O O T D I S E A S E S A N D S T E M D E C AY S
58 USDA Forest Service RMRS-GTR-241. 2010.

R O O T D I S E A S E S A N D S T E M D E C AY S 

Signs and Symptoms—Conks, 

the spore-producing fruiting bodies, 

are evidence of infection, but they 

are not always present on infected 

trees. Conks may form anywhere on 

the stem or branches (fig. 103), but 

are most common at branch stubs, in 

cankers, and near cavity openings.

In addition to conks, symptoms 

may be evident. Openings leading 

to internal hollows (cavities) may 

form, especially at branch stubs or 

openings created by cavity-nesting 

birds (fig. 104). Fomitiporia punctata also causes cankers, or patches of 

killed bark—such a disease caused 

by wood-decay fungi is called a 

canker rot. The fungus decays wood 

inside the stem but grows out along 

branch traces and kills the cambium 

around branch stubs, which results in death 

of the overlying bark. These fungi all cause 

a uniform white rot of the wood.

Disease Cycle—For most stem-decay diseases, the precise point of infection is uncertain. Branch stubs and even small dead 

twigs can be infected in some cases, but 

wounds, especially large ones, greatly increase the likelihood of stem decay. Spores 

germinate at the point of infection, and the 

fungus may grow down a branch stub or into 

a wound surface to reach the inner wood. 

The fungus decays the wood for some years 

and then returns to the surface, often along 

a branch trace, to produce a conk. Spores 

are released from the conk and are blown by 

wind to reach a new infection point.

Impact—Decay columns slowly expand, 

in some cases shrinking sapwood area and 

causing physiological stress and slower 

growth. Decay weakens wood, which 

can lead to structural failure of stems and 

branches. In developed areas, this creates 

hazard to people or structures. The softened 

Figure 103. Perenniporia fraxinophila on green ash. 

Photo: James T. Blodgett, USDA Forest Service.

Figure 104. Two cavity nest openings on ash. 

Stem-decay fungi create conditions necessary 

for excavation of cavity nests. Photo: Jim 

Worrall, USDA Forest Service.
USDA Forest Service RMRS-GTR-241. 2010. 59

Figure 105. Conks of Fomitiporia punctata. Photo: Mike Schomaker, 

Colorado State Forest Service, Bugwood.org.

Figure 106. Small conk of Phellinus 

igniarius. Photo: Jim Worrall, USDA 

Forest Service.

inner wood also creates habitat suitable for excavation of cavity nests by birds 

(fig. 104).

Fomitiporia punctata is the most important stem-decay fungus on green ash 

in North Dakota, and it also occurs in Nebraska (fig. 105). Phellinus igniarius

occurs throughout the Great Plains on a variety of hosts (fig. 106). In Nebraska, 

Perenniporia fraxinophila is the most important stem-decay fungus in green ash, 

fruiting on about 5.5% of trees in windbreaks (as of 1978) and about 10% of the 

green ash in woodlands (as of 1981). Incidence of the latter fungus increased 

consistently with tree diameter, and infected trees had an average of nine conks. 

The incidence of infection and amount of decay in infected trees increased with 

tree age.

Management—Wound prevention is the primary preventive approach to stem 

decays. Wounds from vehicles, machinery, livestock, and fire increase the likelihood of stem decay. Prune early when branches are small. Pruning should be 

done properly to minimize damage and maximize the ability of callus to cover 

the wound quickly. Studies have not shown any benefit from wound dressing 

treatments and, in some cases, have shown negative effects, so they are not recommended. Where hazard to people and property is a consideration, removal of 

infected trees is recommended.

References: 14, 138, 139, 141, 142, 180

R O O T D I S E A S E S A N D S T E M D E C AY S
60 USDA Forest Service RMRS-GTR-241. 2010.

Introduction to Cankers and Overview 

of Aspen Cankers

Diseases of the cambium and bark

Cankers are diseases of the bark. They differ from wounds in that the cause, 

usually a fungus, remains active for some time, causing progressive expansion 

of the lesion over time. Where the cambium and inner bark die, the sapwood 

beneath also dies. If a canker grows all the way 

around a stem or branch, it is said to be girdled, and 

everything distal to the girdle dies. Branches fade and 

die (branch flagging) after girdling. Diffuse cankers 

continue to grow, and the tree does not have time to 

produce callus. Target cankers grow short distances 

periodically, and the host produces callus each year 

in an expanding concentric pattern.

Canker impacts include: deformation of stem 

growth, leading to loss of wood value and a weak 

point that may snap; creation of infection court for 

wood-decay fungi; and mortality.

Some diseases include canker development as one 

of their symptoms but are classified primarily as other 

disease types. This group includes some stem-decay 

fungi that may come to the surface and kill cambium 

and bark (canker-rots) and some rusts and mistletoes.

Aspen Cankers—Aspen has more important canker diseases than any other tree species in the Rocky 

Mountain Region (figs. 107-111). Descriptions of the 

most important ones are available, but here we provide an overview and guide to identification of the 

aspen cankers common in the Rocky Mountains.

Sooty-bark canker is the most widespread, damaging canker of aspen in Colorado and probably other 

Rocky Mountain states. Typically, sooty-bark canker 

kills more mature aspen trees than any other agent 

(this is not the case during unusual episodes of aspen mortality such as from 2004 to 2009). In surveys, 

sooty-bark canker was responsible for 55% of mortality. Next in importance is Cryptosphaeria canker, 

causing 26% of mortality. Black canker is probably 

next most common but usually does not directly kill 

trees. Cytospora canker is quite common, but attributing mortality to it is more problematic because it 

is frequently associated with stress caused by other 

diseases and factors.

Figure 107. Typical sooty-bark 

canker on aspen (from Johnson 

and others 1995).

C A N K E R S 
USDA Forest Service RMRS-GTR-241. 2010. 61

Identification—Generally, well-developed cankers have sufficiently distinct features that they 

can be readily identified in the field. However, 

small, young cankers often do not have all the 

typical features and can be difficult to identify. 

Following are brief descriptions that should enable identification using distinguishing features 

that are usually available in well-developed cankers. A more detailed comparison is presented 

in table 7.

•  Sooty-bark canker: cankers expand rapidly 

so that callus formation is rare; alternating, 

concentric zones of light and dark where the 

periderm remains present versus where it 

falls off, revealing the sooty black inner bark; 

small (1/25-1/13 inch [1-2 mm] in diameter) 

light gray, usually shriveled, cup-shaped 

fruiting bodies (fig. 107).

•  Cryptosphaeria canker: cankers become very 

long and narrow with orange-brown margins; inner bark becomes black but usually 

has small (1/50-1/13 inch [0.5-2.0 mm]) whitish lenticular spots; black, pimple-like fruiting bodies may be peppered on the dead bark 

in patches (fig. 108).

•  Black canker: cankers are diamond-shaped or oval, often with flaring margins; bark usually sloughs off to expose wood at canker center; wood and 

especially bark at margins are often dark to black; concentric annual callus 

rings, often quite narrow, are visible on the wood 

surface (fig. 109).

•  Cytospora canker: canker margins are discolored 

orange; bleeding of black liquid may be present, 

especially in spring; older dead bark is not much 

altered but may become brownish and shriveled 

or sunken; pimple-like fruiting, often with white 

heads, eventually appears in some spots (fig. 110).

•  Hypoxylon canker: cankers often have irregular 

shape; salt-and-pepper checkered appearance; 

fruiting, especially the small, gray-black, bumpy 

stromata, is almost always present; this is the 

least common of the five cankers described here 

(fig. 111).

Figure 108. Typical Cryptosphaeria 

canker on aspen. Photo: Jim Worrall, 

USDA Forest Service.

Figure 109. Typical black canker on aspen. Photo: 

Jim Worrall, USDA Forest Service.

C A N K E R S
62 USDA Forest Service RMRS-GTR-241. 2010. Table 7. Comparison of aspen cankers. Canker type Surface appearance Fruiting Mortality Sooty-bark Diffuse canker; grows rapidly Alternating zones with periderm Gray, cup-shaped; shriveled, Primary killer present versus exposed; black 1/64-1/16 inch (0.5-2 mm) inner bark diameter common Cryptosphaeria Perennial target or diffuse; Long, narrow canker, dead bark Pimple-like, black heads with Primary killer grows much faster adheres tightly; margin is submerged pseudostroma; longitudinally than tangentially orange-brown common Black	 Perennial target canker; 	 Bark sloughs off and/or flares	 Minute, black perithecia in	 Usually does not directly kill trees grows slowly out; concentric annual callus spring; rarely seen ridges in wood are exposed Cytospora Diffuse canker; grows rapidly Orange discoloration at margin; Pimple-like, often with white Secondary killer; attacks stressed on severely stressed trees bleeding, especially in spring heads; common trees Hypoxylon Diffuse canker Irregular shape; dead bark Gray conidial pillars beneath Primary killer but uncommon in sloughs off in patches leading blistered periderm; perithecia Rocky Mountains to a checkered pattern, in small gray-black stromata yellowish-orange margin

C A N K E R S 
USDA Forest Service RMRS-GTR-241. 2010. 63

Figure 111. Example of Hypoxylon canker on aspen. Photo: Rocky Mountain 

Region Archive, USDA Forest Service, 

Bugwood.org.

Figure 110. Typical Cytospora 

canker on aspen. Photo: Jim 

Worrall, USDA Forest Service.

Management—Because wounds are important infection courts for most of 

the canker pathogens, avoiding wounding is important, especially during the 

growing season. This is a major reason why partial cutting in aspen is strongly 

discouraged. It is also a major reason why developing campgrounds in aspen 

stands is strongly discouraged in Regional policy.

For timber management, information on the incidence and types of cankers 

in managed stands can be used in prioritizing stands for treatment. Trees can 

survive with black canker for many years, but most of the other cankers may 

be indicators of stands that are in the process of overstory mortality. Cytospora 

canker is usually evidence of some other stress factor at work.

References: 76, 79, 96, 100

C A N K E R S
64 USDA Forest Service RMRS-GTR-241. 2010.

Black Canker

A common, slowly developing target canker of aspen

Pathogen—Black canker is caused by the fungus Ceratocystis populicola (part of the C. fimbriata complex). The disease is sometimes called 

Ceratocystis canker or target canker of aspen.

Hosts—The C. fimbriata complex occurs all 

over the world on diverse hosts, causing diverse 

types of diseases. The form that causes black 

canker on aspen apparently occurs occasionally 

on other Populus species and has been introduced to eastern Europe where it is more lethal 

than in America.

Signs and Symptoms—Black canker may occur anywhere along a stem or branch. Cankers 

are typically diamond-shaped or oval, and the 

margins are often flared out (figs. 112-114). 

They are generally blackish and the bark tends 

to break off as the cankers develop, leaving an 

exposed canker face. Narrow, concentric, annual 

callus ridges are visible in the wood of the canker face. Fruiting is microscopic and usually not 

seen.

Disease Cycle—The fungus may infect at 

wounds, and when the host is dormant, the fungus kills a patch of cambium and inner bark. 

When the host is active, it produces callus that partially grows over the wood under the killed bark. When 

the host is again dormant, the fungus resumes growth, 

killing the callus and an additional bit of bark and 

cambium. This sequence continues for many years, 

resulting in successive, concentric rings of callus that 

are visible in the wood inside the canker. Because of 

the concentric rings, this type of canker is called a target canker. As the callus is produced, it tends to alter 

the form of the stem, leading to flaring of the margins.

The pathogen produces minute, black perithecia 

in cankers, but they are not always produced and are 

difficult to observe. They are produced in spring at 

the border of the canker on tissues dead for at least 

a year. Ceratocystis species generally have adaptations for dispersal by insects, and this appears to be 

Figure 113. Another large black 

canker. Photo: Jim Worrall, USDA 

Forest Service.

Figure 112. Large, black canker that 

shows diamond shape, flaring margin,

lack of bark over canker, annual callus 

rings in wood, and that cankers are 

not always black. Photo: Jim Worrall, 

USDA Forest Service.

C A N K E R S 
USDA Forest Service RMRS-GTR-241. 2010. 65

functional for black canker. 

Nitidulid beetles are attracted 

to the aroma of the fungus 

and may acquire the sticky 

spores from the fruiting bodies, which can be deposited 

again when the beetles visit 

fresh aspen wounds.

Impact—The incidence of infection varies greatly among 

stands, but it is not known if 

this is due to variation in resistance or due to stand/site 

factors that favor infection. Black canker develops slowly and normally does not 

kill trees directly (figs. 115-116). However, it has a number of impacts:

•Cankers deform the stem, resulting in cull if the trees are harvested for woo

products.

•Cankers can predispose stems to snapping, resulting in premature mortalit

Figure 115. Black canker in heavily-affected aspen stand. Photo: Jim Worrall, USDA Forest Service.

Figure 114. Closeup of black canker, 

showing the narrow, annual callus 

rings. Photo: Jim Worrall, USDA 

Forest Service.

C A N K E R S
66 USDA Forest Service RMRS-GTR-241. 2010.

Figure 116. Black cankers on aspen. The disease develops 

slowly and usually does not kill trees directly. Photo: Jim 

Worrall, USDA Forest Service.

•Cankers frequently serve a

points of infection for wooddecay fungi, further affecting the tree, causing additional cull, and increasing the 

likelihood of snapping and 

mortality.

•Cankers and associated deca

create hazards to people and 

property in developed sites.

Management—Practical means 

of preventing infection are not 

known. Certainly, wounding 

should be avoided, but natural infection courts apparently lead to 

heavy infection in some stands. 

In timber stands, partial cutting 

of aspen is strongly discouraged, 

because the residual stand often 

deteriorates in 5-10 years. Therefore, where the disease threatens 

management objectives, early 

harvest/regeneration should be 

considered.

References: 7, 79, 96, 97, 100

Black Knot of Cherry and Plum

Enlarged black growths on stems

Pathogen—Black knot is caused by the 

fungus Apiosporina morbosa.

Hosts—Hosts of black knot are cherry and 

plum species.

Signs and Symptoms—Elongated swellings or knots (about 1-8 inches (2.5-20 cm) 

long and 1/3-1 inch (8-25 mm) thick) on 

twigs, branches, and small stems are commonly seen on susceptible hosts. The knots 

start out greenish in color and soft but become hard and black over time (fig. 117).

Disease Cycle—Spores are discharged 

from the fruiting bodies (in knots) in the 

Figure 117. Swellings on branches caused 

by black knot. Photo: Joseph O’Brien, USDA 

Forest Service, Bugwood.org.

C A N K E R S 
USDA Forest Service RMRS-GTR-241. 2010. 67

spring as new growth starts 

to elongate. Spore discharge 

ends at about the time terminal 

growth stops. Rain is required 

for spore discharge, and spores 

are carried by both rain and 

wind to new infection sites. The 

fungus enters unwounded twigs. 

Infections are severe under 

moist conditions in the spring at 

the time of spore discharge.

Knots start to appear a few 

months after infection. Some 

appear in late summer, others 

not until the following spring. 

One to 2 years are required for 

the knots to produce fruiting 

bodies.

Impact—Fruit production can 

be significantly reduced in 

heavily infected cherries and 

plums. In heavy infections, 

small branches and sprouts can 

die over time (fig. 118).

Management—Management is generally not needed for black knot. When necessary, prevention of infection is the most efficient means of control. Removing 

infected wild hosts surrounding plantings will reduce infections in plantings. 

Pruning the knots can also reduce infections. Cuts should be made well below 

the knot (3-4 inches [7.6-10 cm]) as the fungus may extend beyond the swelling. Fungicides can also be applied in addition to the sanitation efforts. There is 

considerable variation in cultivar susceptibility to black knot.

Reference: 140

Botryodiplodia Canker

Difficult to detect on most hardwood species

Pathogen—Botryodiplodia canker is caused by fungal species in the genus 

Botryodiplodia, including B. hypodermia and B. theobromae. Botryodiplodia is 

the asexual form of the fungus. The sexual states for some of the Botryodiplodia

species are Botryosphaeria spp.

Hosts—Hosts of Botryodiplodia species in the Rocky Mountain Region include 

elm, oak, and sycamore. Botryodiplodia canker is one of several hardwood 

canker diseases.

Figure 118. Black knot on plum. Photo: William M. Brown, Jr.; 

Bugwood.org.

C A N K E R S
68 USDA Forest Service RMRS-GTR-241. 2010.

Signs and Symptoms—The signs are 

small, black, pimple-like 

fruiting bodies (pycnidia) produced on dying and dead bark at the 

canker margin (fig. 119). 

However, it is difficult to 

identify the pathogen by 

signs. Several fungi produce small, black, pimple-like fruiting bodies 

and occur on hardwood 

hosts. Microscopic examination of the spores 

is necessary for correct 

identification.

Symptoms include discolored and cracked bark (fig. 119). Outer bark may become loose. Inner bark, cambium, and sapwood turn reddish brown to dark brown. 

The discoloration ends at the canker margin. Cankers on trees with rough bark can 

be detected only after removing bark to expose dead inner bark, cambium, and sapwood. Cankers may appear sunken and may be surrounded by callus tissue.

Girdled stems and branches die. The leaves above the cankers wilt, turn yellow 

to brown, and die, and adventitious shoots may form below the canker.

Disease Cycle—Spores (conidia) are released during rain and are dispersed by 

wind, insects, pruning tools, and rain droplets. Wounded bark can be infected 

throughout the growing season during moist conditions but most abundantly in 

spring. Cankers develop at wound sites and can girdle and kill the trees. Fruiting bodies are produced on dying and dead bark at the canker margin. The fungus overwinters in the bark as mycelium or as fruiting bodies. Cankers develop 

quickly during dry periods and during hot summer months. Fruiting bodies can 

be produced yearlong but are mainly produced in the fall on dying and recently 

dead bark.

Impact—These diseases cause branch dieback and tree mortality. Botryodiplodia canker diseases are important diseases of hardwoods in windbreaks.

Management—Preventing wounds is the best way to minimize cankers. Cankered branches should be pruned in the winter to reduce inoculum (fungus available to initiate new infections). Severely diseased trees and branches should be 

removed. Pruning cuts should be made well below canker margins and pruning tools should be disinfected. Because this disease responds to water stress, 

management of competing vegetation, stand thinning, planting techniques that 

reduce drought, and selection of a tree species compatible with a site will reduce 

losses.

References: 114, 137

Figure 119. Signs and symptoms of Botryodiplodia canker on an elm. 

Photo: William Jacobi, Colorado State University, Bugwood.org.

C A N K E R S 
USDA Forest Service RMRS-GTR-241. 2010. 69

Cryptosphaeria Canker

Long, narrow canker kills aspen

Pathogen—Cryptosphaeria canker is caused by 

the fungus Cryptosphaeria lignyota (formerly 

known as C. populina). The anamorph (asexual 

stage) is Libertella sp. The disease is also sometimes called “snake canker.”

Hosts—The disease occurs on quaking aspen 

and other Populus species in the Rocky Mountains. The pathogen has been found throughout 

much of North America and in Europe.

Signs and Symptoms—Cryptosphaeria cankers are usually much longer and narrower than 

other cankers of aspen (fig. 120). Cankers may 

only be 2-4 inches (5-10 cm) wide and 10 ft 

(3 m) long. They may ultimately extend for 

much of the stem length or occur primarily in the 

portion of the stem with live branches.

Bark recently killed near the margin of the 

canker becomes discolored light brown to orange. Within a year or two, dead inner bark 

becomes stringy, black, and sooty, much like 

sooty-bark canker, but usually contains small 

(1/50-1/13 inch [0.5-2.0 mm]), lenticular, lightcolored areas (fig. 121). Annual callus formation may be visible. Dead bark adheres tightly 

to cankers.

Figure 121. Light-colored 

lenticular spots that 

frequently appear in older 

dead bark of canker caused 

by Cryptosphaeria lignyota. 

Live bark is at the left-most 

margin of the cut. Photo: 

Jim Worrall, USDA Forest 

Service.

Figure 120. Cryptosphaeria canker 

killing an aspen. The canker winds 

around this tree all the way up to the 

live crown. Photo: Jim Worrall, USDA 

Forest Service.

C A N K E R S
70 USDA Forest Service RMRS-GTR-241. 2010.

The disease is actually a canker-rot. The pathogen colonizes the heartwood 

and sapwood, causing discoloration and decay. Discoloration may have hues of 

gray, brown, yellow, orange, and even pink and extends up to 3.3 ft (1 m) or more 

beyond the canker. The decay is brownish and mottled and is probably a type of 

soft rot similar to white rot, as is typical of wood-decaying ascomycetes. The 

fungus grows out from the wood to the cambium and bark, causing the canker.

Mortality usually occurs in small trees after only a few years and before the 

canker girdles the stem. Killing of sapwood apparently causes more damage than 

the canker itself. Large trees may be killed more slowly as their branches are 

engulfed in the canker. Especially on large trees, branches may be infected first, 

from which the pathogen can spread to the stem.

Cytospora chrysosperma frequently infects and fruits at the margin of 

Cryptosphaeria cankers and quickly colonizes trees after they die, so the cause 

of death may be misattributed to Cytospora canker. For this reason, and because 

the pathogen may not fruit reliably, especially on small trees, diagnosis should 

be made by looking for the lenticular, light-colored areas in bark and the staining 

in the sapwood.

Disease Cycle—The fungus frequently infects trees through wounds. As 

described above, it appears to progress primarily through wood, then move out 

to cambium and bark. Black perithecia of the pathogen may appear scattered 

on bark that has been dead for a year or two (fig. 122), although fruiting may 

not occur on small trees. Beneath the surface, the perithecia are embedded in a 

widely spread pseudostroma.

Ascospores are forcibly ejected from perithecia during wet weather and presumably dispersed by wind. Conidia can also be produced but may function only 

in fertilization.

Impact—Cryptosphaeria canker is an important killer of aspen. In one Colorado 

survey, 83% of 30 surveyed 

sites had Cryptosphaeria 

canker, and 26% of aspen 

mortality was attributed to 

the disease. Unlike sootybark canker, it seemed to 

kill mostly small to mid-size 

trees. It is estimated that 

about 8% of aspen in Colorado have decay caused by 

C. lignyota. Cryptosphaeria 

canker is one of a number 

of wound-infecting cankers 

that leads to mortality following partial cutting of 

aspen stands and also in 

developed recreation sites 

established in aspen.

Figure 122. Perithecia (fruiting bodies) of Cryptosphaeria lignyota. 

Photo: Jim Worrall, USDA Forest Service.

C A N K E R S 
USDA Forest Service RMRS-GTR-241. 2010. 71

Management—Wounding should be prevented. However, many infections are 

due to natural infection courts, and practical means of preventing such infection 

are not known. In timber stands, partial cutting of aspen is strongly discouraged 

because the residual stand often deteriorates in 5-10 years. Therefore, where the 

disease threatens management objectives, early harvest or regeneration should 

be considered. Similarly, development of recreation sites in aspen stands is 

strongly discouraged by Regional policy because cankers such as this one cause 

rapidly increased mortality following development and use.

References: 77, 78, 100, 181

Cytospora Canker of Aspen

Orange weeping bark

Pathogen—Cytospora canker of aspen is most commonly caused by Valsa sordida, a name that applies to the sexual fruiting stage as well as the whole fungus. 

However, the fungus is frequently called Cytospora chrysosperma, because this 

name applies to the asexual, or conidial, fruiting stage, which is most commonly 

seen. Other, related species of Cytospora-like fungi, such as Leucostoma niveum

(asexual stage Leucocytospora nivea) can occur on dying and dead bark and may 

cause cankers in some cases.

Hosts—Valsa sordida primarily causes disease on Populus species. Related 

fungi occur on many other hosts.

Signs and Symptoms—The small, pimple-like fruitbodies are embedded and 

break through the bark surface. They may be so numerous and small that they 

roughen the bark surface like coarse sandpaper. The exposed surface of the 

asexual fruiting body (the disk) is gray to black and prominent or reduced in 

Valsa sordida and is usually white but sometimes gray-brown in Leucostoma

niveum. During wet weather when temperatures are above freezing, conidia may 

be produced from the fruitbodies in a sticky matrix. If the bark is moist but 

not wet, fine, curly, orangish tendrils of spores project 

from the fruiting bodies, extruded like toothpaste from a 

tube. Otherwise, amber- to 

orange-colored spore masses may adhere to the bark 

surface (fig. 123). Sexual 

fruitbodies are superficially 

similar to those that produce 

conidia.

The most common symptom is rapidly spreading necrosis (death) of bark. On Figure 123. Pycnidia of Cytospora sp. with orange spore masses. 

Photo: Jim Worrall, USDA Forest Service.

C A N K E R S
72 USDA Forest Service RMRS-GTR-241. 2010.

smooth bark, this appears first as an orange discoloration 

that may be accompanied by exudation of a brown liquid (fig. 124). The inner bark turns from green to brown 

to black, and bark begins to slough off after 2-3 years. 

Cankers may be diffuse, continuing their spread until the 

tree is dead without stopping or becoming sunken. Once 

any point on the stem or branch is girdled (killed all the 

way around), everything above that point dies. On trees of 

intermediate susceptibility, cankers may be annual, stopping growth after one season and becoming sunken and 

callused over as the tree grows around them. In rare cases, 

the canker can resume growth in successive dormant seasons, approximating the appearance of a perennial target 

canker.

Disease Cycle—Cytospora species are usually opportunistic pathogens. They are quick to attack plants that 

are stressed by heat, drought, winter injury, and other diseases and insects. They often attack and kill stressed trees 

that may otherwise have survived the stress and recovered. They also colonize dying or recently dead tissues as 

saprobes. Valsa sordida, the primary cause of Cytospora 

canker of aspen, is one of the more aggressive species in 

the group.

Most canker pathogens, including Cytospora species, 

have traditionally been thought to infect primarily through 

wounds. More recent information suggests that they can 

sometimes infect and inhabit apparently healthy bark and 

buds, thus being in a position to rapidly colonize and kill weakened tissues. Such 

infection by a pathogen without symptom production is termed a latent infection. 

In addition to wounds, infection may occur through buds, nodes, and lenticels.

Once the pathogen has infected and is causing disease, it kills the cambium 

and living tissues in the bark. As a result, the underlying wood dies. Cankers 

typically develop when the host is dormant, but can develop during the growing 

season when the host is severely weakened. Cankers develop in a temperature 

range of 36-86 oF and may grow as fast as 1.6 inches (4 cm) per day.

Microscopic conidia are produced by the millions in the asexual fruitbodies. 

Conidia are dispersed by rainsplash and incidentally by insects or other animals. 

Later, ascospores may be produced. Ascospores may be extruded and dispersed 

like conidia, but under some conditions, they are forcibly ejected into the air 

when mature and are dispersed by wind.

Impact—Mortality is the major impact of Cytospora canker. Although stress 

usually precedes severe infection, disease impact can be substantial because 

trees may recover from stress in the absence of the canker.

Figure 124. Cytospora 

canker of aspen. Bleeding is 

usually not this copious and 

may be absent. Photo: Rocky 

Mountain Region Archive, 

USDA Forest Service.

C A N K E R S 
USDA Forest Service RMRS-GTR-241. 2010. 73

The canker also impacts regenerating stands. A study of mortality greater than 

90% in 4- to 5-year-old stands in 1988 found rapidly expanding cankers with 

consistent C. chrysosperma fruiting. Symptoms were reproduced in greenhouse 

inoculations, but it was not known what conditions led to such severe disease in 

these cases.

Management—Avoiding wounds and stress will reduce the likelihood of 

Cytospora canker in individual trees. Clearcutting, prescribed fire, or wildfire 

will stimulate regeneration and will give the best chances for maintaining aspen 

on the site.

References: 23, 89, 158, 162

Cytospora Canker of Conifers

Branch flagging in stressed conifers

Pathogen—Cytospora canker of conifers is caused by Valsa kunzei (= Leucostoma kunzei) (asexual stage is Cytospora kunzei, = Leucocytospora kunzei). The 

disease is commonly referred to as Cytospora, Valsa, or Leucostoma canker.

Hosts—Many coniferous species are hosts, primarily spruce species. Common hosts in the Rocky Mountain Region include Colorado blue spruce, white 

spruce, Engelmann spruce, and Douglas-fir. The disease is particularly prevalent 

in windbreaks and ornamental plantings.

Signs and Symptoms—From a distance, the most obvious symptom is dead 

or dying branches, particularly older branches (fig. 125). Cankers are diamondshaped and are usually very resinous—clear amber resin exudes from canker 

margins and eventually hardens to a conspicuous white crust (this symptom is 

less prominent on Douglas-fir). Cankers on trunks become sunken in the middle 

with expanding, flared edges (fig. 126). Branch cankers are common on all conifer hosts, and Engelmann spruce and Douglas-fir are also susceptible to trunk 

cankers. When a branch is 

girdled, foliage becomes 

discolored, dies in spring 

and summer, and brown 

needles are shed the following winter (fig. 127). The 

disease often progresses upward in tree crowns. Small 

(1/25-1/13 inch [1-2 mm]), 

black fruiting bodies are 

sometimes visible around 

the edges of cankers. During moist weather, yellow 

tendrils of spores ooze from 

pycnidia (fig. 128).

Figure 125. Typical symptomatic branch flagging caused by

Cytospora. Photo: Joseph O’Brien, USDA Forest Service, Bugwood.

org.

C A N K E R S
74 USDA Forest Service RMRS-GTR-241. 2010.

Figure 128. Cytospora spore tendrils exuding from pycnidia. Photo: Michael Kangas, North Dakota 

State Forest Service, Bugwood.org.

Figure 126. Sunken stem canker with flared edges

on Engelmann spruce. Photo: Kelly Burns, USDA 

Forest Service.

Figure 127. Branch canker, showing white, resinous 

pitch. Photo: Michael Kangas, North Dakota State 

Forest Service, Bugwood.org.

Disease Cycle—Cytospora canker is a disease of stressed conifers. The pathogen overwinters in cankered bark. Ascospores and conidia are dispersed during 

wet weather in the spring, summer, and fall by splashing rain, wind, and insects. 

Infection occurs through wounds; tiny openings in the bark created by environmental stresses such as ice or snow may also serve as infection courts. Symptoms develop shortly after infection but latent infections are not uncommon. The 

fungus can remain dormant on the outer bark until an infection court becomes 

available. Pycnidial and later perithecial stromata form around the edges of old 

cankers, and the fungus colonizes adjacent healthy tissue.

Impact—Cytospora canker causes branch and stem cankers, which can deform 

stems. It is one of the most common and damaging diseases of planted spruce. 

Colorado blue spruce sustains the most damage east of its natural range. The 

pathogen is opportunistic and invades trees weakened by other factors such as 

drought, hail, or insects. Older branches are more susceptible, so infections generally start in the lower crown and progress upward. Mortality is rare.

C A N K E R S 
USDA Forest Service RMRS-GTR-241. 2010. 75

Management—The incidence and severity of Cytospora canker may be reduced 

by maintaining tree vigor (fertilize, water, control insects), reducing injuries to 

the stem and bark, and pruning and destroying infected branches.

Reference: 158

Hypoxylon Canker

Uncommon but locally important canker of aspen

Pathogen—Hypoxylon canker is caused by the fungus Entoleuca (Hypoxylon) 

mammatum.

Hosts—Hypoxylon canker occurs on trembling aspen and a few other aspens. 

It occurs over most of the range of trembling aspen in North 

America and also in Europe.

Signs and Symptoms—Stem cankers are often centered 

on dead branches, from which they commonly enter the 

stem. Young cankers, and edges of older cankers, are yellowish orange to orange-brown with an irregular margin. In 

recently killed portions of a canker, black and cream mottling may be found in the inner bark, cambium, and outer 

sapwood. In a year or two, the bark begins to blister and the 

periderm (the thin outer layer of bark) falls in patches, giving the canker a mottled or salt-and-pepper appearance from 

a distance (fig. 129). Eventually, most of the outer layer falls 

off, leaving the blackened inner bark on the surface.

Patches of tiny, gray pillars, like short, coarse hairs, are 

revealed where the periderm falls off. They can often be 

found earlier by peeling back loose, blistering periderm. 

The sexual stage will eventually be present in slightly older 

parts of the canker. It is composed of raised stromata that 

are initially gray and become black when mature, are about 

1/8-1/2 inch (3-13 mm) in diameter, and each has several to 

30, pimple-like, embedded perithecia (figs. 130-131). Small, 

white mycelial fans develop under the bark just behind the 

advancing edge of the canker.

Disease Cycle—The infection court is uncertain, but it appears that dying year-old twigs and small wounds can be 

infected. Various insects, including the poplar borer and the 

aspen tree hopper, make wounds that can also serve as infection courts.

Airborne spores that land on the infection court germinate 

during moist weather and grow into living bark. Two years 

may elapse before symptoms develop. In the year after a visible canker begins to form, the pillars are formed. The pillars 

Figure 129. Hypoxylon 

canker killing an aspen. 

Note the mottled, saltand-pepper appearance 

of the upper, younger portion of the canker where 

outer bark is missing 

in patches. Photo: Jim 

Worrall, USDA Forest 

Service.

C A N K E R S
76 USDA Forest Service RMRS-GTR-241. 2010.

grow up to blister the periderm, which 

flakes off. This exposes the spores that are 

produced all over the surfaces of the pillars. These spores are not infectious; they 

function only in fertilizing other cankers.

Stromata, containing the perithecia, are 

produced in the following year. Spores are 

forcibly ejected from tiny holes at the tips 

of the perithecia and are wind-dispersed. 

Additional cycles of pillar and stromata 

occur each year until the tree is killed and the fungus dies soon after.

Impact—Hypoxylon canker is widespread geographically, but is the least common of the five major cankers of aspen in Colorado and, presumably, in the rest 

of the southern Rocky Mountains. East of the Great Plains, however, it is often 

the most destructive disease of aspen. Even in the southern Rocky Mountains, 

localized areas may suffer serious mortality from this disease. Trees of all sizes 

may be attacked. Damage is most severe in open, widely spaced stands and near 

stand edges or openings. There is some indication that good aspen sites with 

deep soils and adequate moisture have higher levels of Hypoxylon canker than 

poor sites with dry, shallow soils, but this has not been verified. Mortality may 

result from girdling or from snapping due to wood decay beneath the canker.

Management—The canker is favored by stand openings and stands of low density, so maintaining continuous aspen cover should reduce infection. Susceptibility to the disease varies clonally, so consider avoiding further management of 

heavily infected stands.

References: 76, 100, 146, 158

Figure 130. A large stroma of Hypoxylon canker with 

multiple perithecia. Photo: Jim Worrall, USDA Forest 

Service.

Figure 131. Conidial pillars and stromata of 

Hypoxylon canker. Generally stromata are 

produced each year where pillars were produced 

the year before. Photo: Jim Worrall, USDA 

Forest Service.

C A N K E R S 
USDA Forest Service RMRS-GTR-241. 2010. 77

Sooty-Bark Canker

Normally the No. 1 killer of Rocky Mountain aspen

Pathogen—Sooty-bark canker is caused by the fungus Encoelia pruinosa

(formerly known as Cenangium singulare). It is sometimes called Cenangium 

canker.

Hosts—Sooty-bark canker is found primarily on aspen. Other Populus species 

may be attacked occasionally in other areas.

Signs and Symptoms—Sooty-bark canker begins as a small, sunken, oval patch 

of dead bark. It may grow as much as 3.28 ft (1 m) long and 12 inches (30 cm) 

wide each year. Each year’s canker growth typically has an 

alternation of light and dark zones, caused by the appearance 

of the thin, light layer of outer bark contrasting with the black 

inner bark where the outer bark is gone (figs. 132-133). The 

black inner bark becomes dry and powdery, leading to the term 

“sooty.” Eventually, the canker girdles the tree and it dies.

The fruiting bodies are apothecia and often appear in large 

numbers on killed bark. They are cup-shaped, up to 1/8 inch 

(3 mm) in diameter, and have very short stalks (fig. 134). They 

are gray with a fine granular surface (pruinose). When dry, they 

are shriveled and inrolled.

When the bark is completely weathered away, a final sign 

of sooty-bark canker is small patches (several millimeters in 

diameter) of black fungal material on the wood, separated by 

plain wood. The pattern is often likened to that on a leopard’s 

fur (fig. 135).

Disease Cycle—Cankers can be initiated at points with no 

apparent wounds, but the fungus does infect stem wounds. 

However, the type, season, and size of wounds that are suitable 

infection courts are not known. The inner bark and cambium 

are rapidly invaded and killed. As described under “Signs and 

Symptoms” above, vertical growth is somewhat faster than 

horizontal growth. Callus is not produced unless the disease is 

stopped, which is unusual.

The pathogen fruits within a year or two of bark death. 

Ascospores are forcibly shot from apothecia into the air under 

moist conditions. Spores germinate and infect through suitable 

infection courts.

Impact—Sooty-bark canker is the most lethal canker on aspen 

in the West, and it is the most widespread canker in Colorado 

and probably other areas of the West. Under normal circumstances, it is the primary killer of aspen in Colorado and probably other parts of the West. This is not the case during unusual 

Figure 132. The barberpole pattern of sooty-bark 

canker. Each light and dark 

pair represents one year 

of growth. Photo: Thomas 

Hinds, USDA Forest Service.

C A N K E R S
78 USDA Forest Service RMRS-GTR-241. 2010.

events such as sudden aspen decline, a mortality event of 2004-2009 that was associated with drought stress.

The disease usually affects the larger trees in a stand. It kills trees in only 

3-10 years unless its development is arrested, which is uncommon. Damage from 

this canker is most common in stands where the incidence of wounding is high.

Management—As with the other aspen cankers, few practical approaches to preventing infection and canker development are known. Vigorous trees can be attacked and killed, so maintaining host vigor will not affect the disease. Avoiding 

wounds should reduce the likelihood of infection.

Silvicultural approaches can reduce the long-term impact. Because sooty-bark 

canker tends to attack older, larger trees, managing aspen in rotations of 80-100 years 

should be effective. Clearcutting, prescribed fire, and wildfire can all improve aspen 

condition by stimulating dense suckering before diseases like sooty-bark canker 

lead to gradual stand deterioration and, in some cases, conversion to other vegetation types.

References: 79, 96, 100

Figure 135. The “leopard 

spotting” that often remains 

on wood after the bark is 

weathered away from the 

sooty-bark canker. Photo: Jim 

Worrall, USDA Forest Service.

Figure 133. Another 

example of the barberpole pattern of sooty-bark 

canker. The pattern may 

be more obscure than 

these examples. Photo: 

Jim Worrall, USDA Forest 

Service.

Figure 134. Apothecia (fruiting bodies) of Encoelia pruinosa, the sooty-bark canker pathogen. Apothecia are 

light gray, cup-shaped, and inrolled and shriveled during 

dry weather. Photo: Jim Worrall, USDA Forest Service.

C A N K E R S 
USDA Forest Service RMRS-GTR-241. 2010. 79

Introduction to Wilt Diseases

Disruption of water-conducting tissues

In a healthy tree, water in the soil enters roots and is transported through vessels or tracheids in the xylem to leaves. Wilt diseases disrupt this flow of water 

in the xylem, thus causing leaves to wilt. These diseases result from pathogen 

activity in the vessels or tracheids. Wilt pathogens are parasites that can move 

through the vascular tissue of trees. The pathogens can include fungi, nematodes, 

bacteria, or other micro-organisms.

The means of water disruption vary and are often not completely understood. 

In hardwoods, living parenchyma cells may balloon into vessels as a defense response to pathogens, resulting in vessel blockage. Pathogens may directly block 

water flow or cause air bubbles by damaging cell walls in the vessels or tracheids 

that disrupt water transport. Some wilt pathogens produce toxins that damage 

host cells or produce enzymes or other chemicals that disrupt flow. Pathogens 

can invade and kill the living parenchyma cells in the xylem tissues that are associated with water transport, thus disrupting flow. Any of these means can result 

in the typical wilt symptoms.

There are several wilt diseases in the Rocky Mountain Region (table 8). Some 

of the more damaging and/or common wilts in the Region are included in this 

table, but other wilt diseases may occur.

Table 8. Comparison of some more damaging and common wilts in the in the Rocky Mountain Region.

Type of 

Wilt Pathogen organism Common tree host

Dutch elm disease Ceratocystis ulmi Fungus Elm; most damaging on native elm

 (Fig. 136)

Verticillium wilt Verticillium albo-atrum, Fungus Several hardwood genera

 (Fig. 137) V. dahliae

Oak wilt Ceratocystis fagacearum Fungus Oaks; most damaging on red oak group

 (Fig. 138-139)

Pine wilt disease Bursaphelenchus Nematode Pines and other conifers; most

 (Fig. 140) xylophilus damaging on exotic pines

Black stain root Leptographium wageneri Fungus Pinyon

 disease var. wageneri

 (Fig. 141)

Signs and Symptoms—Signs are microscopic for many of the wilts. Therefore, 

microscopic examinations, isolation of the pathogen on agar media, or other 

laboratory tests are often the best ways to identify the causal organisms.

The common name for these diseases, “wilts,” comes from the typical 

wilt symptoms that are attributed to drought stress, including drooping leaves 

and branches. The wilting leaves fade to yellow, then to brown, and then die. 

W I LT S
80 USDA Forest Service RMRS-GTR-241. 2010.

W I LT S 

Depending on the disease, 

leaves may be shed or remain 

on trees. In some cases, wilting 

and yellowing of individual 

branches and branch mortality may occur (figs. 136-141). 

Tree mortality is common with 

these diseases. Trees with wilt 

disease may appear healthy 

when soil moisture is high 

and then suddenly wilt and 

die during hot and dry periods. Vascular discoloration in 

the new sapwood is a common symptom of many wilt 

diseases.

General wilt symptoms 

can have many causes, including wilt disease. Root diseases 

and cankers often cause crown 

wilting. Prolonged periods of 

drought may cause wilt symptoms, especially on younger 

trees.

Disease Cycle—These diseases may spread by vectors, root 

grafts, and wind, or they may 

persist in the soil around infected trees. Vectors are motile 

Figure 139. Crown wilt symptoms of oak wilt. Photo: Joseph 

O’Brien, USDA Forest Service, Bugwood.org.

Figure 138. Leaf wilt symptoms of oak 

wilt. Photo: James T. Blodgett, USDA Forest 

Service.

Figure 136. Crown wilt symptoms of Dutch elm disease. Photo: 

Fred Baker, Utah State University, Bugwood.org.

Figure 137. Crown wilt symptoms of Verticillium wilt. Photo: 

USDA Forest Service Archive, Bugwood.org.
USDA Forest Service RMRS-GTR-241. 2010. 81

W I LT S

organisms, often insects, that carry 

and help spread pathogens. Pathogens 

enter the xylem through wounds on 

the roots, stems, or branches. In some 

cases, wounds are caused by the associated insect vector. The pathogens 

then grow within the xylem, plugging 

vessels or tracheids and/or disrupting 

the integrity of the host cells resulting 

in the disruption of water transport.

Impact—Impacts include reduced 

growth, branch mortality, and tree mortality. Some infected trees can survive 

for many years, or mortality may be sudden. Mortality 

is common for many of the 

important wilt diseases in the 

region including Dutch elm 

disease, oak wilt, pine wilt 

disease, and black stain root 

disease.

References: 140, 158

Dutch Elm Disease

A non-native invasive wilt

Pathogen—The fungus Ophiostoma novo-ulmi is the most aggressive pathogen 

involved, but other Ophiostoma species may occur within the Rocky Mountain 

Region. The pathogen was introduced into the United States in the 1930s and 

subsequently spread throughout the country.

Vectors—The native elm bark beetle (Hylurgopinus rufipes), smaller European 

elm bark beetle (Scolytus multistriatus), and potentially the banded elm bark 

beetle (S.schevyrewi) vector the pathogen. See the Elm Bark Beetles entry in this 

guide for more information.

Hosts—This disease affects trees in the elm family (Ulmaceae). Native elms are 

most susceptible.

Figure 141. Crown wilt symptoms of black stain root disease. 

Photo: James T. Blodgett, USDA Forest Service.

Figure 140. Crown wilt symptoms of pine wilt disease. Photo: James T. Blodgett, USDA Forest Service.
82 USDA Forest Service RMRS-GTR-241. 2010.

W I LT S 

Signs and Symptoms—

Symptoms differ between 

trees that are infected through 

root grafts and those infected 

via beetles. Trees infected by 

root grafts wilt, their leaves 

turn brown, and the trees 

die rapidly, usually in the 

spring (fig. 142). When the 

disease is transmitted by bark 

beetles, the first symptoms 

are often yellowing and 

wilting of leaves on one or 

several branches. The leaves 

eventually turn brown and 

fall in mid to late summer 

(figs. 143-144). As the 

disease spreads to adjacent 

branches, additional 

branches die, and eventually 

tree mortality results. This 

often takes 1 or more years.

Slight symptom differences also occur among the 

beetle vectors. The smaller 

European elm bark beetle 

feeds in small twigs, usually 

high in the crown of mature 

trees. This results in the initial wilt symptoms on higher 

and smaller branches. In 

contrast, the native elm bark 

beetle bores into the bark of 

branches 2-4 inches (5-10 

cm) in diameter to feed.

Brown streaks in the new 

sapwood are characteristic symptoms of infection 

(fig. 145). However, laboratory tests may be needed to confirm the presence of 

the pathogen because signs of the pathogen are usually microscopic. Sometimes 

the white spore-bearing bodies with tiny black stalks (synnemata, also known as 

coremia) of Ophiostoma may be visible in beetle galleries (fig. 146).

Disease Cycle—The pathogen infects new trees when the bark beetle vectors 

feed on twigs or small branches of healthy trees; new trees are also infected 

throughout the growing season by root grafts. As the beetles feed, they introduce 

Figure 143. Typical crown symptoms of Dutch elm disease associated with insect transmission. Photo: Northeastern Area Archive, 

USDA Forest Service, Bugwood.org.

Figure 142. Trees killed by Dutch elm disease that spread 

through root grafts. Photo: Edward L. Barnard, Florida 

Department of Agriculture and Consumer Services, Bugwood.org.
USDA Forest Service RMRS-GTR-241. 2010. 83

W I LT S

the pathogen 

spores attached 

to their body into 

the sapwood. 

The fungus 

quickly spreads 

in the vessels 

of the xylem 

and moves both 

up and, more 

i m p o r t a n t l y, 

down, eventually reaching 

the roots. As the 

disease spreads 

throughout the 

tree, it disrupts water movement, 

causing trees to wilt. The fungus 

continues moving through the root 

system, infecting other adjacent trees 

through root grafts. As trees die, 

the beetles form breeding galleries 

in the recently dead or dying stems 

and branches of infected trees. The 

fungus sporulates in the galleries, and 

spores are picked up by the beetles in 

the galleries and are carried to a new, 

healthy host.

Impact—Since its introduction in the 1930s, this disease has quickly spread 

throughout the range of elms and is now the most damaging disease of elm and 

the leading cause of elm mortality.

Management—Sanitation (removal of infected materials) can significantly 

reduce the disease spread of new infections. Sanitation efforts should focus on 

promptly removing infected trees and prohibiting the storage of elm wood with 

attached bark because beetles can infest cut wood and spread the disease. The 

fungus can persist in the sapwood for several years after trees die.

Figure 144. Close-up with yellow and brown leaves due to Dutch elm disease. Photo: 

Joseph O’Brien, USDA Forest Service, Bugwood.org.

Figure 146. The pathogen Ophiostoma novo-ulmi sporulating along beetle gallery. Photo: Joseph O’Brien, 

USDA Forest Service, Bugwood.org.

Figure 145. Brown streaks in the new sapwood caused 

by Ophiostoma novo-ulmi. Photo: Petr Kapitola, State 

Phytosanitary Administration, Bugwood.org.
84 USDA Forest Service RMRS-GTR-241. 2010.

Breaking root grafts is important to prevent root spread but may be difficult 

and expensive. Root graft disruption should be done before the infected trees are 

removed. Otherwise, the transpirational pull from the healthy trees will draw the 

pathogen from the diseased tree roots to the healthy tree.

A few varieties of American elm with some level of resistance are commercially available. However, many of the resistant elm varieties are susceptible 

to elm yellows. Planting resistant European or Asiatic elm species will reduce 

losses, but the growth forms are not the same as the American elm. Non-elm 

species adapted to the area are another option.

Preventive chemicals are available as tree injections that can protect healthy 

elms from the disease for up to 3 years. Because the chemicals are injected in the 

lower stems or upper roots and can only move upward in trees, these treatments 

are ineffective for root-graft infections. The treatments are also costly but might 

be appropriate for high-value trees. An appropriately registered fungicide for an 

individual state should be selected.

References: 54, 107, 158

Oak Wilt

Red oaks die quickly; white oaks may recover

Pathogen—Oak wilt is caused by the fungus Ceratocystis fagacearum. This 

disease occurs in Kansas, Nebraska, and South Dakota.

Vectors—Beetles in the Nitidulidae family and oak bark beetles (Pseudopityophthorus spp.) can transmit the fungus.

Hosts—Oak wilt is a disease of oak species. Species in the red oak group die 

more quickly than species in the white oak group.

Signs and Symptoms—The only visible signs are the black and gray fungal 

mats (fig. 147) that are common in infected red oaks but less common or less 

apparent in white oaks. The mats form beneath the bark soon after mortality. 

These mats occasionally raise and crack the bark (fig. 148). Other signs of the 

pathogen are microscopic. Laboratory tests and fungal culturing on agar media 

can be used to confirm the pathogen’s presence.

For the red oak group, symptoms are often expressed in spring but can continue into the summer. Symptoms start from the tip and outer edges of leaves 

and move toward the midrib and base of leaves, often with a distinct margin 

(fig. 149). First, leaves turn dull green or bronze, can appear water-soaked, and 

wilt. Later, the leaves turn yellow and/or brown, curl around the midrib, and are 

shed at branch tips. Finally, both green and symptomatic leaves throughout the 

crown fall. Symptoms often develop quickly throughout the crown in red oak 

(fig. 150). Trees may die only 1 or 2 months after symptoms appear and seldom 

survive more than a year.

Disease symptoms are more variable for species in the white oak group. 

Symptoms can be the same as those of red oaks with quick mortality, but 

W I LT S 
USDA Forest Service RMRS-GTR-241. 2010. 85

Figure 147. Fungal mat of oak wilt 

disease. Photo: Fred Baker, Utah State 

University, Bugwood.org.

Figure 148. Cracked bark caused by fungal mat of oak 

wilt disease. Photo: Joseph O’Brien, USDA Forest Service, 

Bugwood.org.

Figure 149. Oak wilt symptoms on red oak 

leaves. Photo: D. W. French, University of 

Minnesota, Bugwood.org.

Figure 150. Typical crown symptoms of oak wilt on 

red oak. Photo: Joseph O’Brien, USDA Forest Service, 

Bugwood.org.

Figure 151. Black streaks in the new sapwood 

caused by oak wilt disease. Photo: Fred Baker, Utah 

State University, Bugwood.org.

typically, white oaks die slowly over several years, with only a few branches 

showing symptoms and dying per year. The leaves often remain attached with 

discoloration only at the margins. Other times, however, white oaks seem to 

recover. Brown to black discoloration commonly develops in the outer sapwood 

of infected white oaks (fig. 151). This symptom is less common in infected red 

oaks.

W I LT S
86 USDA Forest Service RMRS-GTR-241. 2010.

Disease Cycle—Root 

grafts and insect vectors 

spread the oak wilt 

fungus. Root grafts are 

an effective means of 

transmitting the fungus 

from infected to nearby 

healthy trees. These 

grafts are a major factor 

in local spread of the 

oak wilt pathogen, 

especially in areas with 

deep, sandy soils and 

where oaks grow close 

together (fig. 152).

The fungal mats can 

enlarge and crack the 

bark (fig. 153). These aromatic mats 

attract insects such as sap-feeding 

Nitidulidae beetles and the fungal 

spores adhere to their bodies as they 

crawl over the mats (fig. 154). The 

beetles carry the spores to wounds 

on healthy oaks throughout the 

summer.

Oak bark beetles, Pseudopityophthorus spp., can also transmit 

the fungus (fig. 155). The beetles 

form breeding galleries in recently 

dead or dying oak stems and branches, including wilt-infected infected 

trees. The adults carry spores on their 

bodies when they emerge the following spring and infect healthy trees as they feed 

in spring and early summer.

Figure 154. Adult Nitidulidae beetle. Photo: 

Pennsylvania Department of Conservation and 

Natural Resources, Forestry Archive, Bugwood.org. Figure 155. Adult oak bark beetle. Photo: J. R. Baker 

& S. B. Bambara, North Carolina State University, 

Bugwood.org.

Figure 153. Older fungal mat with bark beetle galleries. 

Photo: C. E. Seliskar, North Carolina State University, 

Bugwood.org.

Figure 152. Oak wilt spread by root grafts. Photo: D. W. French, 

University of Minnesota, Bugwood.org.

W I LT S 
USDA Forest Service RMRS-GTR-241. 2010. 87

This disease is a vascular wilt. Trees respond to infection by forming tyloses 

(where living parenchyma cells balloon into vessels as a defense response) that 

restrict water flow in the vessels. The pathogen spreads rapidly within vessels of 

red oaks and often more slowly in white oaks. The fungus overwinters as mycelium in infested trees and as fungal mats on dead trees.

Impact—The disease quickly kills red oaks and can cause reduced growth and 

mortality of white oaks. The disease range is expanding, and, in some areas, the 

incidence of disease is increasing.

Management—Sanitation (removal of infected materials) can significantly 

reduce new infections. If the mats or beetle galleries are present, trees should be 

cut, and the wood should be burned, buried, or chipped. The fungus can persist 

in cut wood with attached bark and in dead trees. After trees die, fungal mats 

continue to form and may attract beetles. Trees should not be pruned in spring or 

early summer when beetles are most active.

Mechanical and chemical barriers and severing root grafts between diseased 

and healthy trees are effective ways to prevent the spread of oak wilt through root 

grafts. New root grafts will not form between dead/dying trees and healthy trees.

References: 5, 47, 136, 173

Pine Wilt

Rapid wilting and death of pine

Pathogen—The pine wood nematode, Bursaphelenchus xylophilus, causes pine 

wilt disease. Nematodes are “roundworms” in the phylum Nematoda, which has 

over 80,000 described species. This disease can be a problem wherever nonnative pines are planted but is most common in Kansas, Nebraska, and South 

Dakota.

Vectors—The pine sawyer beetles, Monochamus spp., transmit the nematode. 

Please see the Roundheaded Wood Borers (Longhorned Beetles) entry in this 

guide for more information.

Hosts—Scots, Austrian, and other non-native pines are often killed by this 

disease. Eastern white pine, a native pine, is also affected and may be killed by 

pine wilt disease. The nematode commonly infects other native pines and some 

native conifer species. However, most native species are resistant to the disease 

(e.g., native conifers may be infected and express little or no disease symptoms).

Signs and Symptoms—As with many wilts, signs are microscopic. The pine 

wood nematode is relatively large compared with other nematodes, but it cannot 

be seen with a hand-lens in infected wood. Laboratory tests are required to 

confirm its presence.

Pine wilt disease causes rapid wilting and death on non-native pines. 

Symptoms are often first expressed in early summer but can occur throughout 

the growing season. Symptoms may first appear on one or a few branches but 

W I LT S
88 USDA Forest Service RMRS-GTR-241. 2010.

often develop quickly throughout the crown, and 

trees may die only 1 or 2 months after symptoms 

appear. Trees seldom survive more than a year. 

Symptoms start with needle discoloration, which 

progresses rapidly from a grayish green to yellow 

and then to brown. Needles can but often do not 

show wilt-type symptoms, and needles are usually retained for a few months (figs. 156-157).

Disease Cycle—The nematode is introduced into 

pines as the pine sawyer beetles feed (fig. 158) or 

lay eggs. Eggs are laid in egg niches chewed by the 

females. When the beetle feeds on a healthy tree 

or chews the egg niches, the nematodes leave the 

beetle and enter the tree through the wounds. The 

nematodes reproduce rapidly in resin canals and go 

from egg to adult in 5 days with many eggs per nematode. They then spread rapidly 

within xylem tracheids and clog the water transport system of the pines, thus causing disruption of water movement throughout the tree (i.e., a vascular wilt disease). 

Nematodes can overwinter in dead or living pine.

Nematodes feed on fungi in the wood, including bluestain fungi (fig. 159) that 

are transmitted by engraver and other bark beetles. The nematodes can survive 

and reproduce with fungi in dead, stressed, and living pine. Therefore, this pathogen can be found on trees killed by other causes.

Nematodes can overwinter in a dormant state. As wood dries, they molt and 

enter a dormant phase high in lipids and do not feed. This phase is resistant to 

unfavorable environmental conditions.

Figure 158. Nematodes can be transmitted to pines when the adult pine 

sawyer beetle feeds on shoots. Photo: 

L. D. Dwinell, USDA Forest Service, 

Bugwood.org.

Figure 156. Symptoms of pine wilt disease 

on Austrian pine branch. Photo: North 

Central Research Station Archive, USDA 

Forest Service, Bugwood.org.

Figure 157. Crown symptoms of pine wilt disease on Scots 

pine. Photo: James T. Blodgett, USDA Forest Service.

W I LT S 
USDA Forest Service RMRS-GTR-241. 2010. 89

The pine sawyer beetles lay eggs in dying or recently dead trees in spring. The eggs 

hatch, the beetle larvae bore into the wood to 

feed, and the beetle overwinters as larvae or 

pupae. Before emerging as young adults, the 

beetles acquire the nematodes. These beetles 

transmit nematodes to healthy trees.

Impact—The disease quickly kills exotic 

pines (especially when grown on poor sites 

with dry, shallow soils) but usually has little 

effect on native pines.

Management—Sanitation (removal of infected materials) can reduce new infections. 

Infected trees should be cut and the wood 

should be burned, buried, or chipped. The nematode can survive for a time in cut 

wood that can attract beetles, which results in subsequent disease spread. Infected 

wood should not be kept for firewood without removing the bark. Anything that 

reduces pine sawyer beetle attraction and breeding success will reduce losses. 

This includes reducing stresses from other diseases, insects, or the environment.

Planting native tree species suitable to the site will reduce losses. Although 

they may be infected, native conifers seldom develop pine wilt disease symptoms. Even native pines planted off-site rarely develop this wilt disease. If exotic 

pines are desired, they should only be grown on favorable sites.

References: 106, 117, 187

Verticillium Wilt

Vascular wilt of hardwoods

Pathogen—Verticillium wilt is caused by two closely related species of fungi, 

Verticillium albo-atrum and V. dahliae.

Hosts—Verticillium wilt is a vascular wilt of hardwoods. Over 300 plant species 

are affected by Verticillium wilt. The disease is particularly destructive to trees in 

landscape plantings. Ash, catalpa, elm, sumac, and maple are the most common 

hosts in the Rocky Mountain Region.

Signs and Symptoms—Typical symptoms include chlorosis, wilting, marginal 

and interveinal necrosis of the foliage, branch dieback, and mortality (fig. 160). 

Symptoms vary among hosts and are not always completely diagnostic. 

Symptomatic wilting is most obvious on warm, sunny days. The disease often 

progresses from the lower crown upward, but it is not unusual for only a branch 

or portion of the tree to be impacted.

The sapwood may exhibit dark streaks or bands along the grain (fig. 161). 

The color of the streaking depends on the host and may be shades of green, red, 

brown, or black.

Figure 159. Bluestain fungal mycelium in 

pine wood is a source of food for the nematode. Photo: North Central Research Station 

Archive, USDA Forest Service, Bugwood.org.

W I LT S
90 USDA Forest Service RMRS-GTR-241. 2010.

Disease Cycle—The fungi that cause Verticillium wilt are soil-borne and gain 

entry through roots or wounds near the ground. Once inside the host, they invade 

the xylem, which disrupts water transport and physiological function. Spread 

can occur throughout the plant by spores transported in the sap stream or by 

vegetative growth of mycelium. Plant-to-plant spread occurs by means of asexual 

spores (conidia) and by resting spore structures (microsclerotia) that allow the 

fungi to persist for long periods of time in the soil independent of any host 

material. Long-distance dispersal is a serious problem because microsclerotia 

can be carried in soil, bare roots, root balls, or equipment within nurseries to 

planting sites.

Impact—Small trees are very susceptible and may be killed rapidly by 

Verticillium wilt. Older trees generally deteriorate over time and can survive 

for several or many years with the disease. Damage is greatest in nurseries and 

landscape plantings. Stressed trees are more susceptible to Verticillium wilt and 

sustain more damage.

Management—The following control strategies can be used to reduce impacts 

of Verticillium wilt:

•Do not plant susceptible hosts in areas where the disease is presen

•Maintain tree vigor by fertilizing with “balanced” fertilizers (10-10-1

[N-P-K]) and watering. High-nitrogen fertilizers may increase damage.

•Prune back branches beyond any streaking in the wood. Destroy infecte

branches and sterilize pruning tools.

•Avoid damage to the roots and base of tree

•Plant resistant or tolerant specie

References: 140, 158

Figure 161. Discolored streaks in the wood of a cherry branch 

infected by Verticillium wilt. Photo: William Jacobi, Colorado State 

University, Bugwood.org.

Figure 160. Verticillium wilt on silver 

maple. Photo: William Jacobi, Colorado 

State University, Bugwood.org.

W I LT S
USDA Forest Service RMRS-GTR-241. 2010. 91

Introduction to Rust Diseases

Specialized foliage and/or canker diseases often with 

alternate hosts

Rust diseases are grouped based on the taxonomic classification of the pathogens. They are fungi in the phylum Basidiomycota, class Pucciniomycetes, and 

order Pucciniales. Rusts are all obligate parasites that depend on the living cells 

of their hosts. Initially, they produce structures (haustoria) that grow into their 

hosts’ living cells, from which they derive nutrients.

The diseases caused by this group are varied. Most of the rust fungi initially 

infect and cause diseases of foliage. However, some spread into branches and 

stems, colonizing the phloem and cambium. Rusts cause cankers (diseases of the 

bark), galls, brooms, foliage diseases, and/or cone diseases.

The majority of the rust diseases in this Region are native pathogens, with 

the exception of white pine blister rust. Table 9 provides examples of some of 

the most damaging and/or common rusts in the region (figs. 162-168). However, 

many other rusts occur within the Rocky Mountain Region.

Signs and Symptoms—The common name, “rusts,” comes from the rust-like 

appearance of the various spore structures that are often rust-colored, yellow, 

and/or orange. These brightly colored signs help to distinguish rusts from other 

diseases. If signs are not present, symptoms may look like other foliage or canker 

diseases, with discolored foliage or branch flagging (faded and dying branches). 

Stem rusts often can be identified by the host and shape of the cankers or galls. 

The cankers usually have distinctive roughness on their margin that are the 

remains of earlier fruiting structures.

Disease Cycle—Rust disease cycles are complicated. Many require two different 

host species (table 9) that usually need to be less than a mile apart. The alternate 

hosts to the tree hosts can be other trees, shrubs, or herbaceous plants. The hosts 

are often specific species or groups within the same genus. Rust fungi may have 

as many as five different spore types or stages (table 9). White pine blister rust 

is an example of a five-stage (macrocyclic) rust with two hosts (fig. 162). There 

are rust diseases, such as western gall rust (Peridermium harknessii), that require 

only one host to complete their life cycle.

Spores are disseminated by wind and often infect hosts through needle stomata. However, some rusts can infect hosts through wounds. Prolonged periods of 

cool, wet weather or high humidity are necessary for infection and germination. 

Therefore, “wave years,” or years with significant increases in new infections, 

occur during years of high rain and increased humidity.

Many rust pathogens have spore types called urediniospores. These spores 

permit the rusts to spread and only re-infect an alternate host group. This allows 

for increased spore levels (increased inoculum) during the summer.

Impact—Impacts include reduced growth, stem deformities, loss of wood 

quality, dead tops and branches, and tree mortality. Stem cankers, especially 

R U S T S
92 USDA Forest Service RMRS-GTR-241. 2010. Table 9. Comparison of the more damaging and common rusts in the Rocky Mountain Region. Number Common alternate of spore Main damages Rust	 Scientific name	 Common tree hosts	 hosts	 types	 on tree host White pine blister rust Cronartium ribicola Five-needle pines Currants and gooseberries 5 Stem and branch cankers (Fig. 163) Comandra blister rust C. comandrae Lodgepole and ponderosa pine	 Bastard toadflax	 5	 Stem and branch cankers (Fig. 164) Stalactiform rust C. coleosporioides Lodgepole pine Indian paintbrush 5 Stem and branch cankers Western gall rust Peridermium harknessii Ponderosa, lodgepole, Scots, None 1 Branch galls and stem cankers (Fig. 165) and pine Spruce broom rust Chrysomyxa arctostaphyli Engelmann and Colorado Bearberry 4 Branch brooms (Fig. 166) blue spruce Fir broom rust Melampsorella White and subalpine fir	 Chickweed	 5	 Branch brooms (Fig. 167) caryophyllacearum Melampsora leaf Melampsora medusae, 	 Aspen, cottonwood, and willow	 Douglas-fir and ponderosa	 5	 Foliage disease rust other Melampsora spp. and lodgepole pine Gymnosporangium Gymnosporangium spp. Junipers Species in family Rosaceae 4 Branch galls, stem swelling, and brooms (Fig. 168)

R U S T S 
USDA Forest Service RMRS-GTR-241. 2010. 93

Figure 163. White pine blister 

rust. Photo: James T. Blodgett, 

USDA Forest Service.

Figure 164. Comandra blister 

rust. Photo: James T. Blodgett, 

USDA Forest Service.

Figure 166. Close-up view of 

spruce broom rust fruiting on 

needles. Photo: James T. Blodgett, 

USDA Forest Service.

Figure 165. Western gall rust. Photo: 

James T. Blodgett, USDA Forest Service.

Figure 162. White 

pine blister rust 

disease cycle as an 

example of a fivecycle rust. Image: Jim 

Worrall, USDA Forest 

Service.

R U S T S
94 USDA Forest Service RMRS-GTR-241. 2010.

when associated with decay fungi, can increase the chance of stem breakage. 

White pine blister rust and comandra blister rust are among the most damaging 

diseases in the Rocky Mountain Region.

References: 1, 6, 158, 201

Broom Rusts of Spruce and Fir

Dense, pale, or dead-looking brooms

Pathogen—Fir broom rust is caused by the fungus Melampsorella caryophyllacearum.

Spruce broom rust is caused by the fungus Chrysomyxa arctostaphyli.

Hosts—Many true fir species are 

susceptible to fir broom rust, including 

white fir and subalpine fir in the Rocky 

Mountain Region. Alternate hosts are 

chickweeds (Cerastium and Stellaria

spp.) (fig. 169).

Spruce broom rust primarily affects 

Engelmann spruce and Colorado blue 

spruce in the Rocky Mountain Region. 

The primary alternate host is bearberry or kinnikinnick (Arctostaphylos

uva-ursi) (fig. 170), but manzanitas 

(Arctostaphylos spp.) are occasional 

Figure 169. Chickweed is the alternate host for fir

broom rust. Photo: Karan A. Rawlins, University of 

Georgia, Bugwood.org.

Figure 167. Gymnosporangium rust. 

Photo: James T. Blodgett, USDA Forest 

Service.

Figure 168. Fir broom rust. 

Photo: James T. Blodgett, 

USDA Forest Service.

R U S T S 
USDA Forest Service RMRS-GTR-241. 2010. 95

alternate hosts on the Uncompahgre 

National Forest.

Signs and Symptoms—These diseases are easily identified by the dense proliferation of branches (witches’ brooms) 

on spruce and fir (fig. 171). Infected 

needles are chlorotic (pale), short, and 

thick and are shed each fall, making the 

broom appear dead over the winter, but 

new chlorotic foliage is produced within 

the broom each spring. Spermogonia form on the underside of needles between 

the cuticle and epidermis in the spring and have a strong, characteristic odor. Aecia appear in the summer. Brooms are most 

obvious at this time 

because the chlorotic 

foliage is covered 

with bright orange aecia, which contrast to 

adjacent healthy foliage (figs. 172-173). 

Witches’ brooms 

commonly lead to 

cankers, dead tops 

and branches, broken 

tops and branches, 

and mortality. Rust 

Figure 170. The alternate host for spruce broom 

rust is kinnikinnick. Photo: Dave Powell, USDA Forest 

Service, Bugwood.org.

Figure 171. Rust broom on Engelmann spruce after 

infected needles produced that year died. Note that 

the tree top has failed. Photo: Kelly S. Burns, USDA 

Forest Service.

Figure 172. Close-up of infected spruce needles sporulating during summer. Photo: William M. Ciesla, Forest Health Management International, 

Bugwood.org.

R U S T S
96 USDA Forest Service RMRS-GTR-241. 2010.

brooms are sometimes confused 

with dwarf mistletoe brooms. 

However, rust brooms are denser, 

yellow, and lack mistletoe shoots. 

Dwarf mistletoes do not occur in 

true firs and spruce in the Rocky 

Mountain Region. Both pathogens cause a leaf spot and shoot 

blight on the alternate hosts.

Disease Cycle—Both pathogens 

require an alternate host to complete their life cycles. Their life cycles are similar to one another and 

to other macrocyclic rust fungi, except C. arctostaphyli does not produce uredinia. Wind-blown spores 

(basidiospores) produced on the 

alternate host are needed to start 

new infections on trees. Infection 

occurs on newly emerging twig and bud tissue. Once a tree is infected, the fungus 

stimulates bud formation, leading to broom development. Spermogonia form on the 

underside of needles in the spring; their strong, fetid odor attracts insects that crossfertilize the fungus. Bright yellow-orange pustules (aecia) of spores (aeciospores) 

rupture through the leaf surface in early summer. Aeciospores disseminate in the 

wind to infect the alternate host. Moist conditions are conducive to infection.

Impact—Broom rusts may cause stem cankers and deformations, growth loss, 

top-kill, and tree mortality. Trees weakened by broom rust may be more susceptible to other insects and diseases. A few brooms on a tree can result in more than 

20% reduction in diameter and height growth. Rust brooms also serve as infection courts for decay fungi such as Porodaedalea (= Phellinus) pini.

Rust brooms are especially damaging when they occur near the stem. Stem 

breakage may occur at the point of infection, creating hazards in recreation areas. 

Portions of the San Juan and Rio Grande National Forests have high levels of 

infection (up to 23-29%) of spruce. Fir broom rust is also common on the San 

Juan National Forest, with some sites having 37-42% infection. However, average incidence in Colorado is 4.2% for spruce and 2.3% for fir.

The incidence of broom rust is associated with the distribution and abundance of hosts, microclimatic conditions, and host susceptibility. Mature stands 

are likely more susceptible simply because there is more susceptible target area, 

but damage is generally greater on small trees because their needles are so close 

to the main stem.

Management—Trees with stem cankers or brooms can be selectively removed 

during stand treatments. Pruning out broom rust infections may reduce the risk 

of stem breakage and maintain tree vigor. Trees with dead tops and adjacent dead 

Figure 173. Rust broom on subalpine fir sporulating in

midsummer. Photo: Susan K. Hagle, USDA Forest Service, 

Bugwood.org.

R U S T S 
USDA Forest Service RMRS-GTR-241. 2010. 97

brooms are often heavily decayed and should be considered hazardous in recreation areas. Consider the following management recommendations (ref. 91).

Tree removal priorities for commercial and precommercial entries:

•Remove trees with spike tops or broken tops while maintaining sufficien

numbers of snags.

•Remove all infected trees that have symptoms of bark beetle attack, decay

and/or root disease.

•Remove trees with one or more bole cankers that girdle more than 33% of th

stem circumference.

•Remove trees with one or more dead brooms and two or more live broom

within 1 ft (30 cm) of the main stem.

•Remove all infected trees while maintaining adequate stockin

On developed sites:

•Remove all dead trees, hazardous trees, and hazardous branche

•Maintain as many live trees as possibl

References: 8, 91, 132, 201

Comandra and Stalactiform Blister Rusts

Branch and stem rusts of lodgepole and ponderosa pines

Pathogen—The rust fungi Cronartium comandrae and C. coleosporioides

cause comandra and stalactiform blister rusts, respectively.

Hosts—Lodgepole and ponderosa pine are hosts of both rusts, but stalactiform 

is uncommon on ponderosa pine. The alternate hosts also differ (figs. 174-175). 

The alternate hosts for comandra blister rust are bastard toadflax (Comandra

Figure 175. Principal alternate hosts for stalactiform blister 

rust are species of Indian paintbrush. Photo: Andrew Kratz, 

USDA Forest Service.

Figure 174. The alternate host for comandra blister rust is bastard toadflax. Photo: 

William Jacobi, Colorado State University, 

Bugwood.org.

R U S T S
98 USDA Forest Service RMRS-GTR-241. 2010.

umbellata) and northern 

comandra (C. livida). The 

alternate hosts for stalactiform rust include several broadleaved plants 

but mainly species of Indian paintbrush (Castilleja

spp.). Comandra rust is 

especially severe in Wyoming and areas of northern 

Colorado. Stalactiform 

rust is less common in the 

Region.

Signs and Symptoms—

Symptoms on pines include branch flagging, 

rough branch cankers, 

elongate stem cankers, 

top-kill (spike tops), and 

tree mortality (figs. 176-

177). Rodents may feed 

on branch and stem canker margins, and damage 

is often accompanied with 

heavy resin flow. Top-kill 

is much more common 

with comandra rust.

Stem cankers initially 

have rough bark and heavy resin flow. With time, 

stem cankers slough the dead bark, revealing perennial stem cankers with concentric ridges of resinous 

sapwood and dead cambium that result from the annual canker growth. Stem cankers caused by the rusts 

are similar in appearance, but comandra rust cankers 

tend to be one and one-half to four times longer than wide, while stalactiform 

rust cankers tend to be eight to many times longer than wide.

For pines, the signs include light yellow to orange pustules (aecia) of spores 

(aeciospores) produced at canker margins in late spring and early summer (figs. 

178-179). Cankers caused by comandra blister rust tend to be yellow-colored 

and cause top-kill after girdling the stem. The fungi are best differentiated by 

microscopic examination of spores.

Symptoms on the alternate hosts can include tan to brown leaf spots. However, 

the signs may be present with few symptoms. Signs, visible on the underside of 

leaves, include orange-colored fruiting bodies (uredinia) with spores (urediniospores), and later in the year, rust-brown, hair-like structures (telia) form.

Figure 176. Comandra blister 

rust canker on an older stem. 

Photo: James T. Blodgett, USDA 

Forest Service.

Figure 177. Stalactiform blister 

rust canker on an older stem. 

Photo: John Guyon, USDA Forest 

Service, Bugwood.org.

R U S T S 
USDA Forest Service RMRS-GTR-241. 2010. 99

Disease Cycle—

Both rusts require 

two different hosts 

to complete their 

life cycle. The incidences of these diseases are correlated 

with presence of 

their alternate hosts. 

Studies have shown 

a high incidence of 

comandra blister 

rust in pine stands 

near sagebrush. Bastard toadflax is an 

obligate parasite of 

sagebrush and many 

other woody perennial plants.

Both rusts have five spore stages. Three spore 

stages (uredinia, telia, and basidia) develop on the 

alternate hosts, and two spore stages (spermogonia 

and aecia) develop on the pines.

Basidiospores are wind-blown from the alternate hosts and infect pine needles and new shoots. The basidiospores cannot 

infect the alternate hosts. Fungal hyphae grow through the phloem and cambium 

of branches, forming perennial cankers that can spread to the stem. Spermogonia 

develop within the canker in the spring and early summer and aecia are produced 

in the same tissue the following spring and early summer. The aeciospores are 

wind-disseminated. These spores can only infect the alternate host’s leaves. The 

urediniospores produced on the underside of leaves re-infect the alternate host 

leaves during the summer but cannot infect pines. Basidiospores develop on the 

telia in late summer or fall for comandra rust or late spring for stalactiform rust 

and infect the pines. Long periods of high humidity are required for infection of 

the pine hosts. Therefore, rust epidemics often involve a burst of infections during a year with a long, moist season.

Impact—Comandra rust is one of the more important diseases of lodgepole 

pine in the region. Both rusts can cause stem deformities, growth reduction, 

and cankers that girdle branches or stems, resulting in top-kill or tree mortality. 

Stem girdling and top-kill are far more frequent with comandra rust. Trees may 

survive several decades with spiked tops, and at times, a lower branch will 

assume terminal dominance and produce a new top. However, the cankers can 

continue to grow down the stem, killing the new top. Infection is occasionally 

heavy, causing high volume losses in stands.

Figure 178. Comandra blister rust 

sporulating on a young stem. Photo: 

James T. Blodgett, USDA Forest 

Service.

Figure 179. Stalactiform blister 

rust sporulating on a young stem. 

Photo: USDA Forest Service Archive, 

Bugwood.org.

R U S T S
100 USDA Forest Service RMRS-GTR-241. 2010.

These rusts affect the form, lumber quality, and growth rate of trees. Although 

these fungi can kill individual trees, they do not kill whole stands. On large trees, 

stem cankers often result in non-merchantability of infected logs. Infected seedlings and young trees are frequently killed. Because fruiting bodies are sparse on 

old stem cankers and rodents (especially porcupines and squirrels) chew canker 

margins, the damage is often attributed entirely to other causes.

Management—Removing infected trees during partial cuts, using disease-free 

trees as leave trees during seed cuts, pruning infected branches, and planting 

non-host species are ways to reduce losses.

References: 1, 72, 95, 175, 201

Gymnosporangium Rusts

Commonly called juniper rust

Pathogen—Rust fungi in the genus Gymnosporangium cause these diseases, 

commonly called juniper rust. There are at least nine (and likely more) 

Gymnosporangium species in the Rocky Mountain Region.

Cedar-apple rust, G. juniperi-virginianae, is often incorrectly credited for 

all rusts in this group and is only common in the eastern part of the Rocky 

Mountain Region, in the Great Plains.

Hosts—The Gymnosporangium rusts require two hosts to complete their 

disease cycle. In this Region, one spore stage (telial stage) occurs on juniper 

species, and the other spore stage (aecial stage) occurs on rosaceous species. 

Different Gymnosporangium rusts can have different hosts.

Various juniper species are affected by these diseases, including Rocky 

Mountain juniper, common juniper, and eastern redcedar. Rosaceous hosts in 

the Region include apple, crabapple, hawthorn, Juneberry, and serviceberry. 

Cedar-apple rust is found on apple and crabapple along with juniper host 

species.

Signs and Symptoms—

On junipers, the telial stage 

looks like exploding, orange “Jell-O” masses on 

branches (figs. 180-181), 

stems, or needles. These 

telial horns are finger-like, 

bright orange, gelatinous, 

and emerge from the areas 

where symptoms occur. On 

rosaceous species, the aecial stage is the most visible sign. During this stage, Figure 180. Sign of Gymnosporangium rust: fresh telial horns on 

branches. Photo: Rocky Mountain Region, USDA Forest Service.

R U S T S 
USDA Forest Service RMRS-GTR-241. 2010. 101

yellow to orange 

fungal tissue can 

be seen within the 

lesions and spots 

(fig. 182). These 

areas first contain 

small, black, pimplelike fruiting bodies 

(spermogonia). Later, 

in the same area, 

tube-like, orange 

fruiting structures 

(aceia) form.

On junipers, 

symptoms include 

branch galls, branch 

knots, brooms, 

stem swelling, and 

small needle lesions 

(figs. 183-184). The type of symptom depends on the rust species involved. 

On rosaceous species, symptoms are lesions and circular spots on leaves, fruit, 

petioles, or young twigs.

Disease Cycle—These rusts have four spore types or spore stages, and two 

hosts are required for the pathogens to complete their disease cycle.

Usually, after a heavy spring rain, telial horns extrude from the branch galls, 

branch knots, brooms, stem swelling, or needle lesions on the juniper host. 

Teliospores in the gelatinous horns produce basidiospores that are wind-blown 

Figure 181. Close-up of 

dried telial horns. Photo: Petr 

Kapitola, State Phytosanitary 

Administration, Bugwood.org.

Figure 182. Signs of Gymnosporangium 

rust: pycnia and aecia on leaves. Photo: 

Clemson University, USDA Cooperative 

Extension, Bugwood.org.

Figure 183. Symptom of Gymnosporangium rust: 

branch galls. Photo: James T. Blodgett, USDA 

Forest Service. Figure 184. Symptom of Gymnosporangium rust: 

stem swelling. Photo: William Jacobi, Colorado State 

University, Bugwood.org.

R U S T S
102 USDA Forest Service RMRS-GTR-241. 2010.

to rosaceous hosts. These basidiospores do not survive long, so the distance 

of spread is generally short. Telial horns dry and re-wet rapidly during spring.

If infection is successful in the rosaceous host, spermogonia develop in orange lesions or leaf spots, often on the upper surface of a leaf. Aecia also develop 

in orange lesions or leaf spots, but often on the lower surface of the leaf. These 

two rust stages can also develop on fruit, petioles, or young twigs; the locations 

where fruiting occurs vary by the rust species and hosts. Aeciospores (spores 

produced in the aecia) are wind-blown to the juniper host the same year from 

early summer to fall, depending on the rust species.

If infection is successful in the juniper host, galls, witches’ brooms, or stem 

swellings form. The telial horns grow from these symptomatic tissues the next 

spring or a year after the next spring.

Impact—The biggest impacts from these rusts are on the rosaceous hosts. 

Damages include growth loss and degraded fruit quantity and quality. Numerous 

infections, which can be common in wet years, can reduce rosaceous host vitality 

and result in attacks by other diseases or insects.

Although the strange appearance and bright color of the telial horns cause 

much concern, damage is usually minor. The aesthetic value is only impacted for about a week and soon fades. Rarely, numerous infections may reduce juniper host vigor, resulting in attacks by other diseases or insects. The 

Gymnosporangium species that cause stem swelling and occasionally the ones 

that cause branch knots can kill the tree host. The rust species that cause brooms 

can deform branches but seldom stems.

Management—It may be possible to reduce new infections in landscape settings 

by eliminating one of the hosts from an area. Rust infections will be minimized 

with a 2-mile separation of the hosts. Because basidiospores (produced on the 

juniper host) are more delicate and short-lived, eliminating juniper at a shorter 

distance will reduce new infections on rosaceous hosts.

Some varieties of both host groups are resistant to some of these rusts and 

can be used in areas where these rusts are a problem. Gymnosporangium rusts 

may also be host-specialized. For example, planting apple near juniper may 

not be a problem if the dominant rust in the area has hawthorn or Juneberry as 

its alternate host. However, host resistance has not been determined for some 

Gymnosporangium rusts.

Pruning the affected parts of the juniper can reduce losses, especially for rust 

species that cause stem swelling, branch knots, or brooms. Preventive fungicides 

are registered to control some of these rusts on both hosts. However, these treatments are costly and are usually not needed. If used, an appropriately registered 

fungicide for the individual state should be selected. Depending on the rust species, fungicides should be applied in spring to the rosaceous host and to the 

juniper host when aecia form on their rosaceous host.

References: 74, 104

R U S T S 
USDA Forest Service RMRS-GTR-241. 2010. 103

Western Gall Rust

Pine-to-pine rust of branches and stems

Pathogen—Western gall rust is caused by the fungus Peridermium (Endocronartium) harknessii.

Hosts—Western gall rust is a pine-to-pine rust with no alternate host. Most two 

and three-needle pines in the West are susceptible, but the disease is especially 

common on lodgepole and ponderosa pine.

Signs and Symptoms—The 

disease causes round to pearshaped woody swellings (galls) 

on branches or stems (fig. 185). 

Galls and cankers are most obvious in the spring and early summer when sporulation occurs. 

Flared target cankers, called 

“hip” cankers, are very common 

on larger lodgepole pine stems 

in the Rocky Mountain Region 

(fig. 186).

Disease Cycle—Peridermium harknessii

does not require an alternate host to complete 

its life cycle but spreads directly from pine 

to pine. White to orange pustules (aecia) full 

of yellow-orange spores (aeciospores) form 

in bark cracks on galls or, less commonly, 

at the edges of stem (hip) cankers in spring. 

Aecia rupture during moist weather and 

release spores that disperse in the wind. Most 

infections occur on the current year’s shoots 

or needles, which are highly susceptible until 

they reach 90% elongation. Infection rarely 

occurs through wounds on older shoots. Trunk 

infection occurs through smaller side branches 

or from branch galls near the main stem 

(figs. 187-188). Galls are produced at the point 

of infection and sporulation typically occurs 

the second or third year. Host cambial cells are 

stimulated by the pathogen to divide rapidly, 

causing spherical gall formation.

Impact—Western gall rust affects trees of 

all ages, causing growth loss, branch death, 

and deformity. Mortality is most common in 

Figure 185. Branch gall sporulating on lodgepole pine. 

Photo: James T. Blodgett, USDA Forest Service, Bugwood.org.

Figure 186. Hip canker on lodgepole pine. 

Photo: William Jacobi, Colorado State 

University, Bugwood.org.

R U S T S
104 USDA Forest Service RMRS-GTR-241. 2010.

seedlings and saplings because 

galls can quickly girdle the 

small stem. Branch galls 

typically only live a few years 

until the branch and the gall 

die. Mortality may result when 

numerous branch galls occur 

throughout the crown.

Hip cankers can severely 

deform larger trees, and wind 

snapping is common. They 

may persist for many years 

(100-200 years), but mortality 

is rare because they usually expand faster in a vertical direction. Stem cankers have very 

little impact on growth but can greatly 

reduce merchantable volume.

Mass infection tends to occur in wave 

years when conditions are particularly favorable. In the Rocky Mountain Region, 

wave years are somewhat rare, occuring 

approximately every 5-15 years. Vigorous 

trees are more prone to infection during 

this time because they have a larger proportion of susceptible foliage.

Management—Management of western 

gall rust is complicated because of the 

lag time between infection and symptom 

development. The following options 

may be useful for reducing disease impacts, but complete sanitation is difficult.

•  Sanitation. This option involves removing all trees with stem infections and 

carefully selecting leave trees that are disease-free or only have branch galls 

or stem cankers high in the crown. Trees with stem cankers can be hazardous 

in recreation areas and should be given priority for removal.

•  Pruning. Pruning infected branches provides little benefit to the tree because 

branches with galls usually die anyway. However, pruning may reduce inoculum levels in some areas.

•  Prepare for disease losses. Regenerate stands at increased stocking levels to 

compensate for future rust-caused mortality.

•  Manage species. Plant non-host species that are adapted to the site.

•  Destroy and regenerate. It may be necessary to start over in areas where infection is severe and managing species is not an option.

References: 122, 130, 133, 177

Figure 187. Stem canker that was initiated by a branch gall 

severely deforms the stem at the point of the canker. Photo: 

James T. Blodgett, USDA Forest Service.

Figure 188. Stem gall on ponderosa pine. Photo: 

James T. Blodgett, USDA Forest Service.

R U S T S 
USDA Forest Service RMRS-GTR-241. 2010. 105

White Pine Blister Rust

Non-native, invasive rust of five-needle pines

Pathogen—White pine blister rust is caused by Cronartium ribicola, an Asian 

fungus that was introduced into North America from Europe in the early 1900s. 

The disease continues to spread to five-needle pines throughout North America.

Hosts—All North American white pines (members of subgenus Strobus) are 

susceptible. In the Rocky Mountain Region, hosts include limber pine, whitebark 

pine, Rocky Mountain bristlecone pine, and southwestern white pine. Alternate 

hosts include currants and gooseberries in the genus Ribes and, occasionally, 

species of Pedicularis and Castilleja.

Signs and Symptoms—Signs of white pine blister rust are visible on Ribes spp. 

in the summer and fall (uredinia and later telia) on the undersurface of leaves. 

Symptoms such as leaf spots and premature defoliation occur on Ribes spp. but 

otherwise, the disease causes little damage.

Bright red, recently killed 

“flagged branches” are the 

most obvious symptom of 

white pine blister rust from a 

distance (fig. 189). However, 

other agents, such as dwarf 

mistletoe and twig beetles, 

can cause flagging. The 

first detectable symptoms 

on pines are yellow needle 

spots. Diamond-shaped stem 

cankers are often swollen and 

resinous and sometimes have 

an orange margin. Cankers 

are most obvious in spring 

and early summer when pustules (aecia) full of orange aeciospores rupture through the 

bark. The cankered bark becomes roughened and dark as it dies following sporulation, but the fungus continues to expand into adjacent healthy tissue. Rodents 

often gnaw the bark off around cankers (fig. 190)

Disease Cycle—White pine blister rust cannot spread from pine to pine but is 

transmitted to pines from basidiospores produced on infected Ribes spp. leaves. 

Basidiospores are short-lived and primarily disperse short distances (usually less 

than 1,000 ft [300 m] but possibly a few miles). Pines are infected through needle 

stomata in the late summer and early fall. Following infection, the fungus grows 

down the needle and into the bark where a perennial canker forms (fig. 191). 

In spring to early summer 2-4 years later, spermogonia (pycnia) form within 

Figure 189. Flagged branches on limber pine. Photo: William 

Jacobi, Colorado State University.

R U S T S
106 USDA Forest Service RMRS-GTR-241. 2010.

the canker (fig. 192), 

and aecia are produced in the same 

tissue the following 

year. Aeciospores 

can travel long 

distances (potentially 

hundreds of miles) 

in the wind to infect 

susceptible Ribes spp. 

Urediniospores are 

produced in orange 

pustules (uredinia) 

on the underside of 

infected Ribes spp. 

leaves throughout 

the summer. These 

spores re-infect other 

Ribes spp. leaves; they 

cannot infect pines. 

In late summer, hairlike columns (telia) 

with teliospores are 

produced on infected 

Ribes spp. leaves (fig. 193). Teliospores form basidiospores that later infect 

pines, completing the cycle.

Impact—White pine blister rust affects trees of all ages and sizes and can 

essentially eliminate white pines from certain ecosystems. Branch and stem 

cankers eventually lead to branch death, top-kill, or whole tree mortality. The 

probability of branch infections reaching the bole declines with distance, and 

branch infections more than 2 ft (61 cm) from the trunk will usually kill the 

Figure 193. Ribes spp. leaf with infection that is producing telia. Photo: Kelly S. Burns, USDA Forest Service.

Figure 192. Infected limber pine seedling with 

needle spots and spermogonia. Photo: Isabelle 

Lebouce, Dorena Genetic Resource Center, USDA 

Forest Service.

Figure 191. White pine blister rust 

stem canker sporulating on limber 

pine. Photo: Kelly S. Burns, USDA 

Forest Service.

Figure 190. Canker with orange 

canker margin and rodent feeding. 

Photo: William Jacobi, Colorado 

State University.

R U S T S
USDA Forest Service RMRS-GTR-241. 2010. 107

branch before reaching the main stem. Small trees are especially susceptible 

because most infections occur close to the main stem, quickly girdling the tree. 

In the Rocky Mountain Region, mortality is common on larger trees without a 

stem infection when numerous branch infections occur throughout the crown. 

White pine blister rust may severely impact reproductive potential by weakening 

and killing cone-bearing branches. Infected trees may become susceptible to 

other damaging agents such as bark beetles.

Management—The most promising strategy for managing white pine blister 

rust is to increase the frequency of rust-resistant individuals across the landscape. 

The conventional approach is to develop a rust resistance breeding program in 

which seed is collected from putatively resistant trees, progeny are screened 

for resistance, and a seed orchard is established to supply seed for restoration 

and reforestation. An alternate strategy for areas not yet challenged by rust is to 

increase age class diversity by establishing young cohorts within mature stands. 

Upon invasion, natural selection would occur more quickly in the young cohort 

than in the mature cohort, ultimately accelerating the development of a more 

resistant stand.

Management strategies such as pruning and Ribes spp. removal to decrease 

inoculum potential may be used to reduce infections and prolong the life of existing trees. Ribes spp. removal may be feasible in certain high-value areas (for 

example, campgrounds) where all plants within 0.6 miles (1 km) of the area to 

be protected can be located and completely removed. Sanitation pruning is used 

to remove the infection before it reaches the main stem.

Burns and others (ref. 21) present detailed management options for white pine 

blister rust in the Rocky Mountain Region.

References: 21, 25, 87, 116, 119, 135, 144, 151, 152

Introduction to Foliage Diseases

Spots, blights, and casts

Diseases of tree foliage can be broken roughly into three groups: hardwood 

foliage diseases, needle casts, and needle blights (figs. 194-197). Hardwood foliage diseases are diverse.

General Features—General features of foliage diseases:

•Foliage pathogens penetrate either directl

through the cuticle and epidermis or enter 

through stomates. Wounds are not normally 

involved.

•Hardwood foliage diseases usually hav

minor impact unless they cause repeated 

defoliation. Figure 194. Example of a foliage disease: 

early stage of inkspot of aspen. Photo: Jim 

Worrall, USDA Forest Service.

F O L I A G E D I S E A S E S
108 USDA Forest Service RMRS-GTR-241. 2010.

•Conifer foliage diseases (table 10) can significantly affect growth. Conifers canno

refoliate like hardwoods; conifers depend 

on several years of foliage. A defoliated 

hardwood will have a full complement of 

leaves the next year if not the same year.

•Foliage pathogens are usually damagin

only under certain circumstances such as 

in trees that are off-site or out of native 

range; pure, dense stands; small trees, usually seedling to pole size; and particular 

weather patterns.

•Most overwinter in foliage, either on th

tree (conifers) or on the ground (mostly hardwoods). Some hardwood foliage 

pathogens also grow into the twig, causing twig cankers, and can overwinter 

there.

•Although leaf wetness is required for spore production, germination, and infection, the required duration is only a few days or less. So foliage disease

can be successful even during dry years if sufficient moisture occurs at the 

right time. Some (e.g., Davisomycella needle cast) are even more damaging 

during hot, dry conditions, if infection occurred earlier in a wet period.

Figure 195. Marssonina blight of aspen. 

Photo: Jim Worrall, USDA Forest Service.

Figure 197. Unusually severe damage and mortality due to red band needle blight of lodgepole 

pine in British Columbia. Photo: Jim Worrall, 

USDA Forest Service.

Figure 196. Discoloration and reduced needle retention due to needle cast of ponderosa pine. Photo: Jim 

Worrall, USDA Forest Service.

F O L I A G E D I S E A S E S 
USDA Forest Service RMRS-GTR-241. 2010. 109

Hardwood Diseases—Hardwood foliage diseases typically overwinter in 

foliage on the ground. The primary inoculum in spring is ascospores that are shot 

into the air from the foliage. Some produce secondary inoculum from lesions 

on current-year leaves for further infections during the year. Others grow down 

the petiole, invade twigs, and cause cankers there in which they can overwinter.

Needle Casts—Characteristics of needle casts:

•Needles are often lost, or cast, prematurely. However, there are some needl

casts (for instance, on larch) where the needles are kept longer than normal.

•Needle casts have only one infection period per yea

•Most infections are caused by a group of fungi in the family Rhytismataceae

order Rhytismatales, but some pathogens are in other groups of the phylum 

Ascomycota.

Table 10. Conifer foliage pathogens (other than rusts) and their hosts documented in Region 2 and nearby 

areas. Most diseases do not have well-established names, so they are not provided. Alternate names 

are provided where confusion is likely. From refs. 9, 44, 164, and 165. DF = Douglas-fir, F = true firs, LP

= limber pine, LPP = lodgepole pine, P = pine, PiP = pinyon, PP = ponderosa pine, S = spruces, SAF = 

subalpine fir, WF = white fir, WP = white pines.

Pathogen	 Host

Bifusella linearis LP

Bifusella pini PiP

Bifusella saccata PiP, LP

Cyclaneusma niveum LPP, PP

Davisomycella medusa LPP, PP

Davisomycella ponderosae PP

Dothistroma pini P 

Dothistroma septosporum (Mycosphaerella pini) P 

Elytroderma deformans PiP, PP

Hemiphacidium planum LP, PP

Herpotrichia juniperi (H. nigra) P, S, F

Isthmiella (Bifusella) abietis SAF

Isthmiella (Bifusella) crepidiformis F, S

Lecanosticta (Coryneum) cinerea LPP, PP

Lirula abietis-concoloris F

Lirula macrospora (Lophodermium filiforme) S

Lophodermella (Hypodermella) arcuata LP

Lophodermella cerina PP

Lophodermella concolor LPP

Lophodermella montivaga LPP

Lophomerum (Lophodermium) autumnale WF

Lophodermium decorum WF

Lophodermium juniperinum Rocky Mountain juniper

Lophodermium nitens WP

Lophodermium piceae S

Neopeckia coulteri P, S

Phaeocryptopus gaeumannii DF

Phaeocryptopus nudus (Adelopus balsamicola) SAF

Rhabdocline pseudotsugae DF

Rhizosphaera kalkhoffii S

Virgella (Hypoderma) robusta WF

F O L I A G E D I S E A S E S
110 USDA Forest Service RMRS-GTR-241. 2010.

•Needle-cast pathogens in the family Rhytismataceae usually have specialize

fruiting structures called hysterothecia. Hysterothecia may be elongated and 

have a covering that develops a longitudinal slit in the middle. Special cells 

at the outer edges absorb water under wet conditions and force the slit open 

to expose the spore-producing surface. When the weather is dry, they close 

again, functioning as automatic doors.

•Ascospores are forcibly shot into the air. They are usually long and narrow

which may increase the likelihood of hitting a needle. They have a sticky 

sheath that helps them stick to needles.

•Most needle casts infect young, current-year needles, and their sporulation i

synchronized with needle elongation. Some infect mostly older needles, but 

they are less serious diseases and verge on the saprobic species.

•Symptoms usually do not appear during the year of infection. If they do, it i

not until autumn.

Needle Blights—The term “blight” is used various ways, and needle blights 

vary considerably. In general, however, they can infect multiple times during 

the year, whenever temperature and moisture are favorable. They usually have 

two kinds of spores: sexual spores that cause the initial infection and asexual 

spores that can propagate the epidemic during suitable weather. Dead needles 

may remain attached to twigs instead of being prematurely cast.

References: 9, 44, 164, 165

Anthracnose

Common foliage disease of deciduous trees

Pathogen—Anthracnose diseases are caused 

by a group of morphologically similar fungi 

that produce cushion-shaped fruiting structures 

called acervuli (fig. 198). Many of the fungi 

that cause anthracnose diseases are known for 

their asexual stage (conidial), but most also have 

sexual stages. Taxonomy is continually being 

updated, so scientific names can be confusing. 

A list of common anthracnose diseases in the 

Rocky Mountain Region and their hosts is 

provided in table 11.

Hosts—A variety of deciduous trees are susceptible to anthracnose diseases, including ash, 

basswood, elm, maple, oak, sycamore, and walnut. These diseases are common on shade trees. 

Marssonina blight of aspen (see the Marssonina 

Leaf Blight entry in this guide for more information) is an anthracnose-type disease. The fungi 

Figure 198. Apiognomonia quercina

acervuli on the mid-vein of an oak leaf. 

Photo: Great Plains Agriculture Council.

F O L I A G E D I S E A S E S 
USDA Forest Service RMRS-GTR-241. 2010. 111

that cause anthracnose diseases are host-specific such that one particular fungus 

can generally only parasitize one host genus. For example, Apiognomonia errabunda causes anthracnose 

only on species of ash, and 

A. quercina causes anthracnose only on oaks.

Signs and Symptoms—

Symptoms of anthracnose 

vary considerably from 

host to host. Most anthracnose fungi cause blotchy, 

necrotic spots on leaves 

associated with veins and 

sometimes cause leaf distortion and premature defoliation (figs. 199-201). 

On oak and sycamore, the 

disease also impacts twigs, 

shoots, buds, and fruits and 

occasionally causes stem 

cankers and brooming. 

When leaves are infected in 

early spring, they often turn 

black and may be confused 

with frost-damaged leaves.

Disease Cycle—These 

fungi overwinter on infected leaves and branches 

on the tree or the ground. 

Table 11. Common anthracnose pathogens in the Region by host and part of tree impacted (ref. 163).

Host	 Pathogen	 Part of tree impacted

Ash (especially green) Apiognomonia errabunda Leaves and twigs

conidial state = Discula spp.

Basswood Apiognomonia tiliae Leaves and twigs

Elm Stegophora ulmea Leaves

conidial state = Gloeosporium ulmicolum

Maple Kabatiella apocrypta Leaves

conidial state unknown

Oak (especially white) Apiognomonia quercina Leaves, twigs, shoots, and buds

conidial state = Discula quercina

Sycamore and Apiognomonia veneta Leaves, twigs, shoots, and buds

London plane-tree conidial state = Discula spp.

Walnut Gnomonia leptostyla Leaves, twigs, and nuts

conidial state = Marssoniella juglandis

Figure 199. Crown symptoms caused by Apiognomonia veneta

on sycamore. Photo: William Jacobi, Colorado State University, 

Bugwood.org.

F O L I A G E D I S E A S E S
112 USDA Forest Service RMRS-GTR-241. 2010.

In spring, spores are produced during periods of cool, moist weather. These may 

be conidia or ascospores, but conidia appear to play the most important role in 

dissemination and infection. Spores are spread by wind and splashing rains and 

infect buds, twigs, and newly emerging leaves if conditions are conducive. Lesions eventually form and symptoms become visible in mid to late summer. In 

some species (e.g., ash and walnut), secondary spores (called “summer spores”) 

are produced on primary lesions during periods of cool, wet weather, allowing 

the diseases to intensify throughout the summer.

Impact—Disease severity appears to be associated with prevailing weather. Periods of cool, wet weather are conducive to spore production, spread, infection, 

and disease development, and warm temperatures (above 55 °F) inhibit the fungus. In the Rocky Mountain Region, damage is usually minor and vigorous trees 

can recover fairly rapidly. Trees weakened by other factors such as environmental stress, nutritional imbalance, or insects and diseases may be more susceptible 

to the diseases. Anthracnose can also stress trees, making them more susceptible 

to other diseases or insects.

Management—Anthracnose damage is generally insignificant in the Region, 

but management may be necessary in certain high-value areas. The following are 

control strategies that can be used to reduce disease severity:

•Plant resistant or tolerant species in areas where the disease is a problem o

where air circulation is poor. Species vary in their susceptibility to anthracnose 

diseases. For example, true London plane is more resistant than American 

sycamore, and the red oaks tend to be more tolerant than the white oaks.

•Maintain proper spacing between trees to increase air circulation and improv

tree vigor.

•Proper fertilization may increase disease toleranc

•Raking infected leaves and litter in the fall may help reduce infections in th

spring.

•Effective protective fungicides labeled for anthracnose control are availabl

References: 15, 159, 163

Figure 201. Apiognomonia quercina-infected oak leaves with 

irregular necrotic area associated with veins. Photo: Joseph 

O’Brien, USDA Forest Service, Bugwood.org.

Figure 200. Ash leaves infected with 

Apiognomonia errabunda have irregularlyshaped blotches and are distorted. Photo: 

Joseph O’Brien, USDA Forest Service, 

Bugwood.org.

F O L I A G E D I S E A S E S 
USDA Forest Service RMRS-GTR-241. 2010. 113

Brown Felt Blight

Snow mold of conifers (also called bear wipe)

Pathogen—Two fungi cause brown felt blight: Neopeckia coulteri and 

Herpotrichia juniperi. These fungi are commonly referred to as snow molds.

Hosts—Almost all high-elevation conifers are susceptible to brown felt blight. 

Neopeckia coulteri generally infects only pines. Herpotrichia juniperi infects 

a variety of conifers, including Engelmann spruce, Colorado blue spruce, 

subalpine fir, Douglas-fir, lodgepole pine, and whitebark pine.

Signs and Symptoms—The most obvious sign of snow mold is the felt-like 

mat of mycelium that grows on infected twigs and branches (figs. 202-203). 

Under careful inspection, black, globose fruiting bodies may be visible on 

the mycelium. The two fungi can be differentiated based on host preference 

or by microscopic examination of spores. The latter is necessary for specific 

identification in pines.

Disease Cycle—The fungi develop beneath the snow and produce thick, gray, 

felt-like mats of mycelium that smother needles, branches, and twigs. Neopeckia

coulteri is able to penetrate living needles, while Herpotrichia juniperi enters 

needles and twigs after they are dead. As snow melts, mycelium turns gray to 

black, and fungal growth ceases. Spherical, black fruiting bodies (perithecia) 

may be visible protruding from mycelium during the second summer. Spread 

occurs when needles come into contact with infected litter or spores under snow. 

The disease is favored by deep, long-lasting snow packs.

Impact—Damage from brown felt 

blight may result in branch death 

and growth loss. Severe infections 

Figure 202. Close-up of brown felt blight on 

a lodgepole pine branch. Photo: Kelly Burns, 

USDA Forest Service.

Figure 203. Snow mold on Engelmann spruce. Photo: 

Whitney Cranshaw, Colorado State University, Bugwood.org.

F O L I A G E D I S E A S E S
114 USDA Forest Service RMRS-GTR-241. 2010.

on seedlings and saplings may lead to mortality, but the disease has little effect 

on larger trees.

Management—Brown felt blight usually does not warrant management. Possible management options for high-value areas include installing snow breaks 

to divert snow pack from target areas and removing infected branches to reduce 

inoculum.

References: 71, 155, 157, 159

Cercospora Blight of Junipers

Serious disease in shelterbelts

Pathogen—Cercospora blight of junipers is caused by the fungus Pseudocercospora juniperi (Cercospora sequoiae var. juniperi).

Hosts—Many members of the Cupressaceae family are, but Rocky Mountain 

juniper and eastern redcedar are the most severely impacted in the Great Plains.

Signs and Symptoms—Foliage of the inner branches of the lower crown becomes bronzed, then necrotic, and eventually sheds, leaving the inner crown 

devoid of foliage (fig. 204). The extremities remain green. Small, gray, fuzzy 

asexual fruiting bodies (sporodochia) may be visible with a hand lens on dead 

needles (fig. 205). Defoliation progresses from the lower portion of inner crown 

outward and upward.

Disease Cycle—Sporodochia are produced on infected foliage in September. 

Sporulation occurs continuously from April to October during periods of warm, 

wet weather. Conidiospores disperse short distances (<6 ft [1.8 m]) by means 

of rain splash. Free moisture is required for sporulation and infection. Infection 

occurs through stomata or directly through the cuticle. Infection is initiated on 

current and previous years’ juvenile needles (sharply pointed young needles) 

and previous years’ spur leaves (scale-like mature leaves) of the inner crown and 

eventually spreads to the entire branchlet. Whip foliage (elongated shoot tips) 

appears to be resistant. Infection usually occurs in June and July, and symptoms 

develop about 2-3 weeks after infection. The fungus overwinters in infected needles on living trees. The sexual state of P. juniperi is not known.

Impact—Cercospora needle blight can cause devastation and mortality to juniper 

in shelterbelts and other established plantings, particularly with repeated years of 

infection. Impacts are more severe on Rocky Mountain juniper than on eastern 

redcedar and in areas or years with significant moisture and humidity.

Management—Several management strategies may be used to control Cercospora needle blight:

•Do not plant Rocky Mountain juniper and eastern redcedar in areas where th

disease is a problem.

•Maintain wide spacing between trees to promote air circulation and reduc

humidity.

F O L I A G E D I S E A S E S 
USDA Forest Service RMRS-GTR-241. 2010. 115

•Protective fungicides labeled for Cercospora control (e.g., Mancozeb an

Bordeaux mixture) can be used to protect susceptible trees. Two applications 

are usually necessary, depending on the type of foliage present (juvenile, 

spur, or both).

References: 159, 172

Davisomycella and Lophodermella 

Needle Casts

Occasionally epidemic diseases of pines

Pathogen—Davisomycella is a genus of needle-cast pathogens in the family 

Rhytismataceae, order Rhytismatales. In the Rocky Mountain Region, D.

(Lophodermium) ponderosae infects ponderosa pine; D. (Hypodermella) 

medusa infects both ponderosa and lodgepole pines.

The related genus Lophodermella also contains several pathogens that are 

occasionally important in this Region. They were first described in the genus 

Hypodermella and occur in older literature under that name. Lophodermella

arcuata occurs on limber pine, L. cerina occurs on ponderosa pine, and L. concolor and L. montivaga occur on lodgepole pine.

Figure 205. Small, gray, fuzzy fruiting bodies of Cercospora on infected leaves may be visible with a hand 

lens. Photo: Ned Tisserat, Colorado State University.

Figure 204. Inner branches of the lower crown of Rocky Mountain junipers infected with Cercospora needle 

blight are defoliated while the extremities remain green. Photo: Ned Tisserat, Colorado State University.

F O L I A G E D I S E A S E S
116 USDA Forest Service RMRS-GTR-241. 2010.

Hosts—Ponderosa, lodgepole, and limber pines can be infected by different 

species and combinations of species of Davisomycella and Lophodermella (see 

“Pathogen” above).

Signs and Symptoms—These fungi cause browning and premature casting of 

needles. Symptoms of Davisomycella species may appear on needles of any age 

class except current-year needles (fig. 206). Whole needles or portions may be 

affected. After symptoms appear, fruiting structures (hysterothecia) may begin 

to form. In D. ponderosae, they begin to form in spring and mature in summer. Hysterothecia are dark brown and elliptical to linear; they may be partly 

fused end-to-end, forming long, sometimes sinuous or forked lines (figs. 207-

209). They open by a longitudinal slit and release spores into the air during wet 

weather. Hysterothecia of D. medusa are also dark and raised but are considerably shorter. They occur in greenish straw-colored areas of the needle. As older 

needles are lost and only current needles are present, twigs may take on a lion’s 

tail appearance.

With Lophodermella concolor, needles infected the previous year die and 

turn reddish brown by spring, becoming straw-colored during the summer. 

Hysterothecia are colorless (the specific epithet means that they blend with their 

background). They are usually evident as shallow, elliptical depressions up to 

1/25 inch (1 mm) long.

Disease Cycle—The disease cycles of Davisomycella needle casts are somewhat 

unusual. Primarily current-year needles are infected after hysterothecia mature 

in late summer. As with most foliage diseases, wet weather is required for spore 

dispersal and infection. Symptoms may not appear the next year; in fact, the 

needles may appear healthy and may be retained for the normal time. This is 

a latent infection. Evidence suggests that symptom development is triggered 

by drought stress. When drought occurs, any age classes of needles that were 

infected (i.e., had suitable weather conditions and inoculum during their first 

year) may develop symptoms.

Disease cycles of Lophodermella needle casts vary somewhat (table 12). 

Lophodermella concolor

and L. montivaga have a 

1-year cycle. Infection occurs in current-year needles 

in early or late summer, respectively. No symptoms 

develop until late winter. 

By spring, infected needles 

are dead (figs. 210-211). 

Hysterothecia develop in 

them and mature in early 

or late summer, after which 

needles are cast. Figure 206. Davisomycella ponderosae: discoloration and needle 

loss to large, open-grown ponderosa pines. Photo: Jim Worrall, 

USDA Forest Service.

F O L I A G E D I S E A S E S 
USDA Forest Service RMRS-GTR-241. 2010. 117

Impact—Epidemics of needle casts are sporadic. Damage may be severe and 

widespread in some years, while infections may be hard to find in others. Nearby 

trees may contrast greatly in infection levels, suggesting genetic variation in 

resistance. Often, damage is most severe on smaller trees, in the lower crown 

of larger trees, and in dense stands. However, damage is sometimes severe 

throughout the crown of large, open trees. Growth losses have been documented 

for needle casts and can be significant with successive years of infection. During 

an epidemic of Davisomycella medusa associated with drought, 10-year growth 

Figure 207.

Davisomycella ponderosae: immature 

hysterothecium in 

late May. Photo: 

Jim Worrall, USDA 

Forest Service. Figure 208. 

Davisomycella ponderosae: mature, 

open hysterothecia 

in early October following wet weather. 

Photo: Jim Worrall, 

USDA Forest 

Service.

Figure 209.

Davisomycella 

ponderosae: 

another view of 

mature, open hysterothecia in early 

October following 

wet weather. 

Photo: Jim Worrall, 

USDA Forest 

Service.

Table 12. Davisomycella and Lophodermella species that cause needle cast in the Region (includes 

hosts and phenology).

Pathogen	 Host	 Dispersal period	 Symptoms appear

Davisomycella medusa Lodgepole, July-September Following year to not at all

ponderosa

Davisomycella ponderosae Ponderosa July-October Following year to not at all

Lophodermella arcuata Limber July-August Following spring

Lophodermella cerina Ponderosa Spring Beginning that autumn

Lophodermella concolor Lodgepole June-July Following spring

Lophodermella montivaga Lodgepole July-September Beginning that autumn

F O L I A G E D I S E A S E S
118 USDA Forest Service RMRS-GTR-241. 2010.

Figure 210. Lophodermella montivaga: discoloration 

of sapling lodgepole pine. Photo: Leanne Egeland, 

USDA Forest Service.

Figure 211. Lophodermella montivaga: single 

shoot showing symptoms and fruiting on last 

year’s needles in late June. Photo: Leanne 

Egeland, USDA Forest Service.

of infected trees was reduced by 69%, while growth of uninfected trees was 

reduced by only 29%. As noted above, Davisomycella species may be most 

damaging when drought follows a year that is favorable for infection.

Management—Direct management approaches are not known for needle casts 

and are generally not needed. Growth impacts are usually localized and temporary. 

Thinning may be helpful where epidemics recur frequently enough to cause concern 

in high-value forests. Thinning would speed drying of the foliage, thereby reducing 

the periods available for infection. Also, growth increases associated with thinning 

may compensate for reductions caused by the disease.

References: 158, 166, 178, 198

Elytroderma Needle Cast

Perennial, compact brooms in pines

Pathogen—Elytroderma needle cast is caused by the fungus Elytroderma

deformans.

Hosts—The disease is most severe on ponderosa pine, but lodgepole pine, jack 

pine, and pinyon pine are also susceptible.

Signs and Symptoms—Perennial infections on vigorous branches often cause 

small to large compact brooms with upward turning branches and many dead, 

straw-colored needles. This is the only needle cast that is perennial in host twigs.

The fungus invades twigs and branches, causing stunting of needles, reddened foliage, defoliation, and conspicuous brooming (figs. 212-213). Brown, 

F O L I A G E D I S E A S E S 
USDA Forest Service RMRS-GTR-241. 2010. 119

Figure 212. Large, compact witches’ broom typical 

of Elytroderma disease on ponderosa pine. Photo: 

Thomas R. Erikson, USDA Forest Service.

Figure 213. A perennial infection on ponderosa pine 

with upward-turning branches. Photo: USDA Forest 

Service Archive, Bugwood.org.

resinous lesions often develop in the phloem of infected twigs (fig. 214). Infected 

needles are red-brown, becoming pale over the summer, but the basal portion 

(1/4-3/8 inch [6-10 mm]) remains green. Fruiting bodies appear near the base 

of dead and dying needles in the spring and are thin, elongated (approximately 

1/2 inch [13 mm] long), and dull black (fig. 215). Brooms are compact, globose, 

and contain discolored needles, most of which are shed by fall.

Dwarf mistletoe brooms on pines sometimes look similar to Elytroderma 

brooms. However, dwarf mistletoe plants or remnant basal cups should be visible on dwarf mistletoe infections and Elytroderma brooms are more compact 

and globose.

Disease Cycle—Airborne spores infect needles 

in late summer and fall and occasionally in spring, 

following rainstorms. When free moisture is available, 

Figure 214. Necrotic, resinous flecks are often visible in the inner bark

of infected twigs. Photo: John Schwandt, USDA Forest Service, Bugwood.

org.

Figure 215. Black, elongated 

fruiting bodies appear in the 

spring near the base of needles. 

Photo: John Schwandt, USDA 

Forest Service, Bugwood.org.

F O L I A G E D I S E A S E S
120 USDA Forest Service RMRS-GTR-241. 2010.

spores germinate and penetrate young needles. The fungus grows within the 

needle tissue and eventually invades twigs and becomes systemic within the 

host, infecting new shoots and needles each year. The following spring, infected 

needles turn reddish brown, and by early summer, fungal fruiting bodies develop 

on dead needles if conditions are conducive. These fruiting bodies, which appear 

as black, elongated lines or slits, split open during wet weather and release spores.

Impact—Impacts are greatest in areas with moist environmental conditions 

such as drainages. Dry spring weather tends to suppress spore production and 

infection. Infection is favored by dense stand conditions and injury is greatest 

on poor crowns. Elytroderma needle cast is more common and damaging in the 

northern Rockies than in the southern and central portions of the range. The 

disease is occasionally found in the Colorado Front Range, but damage in the 

Rocky Mountain Region is greatest on ponderosa pine in the Black Hills of 

South Dakota. Infection results in loss of the previous year’s needles and death 

of branch cambium. Stem infections can severely damage and deform small 

trees. When infection is severe, growth loss occurs and entire tree tops may be 

deformed. Severely infected trees may be weakened, predisposing them to attack 

by bark beetles or other damages, or killed outright.

Management—In young stands, damage can be reduced by maintaining good 

spacing through thinning. Selectively remove moderately and severely infected 

trees and trees with infections high in the crown. Accelerated logging in heavily 

infested mature stands not only serves to salvage valuable timber but also 

prevents the establishment of secondary pests. Remove all trees with more than 

one-quarter of their twigs killed.

References: 26, 159

Ink Spot

Dark, raised blotches or shotholes in aspen leaves

Pathogen—Ink spot is caused by the fungus Ciborinia whetzelii.

Hosts—Aspen is the predominant host, although other poplars are also 

susceptible.

Signs and Symptoms—Ink spot is characterized by brown to black spots on 

blighted leaves (figs. 216-218). Early summer symptoms of ink spot can look like 

leafminer insect damage with concentric zones that are light and dark. Reddish 

brown blotches become visible on infected leaves within a few weeks, and ink 

spots (sclerotia) develop several weeks later. Infected leaves quickly turn brown, 

and sclerotia begin to drop out, leaving shotholes in the dead leaves. Ciborinia

whetzelii forms well-defined, circular, stalked fruiting structures called apothecia 

that produce and release spores.

Disease Cycle—Ciborinia whetzelii overwinters as sclerotia (hardened masses 

of fungal hyphae) in forest litter. In the spring, sclerotia produce fruiting bodies 

F O L I A G E D I S E A S E S 
USDA Forest Service RMRS-GTR-241. 2010. 121

(apothecia) that release wind- and raindisseminated ascospores that infect developing leaves. Infected leaves develop sclerotia (ink spots) that eventually drop out. 

Infected leaves begin to die midsummer, 

but defoliation may not take place until autumn. 

Cool, moist weather in the spring is conducive 

to disease spread.

Impact—Ink spot is more severe on small 

trees and in the lower crowns of larger trees. 

The incidence of the disease is greatest in 

dense stands. Occasionally, this disease 

is responsible for 25-100% defoliation in 

localized areas. This disease rarely causes 

long-term damage or mortality. As with many 

aspen diseases, some clones appear to be more 

susceptible than others.

Management—Spring infections may be reduced if infected leaves are raked up 

and removed the previous fall. Increasing the spacing between trees may create 

a less favorable microclimate for disease spread and infection. Fungicides can 

be used to prevent infection, but they must be applied before infection occurs.

References: 79, 159

Marssonina Leaf Blight

Midsummer spots and defoliation of aspen

Pathogen—Marssonina leaf blight is caused by at least two fungi in our area: 

Marssonina brunnea (sexual stage Drepanopeziza tremulae) and Marssonina

Figure 216. Early season ink spot on aspen leaf. 

Photo: William Jacobi, Colorado State University, 

Bugwood.org.

Figure 217. Brown, immature sclerotia on 

blighted leaves. Photo: Jim Worrall, USDA 

Forest Service.

Figure 218. Black ink spots (sclerotia) 

on aspen leaf. Photo: Jim Worrall, USDA 

Forest Service.

F O L I A G E D I S E A S E S
122 USDA Forest Service RMRS-GTR-241. 2010.

populi (Drepanopeziza

populorum). The disease is 

also commonly referred to 

as black leaf spot.

Hosts—Aspen is the predominant host, but narrowleaf 

cottonwood and other poplars 

are also susceptible.

Signs and Symptoms—

Small, brownish spots 

appear on infected leaves 

as early as the end of leaf 

expansion but are especially 

noticeable in July and August (figs. 219-222). 

Marssonina brunnea often causes small spots, 

initially about 1/25 inch (1 mm) in diameter, 

while M. populi causes larger spots. The spots 

later enlarge to various sizes and shapes, turn 

brownish black, and often have a yellow margin 

and a white center. Spots may coalesce to 

form vein-limited, necrotic blotches. Acervuli 

(asexual fruitbodies) appear as tiny, ring-like 

blisters in the center of spots when conditions 

are moist. Infected leaves often fall during the 

summer. This may be more common when 

infections are on or near the petiole, even if the 

rest of the leaf is relatively healthy.

Symptoms intensify throughout the summer and 

progress upward in the crown as the season advances. From a distance, canopies of heavily infested 

stands often have a bronze or brown hue. Crowns 

may appear thin from defoliation, and infected 

leaves are often seen on the forest floor and roads 

(figs. 223-224). Fall colors are usually less intense 

in such stands.

Disease Cycle—Primary infection occurs in spring 

when spores produced on twig lesions or infected 

fallen leaves infect developing leaves. Secondary infection occurs throughout the summer when 

spores from fruiting bodies on primary lesions are released with wet weather. 

Disease incidence is closely associated with warm, wet conditions. The pathogens overwinter as tiny stromata in fallen leaves and in twig lesions.

Impact—Marssonina leaf blight is the most common leaf disease of aspen in the 

Rocky Mountain Region. Because of the dependence of the disease on weather, 

Figure 219. Marssonina blight on aspen. Photo: William Jacobi, 

Colorado State University, Bugwood.org.

Figure 220. Close-up of an infected 

aspen leaf, showing irregularly-shaped 

spots with yellow margins, probably 

Marssonina populi. Photo: Jim Worrall, 

USDA Forest Service.

Figure 221. Early stage of infection 

by Marssonina brunnea. Photo: Jim 

Worrall, USDA Forest Service.

F O L I A G E D I S E A S E S 
USDA Forest Service RMRS-GTR-241. 2010. 123

it may be severe one year and difficult to find the next. Infected leaves are often 

stunted and shed prematurely. Defoliation can be severe, with nearly complete 

defoliation by early August. Trees may refoliate in late summer and early fall, 

but twig dieback often occurs the following winter because late shoots lack sufficient cold hardiness. Mortality is rare unless the outbreak occurs over consecutive years or is combined with other stresses. However, damaged trees may be 

more susceptible to other damages, and wood production is reduced for 1 or 

more years following an epidemic. Clonal variation in suceptibility may be visible during severe epidemics.

Management—Because the disease rarely leads to mortality and management 

approaches are often impractical, management of Marssonina leaf blight is not 

often contemplated in forests managed for multiple objectives. In high-value 

sites, the most effective way to manage the disease is by planting or managing 

for resistant or tolerant clones. Removing and destroying diseased material from 

trees and the ground may help reduce infections. Increasing space between trees 

may create a less favorable microclimate for spread and infection. Fungicides 

can be used to prevent infection, but they must be applied at bud break before 

infection occurs.

References: 140, 158

Figure 224. Fresh leaves on the 

ground in mid-July due to infection 

by Marssonina brunnea. Photo: Jim 

Worrall, USDA Forest Service.

Figure 223. Defoliation caused by Marssonina brunnea

in early July. Photo: Suzanne Bethers, USDA Forest 

Service.

Figure 222. Close-up of leaf spots on aspen, 

showing vein-limited margins and tiny blisters 

(acervuli) in the center. Photo: Jim Worrall, 

USDA Forest Service.

F O L I A G E D I S E A S E S
124 USDA Forest Service RMRS-GTR-241. 2010.

Melampsora Rusts

Common leaf rusts of poplars and willows

Pathogen—Several Melampsora species occur in the Rocky Mountain Region, 

but the most common and important is M. medusae (= M. albertensis), the fungus 

that causes conifer-aspen leaf rust. Less common species include M. occidentalis, 

which causes conifer-cottonwood rust, and M. epitea, which causes willow rust.

Hosts—Aspen, cottonwood, and willow are hardwood (telial) hosts. Aecial 

hosts include Douglas-fir, lodgepole pine, ponderosa pines, true firs, Saxifraga

species, and Ribes species. Host relationships are described in table 13.

Signs and Symptoms—Melampsora rusts are the common leaf rusts of poplar 

and willow. The most obvious indicators are the yellow leaf spots, which eventually become necrotic, and the orange powdery pustules on the underside of 

poplar and willow leaves. On conifers, infected needles shrivel and die soon after 

sporulation, but yellow aecia are sometimes visible on cones.

Disease Cycle—The life cycles of Melampsora species are complex and generally require two unrelated host plants and five different spore stages. Teliospores 

overwinter on dead poplar or willow leaves on the ground. In the spring, teliospores germinate, producing wind-disseminated basidiospores that infect current 

year’s conifer needles and cones. Spermogonia (pycnia) and aecia (yellow pustules) develop within several weeks on the underside of needles. Orange-yellow 

aeciospores are dispersed in the wind to infect hardwood hosts. Conifer needles 

become necrotic, shrivel, and are shed shortly after sporulation. Infected poplar 

and willow leaves produce urediniospores that infect other leaves of the same 

species throughout the summer, intensifying the disease (figs. 225-227). In the 

fall, brown, crust-like telia form in place of uredinia. Mild temperatures (65-

70 °F) and moist conditions (continuous moisture on leaf surface for 2-24 hours) 

are required for infection.

Impact—Impacts are more severe on hardwoods than on conifers. The disease 

causes discoloration, shriveling, and premature dropping of poplar and willow 

leaves. Severe defoliation may reduce growth and increase susceptibility to other 

diseases or insects. Impacts to conifer hosts are minimal.

Table 13. Hosts and alternate hosts for Melampsora rusts that occur in the Rocky Mountain Region.

Fungus	 Common name	 Host (aecial)	 Alternate host (telial)

Melampsora medusae	 Conifer-aspen rust	 Douglas-fir, lodgepole, 	 Aspen (most common)

and ponderosa pine and cottonwood

M. occidentalis	 Conifer-cottonwood rust	 Douglas-fir and 	 Cottonwood and other

possibly lodgepole Poplars 

and ponderosa pines

M. epitea Fir-willow rust and 	 True firs and Ribes spp. Willow

Ribes-willow rust

F O L I A G E D I S E A S E S 
USDA Forest Service RMRS-GTR-241. 2010. 125

Management—Plant resistant or tolerant clones, where available. Removing 

and destroying diseased leaves from the 

ground may help reduce infections. Wide 

spacing between trees may create a less 

favorable microclimate for spread and 

infection. Chemical controls are ususally 

not warranted.

References: 140, 159, 201

Red Band and Brown Spot 

Needle Blights

Discolored lower crowns of pines

Pathogen—Red band needle blight (also called Dothistroma needle blight) is 

caused primarily by Dothistroma septosporum and, to a lesser extent, by D. pini

(= Scirrhia pini). The sexual form of D. septosporum is Mycosphaerella pini, 

while Dothistroma pini has no known sexual stage.

Brown spot needle blight is caused by Scirrhia acicola (= Systremma acicola). The sexual form is Mycosphaerella acicola.

Hosts—Most pines are hosts, but the diseases are common on ponderosa, 

lodgepole, and Austrian pine in the Region. Brown spot needle blight is also 

common on Scots pine. Needle blights can occur throughout the range of the 

Figure 226. Close-up of uredinial pustules on the underside 

of an aspen leaf. Photo: William Jacobi, Colorado State 

University, Bugwood.org.

Figure 225. Orange, powdery pustules (uredinia) on the underside of an aspen leaf. Photo: 

Mike Schomaker, Colorado State Forest Service, 

Bugwood.org.

Figure 227. Yellow and necrotic spots on the upper 

surface of infected aspen leaves. Photo: Whitney 

Cranshaw, Colorado State University, Bugwood.org.

F O L I A G E D I S E A S E S
126 USDA Forest Service RMRS-GTR-241. 2010.

hosts but are most common in areas that trap high humidity, in dense stands, and 

in off-site plantings.

Signs and Symptoms—With both diseases, symptoms are usually most severe 

in the lower crowns (especially on the north side) and on small trees and saplings 

(fig. 228). Symptoms first appear on older needles as yellow or tan bands or 

spots (figs. 229-230). Red band needle blight develops distinctive red transverse 

bands (fig. 231). Brown spot needle blight produces brown spots (fig. 232) but 

can form bands. Both diseases produce clearly defined margins on the needles. 

Needles progressively turn light green, yellow, tan, and brown from the tips back. 

With both diseases, black dots (fruiting bodies of the fungi) can occasionally be 

seen in the bands or spots throughout the year. Unlike needle casts, previousyear needles (dead needles) are usually retained in needle blights. The needles 

can have a drooping appearance. Needle blights look similar to early needle 

cast diseases, winter desiccation, drought, chemical damage, air pollutions, and 

symptoms caused by root diseases.

Symptoms of these diseases are quite similar. They can only be distinguished 

by microscopic examination of the spores (asexual spores called conidia). Even 

the spores are similar in shape and size, and both have cross-walls. However, 

conidia of Dothistroma species are clear, and conidia of Scirrhia acicola are 

usually greenish brown.

Disease Cycle—These fungi can 

infect multiple times during the year 

(from April to November) when 

temperatures are favorable during 

moist periods. Most have both 

sexual and asexual spores, but only 

the asexual forms are common in 

this Region. Under moist conditions, 

the asexual spores are dispersed by 

rain splash and, to a much lesser 

degree, by wind. The pathogens enter 

Figure 228. Needle blight on a ponderosa pine sapling. 

Photo: Robert L. James, USDA Forest Service, Bugwood.

org.

Figure 229. Infected ponderosa pine branch. 

Photo: Susan K. Hagle, USDA Forest Service, 

Bugwood.org.

F O L I A G E D I S E A S E S 
USDA Forest Service RMRS-GTR-241. 2010. 127

needles through stomata and colonize the needles. These pathogens overwinter 

in the dead or infected foliage that often remains attached to the branches for a 

year or two. Fruiting bodies develop below the epidermis, split the epidermis, 

and erupt through the needle surface in the bands or spots.

Impact—Impacts include growth reduction and, in small trees with extensive infections, mortality. Occasional outbreaks appear dramatic, but damage is almost 

always minor and trees recover.

Management—Management is often not needed. Growth losses are usually localized and only occur in moist years. In dense stands, thinning can increase 

airflow, which will promote foliage drying and reduce infections. Thinning can 

also be used to remove the most damaged trees and the most susceptible species. 

In high-value sites, applying an appropriately registered fungicide to pines can 

reduce symptoms. Given the occasional nature of outbreaks and the high costs 

involved, fungicide treatments are often inappropriate in forest situations.

References: 9, 49, 129, 145, 159

Figure 230. Infected lodgepole pine branch. Photo: Jane E. Taylor, USDA Forest 

Service, Bugwood.org.

Figure 231. Distinctive 

red, transverse bands of 

red band needle blight on 

ponderosa pine. Photo: 

Robert L. James, USDA 

Forest Service, Bugwood.org.

Figure 232. Typical spots caused by brown spot needle blight on Scots pine. 

Photo: Darroll D. Skilling, USDA Forest Service, Bugwood.org.

F O L I A G E D I S E A S E S
128 USDA Forest Service RMRS-GTR-241. 2010.

Septoria Leaf Spot and Canker

Premature defoliation in windbreaks and other plantings

Pathogen—Septoria leaf spot is caused by fungi in the genus Mycosphaerella

(M. populorum and M. populicola). The asexual stages are Septoria musiva and 

S. populicola, respectively.

Hosts—Septoria diseases are most common in planted windbreaks in the Great 

Plains. All North American poplars and aspens are susceptible to leaf spots 

(fig. 233) caused by Mycosphaerella

populorum, but the fungus also causes 

cankers on cottonwood, hybrid poplars, 

and introduced poplars (fig. 234). 

Hybrid poplars and some introduced 

poplars are most susceptible.

Hosts for M. populicola include 

black and eastern cottonwoods, balsam 

poplar, and narrowleaf cottonwood.

Signs and Symptoms—The appearance of foliar lesions varies within and 

among hosts. Typically, lesions appear 

as small, circular to angular, white, 

grayish, tan, brown, or purplish spots 

with a dark border. Spots may coalesce 

to form blotches but remain small on 

more resistant species. 

Dark specks (pycnidia) 

are scattered in older 

lesions. Pink tendrils 

of conidia may exude 

from pycnidia during 

moist weather. Microscopic examination of 

conidia is necessary to 

positively identify the 

species.

Cankers only occur on trees with leaf 

spots, and their severity is proportional 

to leaf spot severity. 

Cankers usually occur 

Figure 234. Septoria cankers on young poplar branches. Photo: T.H. 

Filer, Jr.; USDA Forest Service; Bugwood.org.

Figure 233. Close-up of an irregularly-shaped 

Septoria leaf spot lesion with fruiting bodies 

(pycnidia) in the center. Photo: Michael Kangas, 

NDSU-North Dakota Forest Service, Bugwood.org.

F O L I A G E D I S E A S E S 
USDA Forest Service RMRS-GTR-241. 2010. 129

on the lower portions of the tree within 5 ft (1.5 m) of the ground and originate 

at wounds, lenticels, stipules, or leaf bases. Cankered bark initially darkens but 

becomes tan in the center. Pycnidia may be visible on young cankers but are 

rare on older cankers.

Disease Cycle—The pathogens overwinter on fallen infected leaves and in 

infected branches and stems. Primary infections occur in the spring by means 

of ascospores produced in fruiting bodies (pseudothecia) on fallen leaves or 

in infected branches and stems. Lesions develop about 1-2 weeks after infection, and asexual fruiting bodies (pycnidia) are produced in about 3-4 weeks. 

Secondary infections may occur throughout the summer during warm, moist 

conditions when conidia are released from fruiting bodies (pycnidia) and are 

spread by wind or rain splash to infect new leaves and stems.

Impact—Mycosphaerella populicola is less virulent than M. populorum. The 

diseases cause little to no damage in natural stands, but M. populorum can 

cause severe damage in plantings and windbreaks. Premature defoliation may 

occur in more susceptible species, and cankers can girdle stems. Cytospora

chrysosperma and other canker fungi may invade the edges of Septoria cankers.

Management—Control strategies for Septoria diseases include: planting only 

resistant or tolerant clones; increasing spacing between trees to increase air 

circulation and reduce humidity; raking and destroying infected overwintering leaves and stems; and applying fungicides to protect propagation beds and 

landscape trees.

References: 108, 159

Pine Shoot Blight and Canker

Branch tip flagging and black dots on cone scales

Pathogen—The fungus Diplodia pinea (= Sphaeropsis sapinea) causes pine 

shoot blight and canker. This disease is also known as Diplodia or Sphaeropsis 

shoot blight. A second species, Diplodia scrobiculata, occurs in the United States 

and may be present in this region.

Hosts—Pine shoot blight is an important disease of pines and other conifers. 

Seedlings to fully mature pines are affected, and damage occurs in nurseries, plantations, Christmas tree and ornamental plantings, and natural stands. 

This disease is widely distributed and locally severe in much of South Dakota, 

Nebraska, and Kansas where it affects mostly ponderosa, Austrian, and Scots 

pine.

Signs and Symptoms—Diplodia pinea can cause shoot blight, canker, crown 

wilt, collar rot, and root disease. New shoots are killed rapidly by the fungus 

(fig. 235). Damage may be confined to the new shoots, particularly on trees with 

shoots that are infected for the first time. Infected needles become discolored (tan 

S H O O T B L I G H T S
130 USDA Forest Service RMRS-GTR-241. 2010.

to gray) while still encased in 

fascicle sheaths. Dead needles 

usually remain attached to the 

twig. Cankers in new shoots can 

cause stunting or crooking of 

the shoots. Repeated infections 

result in dead branches and tree 

mortality (fig. 236). Resinous 

cankers with brown phloem 

and cambial tissue develop on 

branches and stems (fig. 237).

Figure 237. Branch with cankered 

tissue. Photo: James T. Blodgett, USDA 

Forest Service.

Figure 235. Diplodia infected shoot-tip. Photo: James T. 

Blodgett, USDA Forest Service.

Figure 236. Infected ponderosa pine with hail damage. 

Photo: James T. Blodgett, USDA Forest Service.

S H O O T B L I G H T S
USDA Forest Service RMRS-GTR-241. 2010. 131

Small, black fruiting bodies (pycnidia) are produced abundantly on needles 

and cones (figs. 238-239). The fruiting 

bodies can be seen with a hand lens. 

However, microscopic examination 

of the spores is necessary for accurate 

identification.

Disease Cycle—Spores develop in 

the fruiting bodies on needles, needle 

fascicle sheaths, scales of second-year 

cones, and bark. These black fruiting 

bodies, which erupt through the epidermis, are often numerous at the base of 

needles and on scales of second-year 

seed cones.

Spores are dispersed from spring 

to fall, with large numbers of spores 

being dispersed during rain and high 

relative humidity. Rain splash is a 

common means of spread, but spores 

can be carried by high wind or by 

insects.

Moderate temperatures and very 

moist conditions are needed for spore 

germination and infection. Young 

shoots are most susceptible to infection. Spores can penetrate needles and 

young shoots, or D. pinea can infect 

both current-year and older tissues through wounds. Because this pathogen can 

persist in stems and branches of pines with no symptoms, wounding might act 

as a trigger, allowing the fungus to rapidly infect. This could explain the rapid 

disease development often associated with hail events. Both planting stress and 

water stress can also result in rapid disease-associated mortality of already infected trees.

The large numbers of spores produced on cones can contribute to infection 

and branch mortality of older trees or infection of seedlings and young trees 

growing near the older infected trees. The fungus commonly kills the entire new 

shoot by the end of summer. Infections may continue to develop into year-old 

tissues.

Increased disease development has been associated with water deficits; competing vegetation; high soil nutrition; poor site; and damage by hail, snow, and 

insects. Seasonal variations and climate differences also have been related to 

differences in the amount of disease caused by this pathogen.

Figure 238. Pycnidia of Diplodia on needles. Photo: 

James T. Blodgett, USDA Forest Service.

Figure 239. Pycnidia on cone scales. Photo: James T. 

Blodgett, USDA Forest Service.

S H O O T B L I G H T S
132 USDA Forest Service RMRS-GTR-241. 2010.

S H O O T B L I G H T S 

Impact—Pine shoot blight kills current-year shoots and major branches. Severe 

infections may lead to death of trees of all sizes. The disease can predispose trees 

to attack by other diseases and insects, including the pine engraver beetles. This 

pathogen often severely affects trees wounded by hail, insects, or other damage. 

Pruning and shearing wounds may become infected and are susceptible to infection for several days.

Management—Reducing water stress is an important factor in controlling pine 

shoot blight. Techniques to reduce disease losses include managing competing 

vegetation, stand thinning, using planting techniques that reduce drought, and 

selecting tree species that are adapted to the site.

Pruning or shearing should be avoided during periods when conditions are 

highly favorable for infection because of the danger of infection through wounds. 

Pruning tools should be disinfected.

Seedlings and small trees can become infected when they are located close 

to infected cone-bearing pines. Avoid planting adjacent to infected pines, or remove infected trees.

Fertilization of pines at levels recommended for ornamental and shade trees 

can result in increased disease and should be avoided.

Infection of new shoots can be reduced significantly by applying an appropriately registered fungicide to pines during the 2-week period when shoots are 

highly susceptible to infection. This period begins with bud opening, usually 

from late April to mid-May. Fungicide applications during late April and midMay will protect new shoots but will not prevent infection of cones.

References: 20, 48, 126, 127, 128, 175

Shepherd’s Crook

Leaf and shoot blight of young aspen

Pathogen—Shepherd’s crook, also known as aspen shoot blight, is caused by 

the fungus Venturia tremulae var. grandidentatae (= V. moreletii). The asexual 

stage (anamorph) of the fungus is Fusicladium radiosum var. lethiferum (= Pollacia radiosa). A similar disease is caused by Venturia populina but usually on 

other Populus species. Several other less common Venturia species cause similar 

diseases in other parts of North America.

Hosts—Venturia tremulae infects Populus tremuloides, P. alba, P. grandidentata, and hybrids. It has also been reported on P. angustifolia and occasionally 

infects other Populus species. The pathogen occurs across North America and 

in Europe.

Signs and Symptoms—Dark brown to black lesions first appear in the spring 

on leaf blades, petioles, or current-season shoots. Affected leaves and shoots often droop and curl, leading to the name of the disease (figs. 240-241). Early 

symptoms can look similar to frost damage, but patterns in the landscape differentiate the two. Infections can progress from leaves to petioles to shoots. Dead 
USDA Forest Service RMRS-GTR-241. 2010. 133

tissues eventually become dry and brittle and break off. During wet weather, 

lesions develop a fine, olive-colored fuzz due to production of asexual spores on 

the surface.

Disease Cycle—The pathogen can survive the winter as mycelium in the stubs of 

blighted shoots. It also overwinters in blighted foliage on the ground, where pseudothecia (sexual fruiting bodies) develop during the dormant season. In the spring, 

sexual spores are released into the air and can cause initial infections. The mycelium 

in stubs of blighted shoots may also produce infective asexual spores that are dispersed by rain splash. If the weather remains wet, subsequent cycles of infection are 

caused by asexual spores produced on diseased tissues. Lateral shoots cease growth 

early in the season and become resistant, so most infections later in the season are 

on terminals that continue to grow and remain succulent.

Impact—During very wet years, particularly early in the growing season when 

tissues are succulent, the disease can kill nearly all terminal shoots of suckers 

and small saplings. Growth rate is reduced and the stem may become crooked as 

lateral shoots assume dominance. If infection is repeated for years, growth may 

take on a shrubby form as the terminals are repeatedly killed back. Smaller plants 

may be killed when disease is severe. The disease is most severe on moist sites.

A quantitative study using wounding to simulate the disease concluded that it 

significantly reduces height growth (ref. 16). However, the effect was temporary; 

differences between treated and control plants diminished over several years.

There is a high degree of genetic variation in resistance to shepherd’s crook. 

Resistant genotypes have higher concentrations of condensed tannins in their 

tissues.

Figure 241. A group of aspen suckers, all with blighted 

terminals. Photos: Jim Worrall, USDA Forest Service.

Figure 240. Shepherd’s crook on young aspen, showing the typical “crook” (drooping) 

of the young, dead apical shoots and lesions 

on lower, living foliage. Photo: Jim Worrall, 

USDA Forest Service.

S H O O T B L I G H T S
134 USDA Forest Service RMRS-GTR-241. 2010.

A B I O T I C I N J U R I E S A N D M I S C E L L A N E O U S 

Management—No practical management approaches are available for this disease in a forest environment. Because the effects are usually temporary and aspen populations recover, the disease is usually not a major management concern.

References: 16, 84, 158

Burls, Galls, and Tumors

Abnormal swellings on stems and branches

Cause—Burls, galls, and tumors may be caused by bacteria, fungi, insects, environmental stress, or genetic predisposition. In the Rocky Mountain Region, 

insect and pathogen-induced burls and galls are common on hardwoods, but the 

cause of conifer stem galls is often unknown. Galls on ponderosa and lodgepole 

pines, caused by western gall rust, are discussed separately in this guide.

Hosts—Many conifer and hardwood species may be hosts. In the Rocky Mountain Region, incidence is especially common in lodgepole pine, Douglas-fir, Engelmann spruce, subalpine fir, and aspen.

Signs and Symptoms—Burls, galls, and tumors are abnormal swellings that typically occur on stems and branches (figs. 242-244). They often form in succession 

along the stem. Swellings vary in size and may be smooth or rough with rounded 

or flared edges. Adventitious buds or sprouts sometimes protrude from the 

surface. Trees of all ages 

and sizes may be affected, 

and trees may occur singly 

or in groups. Incidence is 

especially frequent in highelevation sites of the Rocky 

Mountain Region.

Impact—Growth impacts 

are minimal, but burls 

and other abnormal swellings may cause significant 

defect and deformation, 

which lowers timber value. On the other hand, researchers found less decay 

in aspen trees that had black 

stem galls in Alberta, Canada, suggesting that galled 

trees may be more valuable 

from a timber perspective in certain situations. 

Figure 242. Several burls 

on an Engelmann spruce 

stem, cause unknown. 

Photo: Kelly S. Burns, 

USDA Forest Service.

Figure 243. Black stem gall on 

quaking aspen, cause unknown. 

Photo: William Jacobi, Colorado State 

University.
USDA Forest Service RMRS-GTR-241. 2010. 135

A B I O T I C I N J U R I E S A N D M I S C E L L A N E O U S

The distinctive wood is also treasured by artists who use it for sculptures, furniture, 

and other forms of woodworking (fig. 245).

Management—Control strategies have not been developed for these swellings, 

and management is not generally warranted. However, avoiding wounding and 

removing damaged trees during intermediate stand treatments and maintaining 

host vigor may reduce impacts.

References: 31, 131, 159

Animal Damage

Various animal species cause physical damage

Cause—A small number of animal species cause significant damage to trees. 

The animals that primarily damage trees in the Rocky Mountain Region include 

deer, elk, porcupines, beaver, mice, squirrels, gophers, rabbits, and birds.

Hosts—All tree species can be damaged by animals, and damage occurs 

throughout the Region. The type and extent of damage varies by animal and tree 

species because animals differ in their preference for tree species.

Signs and Symptoms—In some situations, many trees in an area are affected, 

and in other situations, only individual trees are damaged. Damages include 

removal of bark, wood, foliage, and twigs. Effects of the damage include tree 

girdling, scarring, deforming, brooming, stunting, and callus ridge formation. 

Callus ridges are the result of natural healing of trees as new bark forms around 

damaged bark.

Figure 245. A sculpture created from a burled spruce tree. Photo: 

Jim Worrall, USDA Forest Service.

Figure 244. Galls on a quaking aspen 

caused by the poplar vagabond aphid 

(Mordwilkoja vagabunda). Photo: 

Whitney Cranshaw, Colorado State 

University, Bugwood.org.
136 USDA Forest Service RMRS-GTR-241. 2010.

A B I O T I C I N J U R I E S A N D M I S C E L L A N E O U S 

Common damages and their causes in the Region include: antler rubbing of tree bark by deer and elk (fig. 246); 

browsing of foliage, buds, and young 

shoots by ungulates (figs. 247-248); 

bark feeding by elk (fig. 249) or bear; girdling by various 

mammals (fig. 250), including porcupines (fig. 251) and beavers; rabbit and gopher chewing; and debarking or scratching 

by bears (fig. 252). Porcupines chew through the outer bark 

of branches and boles of conifers, especially in the winter. 

Bears may rip wide strips of bark from trees and scrape bark 

with their claws. The sapsuckers, large birds in the genus 

Sphyrapicus, make uniform horizontal rows of holes in the 

bark of conifer and hardwood species (fig. 253). Sapsuckers 

feed on sap that oozes from holes they make on the stem and 

on insects attracted to the sap. Tooth, claw, or antler scrape 

marks are usually visible in the sapwood of damaged trees. 

Rubbing by deer and elk often produces shredded bark attached to the damaged areas. Mechanical damage can be 

difficult to differentiate from animal damage unless the 

scrape marks or shredded bark are evident. Circumstances 

can provide clues to the cause of the damage.

Squirrels, porcupines, and mice often chew the bark 

around rust-induced cankers. However, they also feed in the 

top or mid-stems of healthy trees. Squirrels also clip small 

Figure 247. Repeated browsing can cause 

the production of multiple stems or brooming. Photo: Susan K. Hagle, USDA Forest 

Service, Bugwood.org. Figure 248. Aspen sucker browsed by 

elk. Photo: James T. Blodgett, USDA 

Forest Service.

Figure 249. Elk feeding 

damage on aspen bark, 

Salida Ranger District, 

San Isabel National 

Forest. Photo: Dave 

Powell, USDA Forest 

Service, Bugwood.org.

Figure 246. Antler rubbing 

damage with shredded bark 

and callus ridges. Photo: 

Susan K. Hagle, USDA 

Forest Service, Bugwood.org.
USDA Forest Service RMRS-GTR-241. 2010. 137

A B I O T I C I N J U R I E S A N D M I S C E L L A N E O U S

branches from trees, leaving green branches on the ground. Rabbits and grounddwelling rodents feed on the bark of young trees near the ground and can scar or 

girdle trees. Beavers cut down larger trees for construction and/or collect branches from trees and saplings to 

eat. This results in damage near the ground, including 

girdling and felling of trees. Underground rodents such 

as pocket gophers feed on roots.

Seeds and cones can be damaged by rodents and 

birds and occasionally larger mammals such as bears.

Impact—The overall losses from wild animal damage 

are usually far less than losses caused by diseases, insects, fire, or even domestic animals. However, large 

losses can occur in limited areas, especially to aspen 

regeneration. The wounds allow entry of diseases (especially canker and wood decay fungi) and may attract 

insects.

If stems or branches are girdled, mortality of the 

tree part above the girdle results. Extensive damage 

without girdling can result in growth loss or stunting. 

In most cases, callus will 

form and trees will recover 

if not attacked by diseases or 

insects.

Figure 251. Tooth marks from 

porcupine feeding within the 

margins of a sporulating rust 

canker. Photo: Brian W. Geils, 

USDA Forest Service, Bugwood.

org.

Figure 252. Bear claw marks are 

distinct when fresh. Photo: USDA 

Forest Service, Bugwood.org.

Figure 250. Tooth marks 

from vole feeding on a root. 

Photo: Paul Bachi, University of 

Kentucky Research & Education 

Center, Bugwood.org.

Figure 253. Sapsuckers make 

distinctive holes on the stem 

that are regularly spaced in 

rows. Photo: James T. Blodgett, 

USDA Forest Service.
138 USDA Forest Service RMRS-GTR-241. 2010.

A B I O T I C I N J U R I E S A N D M I S C E L L A N E O U S 

Regeneration can also be impacted. Browsing can cause brooming, mortality, 

and vegetation removal, resulting in severe loss of forest regeneration. This is 

especially common with aspen and other hardwood species. Rodents can girdle 

seedlings and saplings. Rodents and birds eat and disperse seeds, which can both 

negatively and positively affect regeneration.

Management—Numerous management methods and tools are available, including chemical deterrents/repellents, exclusion with fencing or other means, 

devices to frighten animals, habitat modification, relocation, traps, or lethal control. The application of any of these methods depends on the pest involved, the 

setting, and local laws and regulations.

References: 61, 112

Abiotic Foliage Damage

Often affects many host species in an area

Cause—Common causes of injury or damage to foliage by non-living agents 

include fire, drought, frost, and chemicals.

Hosts—All tree species throughout the Rocky Mountain Region can be damaged.

Signs and Symptoms—Identification of abiotic foliage damage often requires 

learning the history of the site, comparing injury between trees and other plants 

of the same and different species, looking for patterns across the site, and ruling 

out damage from other agents such as diseases and insects. The factors causing 

these damages often operate for a brief period, but some can persist for much of 

the growing season as with drought.

Common damages in the region:

•Fire injury to foliage can be from direct burning or from radiant heat. Mortality is common, especially if stems are affected. See the Abiotic Ste

Damage entry in this guide. The damage can look similar to some foliage 

diseases or other abiotic foliage damage, but charred or burned vegetation 

around the trees is often evident.

•Drought symptoms are usually expressed in the foliage, but drought affect

the whole tree (figs. 254-255). Direct drought mortality is not common 

in older, well-established trees, but damage to young trees and especially 

regeneration is common. Drought stress is a normal phenomenon in trees 

when water loss by transpiration exceeds the rate of absorption from soil. In 

this Region, drought stress occurs frequently in mid to late summer. Direct 

mortality from drought only occurs when a tree’s water potential falls below the permanent wilting point. Drought stress at levels that do not cause 

mortality can predispose trees to some diseases (especially cankers and root 

diseases), insects (especially bark beetles), and fire.
USDA Forest Service RMRS-GTR-241. 2010. 139

A B I O T I C I N J U R I E S A N D M I S C E L L A N E O U S

Symptoms of drought include reduced growth, wilting, foliage discoloration 

and damage, premature leaf fall, and/or early dormancy. With extreme drought, 

new foliage is shed, shoots die back to lateral buds, and tree mortality may occur. 

In conifers, common symptoms are premature browning and shedding of older 

needles, stunted needles, and brown tips of new foliage. In hardwoods, common 

symptoms include wilting, yellowing of foliage, reddish brown to brown leaf 

tip and/or margins (scorch), and premature leaf drop. Injury is most severe on 

south and southwest slopes, shallow or sandy soils, and during periods of heat 

and unusually low rainfall. Symptoms are similar to those of trees suffering from 

root, wilt, or canker diseases. With root diseases, roots die before the foliage and 

a root disease pathogen can be found in the roots. Trees with wilt diseases may 

have vascular discoloration and a wilt disease pathogen can be found in the outer 

xylem tissues. Canker diseases cause the same crown symptoms when a canker 

pathogen girdles stems and/or branches.

•Winter desiccation injury occurs when solar warming of south and southwest tree crowns causes leaves to transpire when roots are frozen and unable to replace moisture (fig. 256). Foliar damage may be predominantly o

the southwest aspect of the crown, but occasionally, damage is located more 

on the side of prevailing winds. The portions of crowns covered by snow 

are not damaged.

Winter desiccation can damage or kill conifer foliage but seldom damages 

the buds and branches. Needles damaged or killed often remain green when 

Figure 255. Aspen after chronic drought. Photo: 

Southwestern Region, USDA Forest Service.

Figure 254. This tree shed older needles first 

due to drought. Photo: Susan K. Hagle, USDA 

Forest Service, Bugwood.org.
140 USDA Forest Service RMRS-GTR-241. 2010.

A B I O T I C I N J U R I E S A N D M I S C E L L A N E O U S 

temperatures are cold, turn yellow 

or red-brown when temperatures 

increase in spring, and are shed in 

early summer. New foliage usually 

emerges in late spring or early summer. Because buds and branches 

usually survive, winter desiccation 

typically results in minor growth 

loss and aesthetic problems of conifers. If buds and/or branches die, 

multiple stems often form, resulting 

in a bushy tree form. Although not 

common, top-kill or mortality has 

been reported. Symptoms occur 

across the landscape, but individual 

trees vary in symptom expression.

•Red belt is a type of winter desiccation (figs. 257-258). This damag

affects distinct patches of conifers, 

often in well-defined horizontal bands (belts). South and west 

slopes are usually affected more 

than north and east slopes. This is 

a weather-related winter phenomenon on slopes where rapid temperature changes cause desiccation of foliage but rarely cause bud mortality. 

Red belt is believed to be caused by air inversions in valleys. A thin layer 

of warm air is trapped and cannot mix downward, so it continues across the 

slopes.

Symptoms are expressed in the spring when needles turn red-brown along 

distinct patches or horizontal bands. Needles can become entirely red-brown 

Figure 257. Red belt damage on hillside in early spring. Photo: USDA Forest Service, Bugwood.org.

Figure 256. Winter desiccation damages the foliage 

exposed above the snow. Photo: Scott Tunnock, 

USDA Forest Service, Bugwood.org.
USDA Forest Service RMRS-GTR-241. 2010. 141

A B I O T I C I N J U R I E S A N D M I S C E L L A N E O U S

or, in less severe cases, discolored only at their 

tips. Upper portions of trees and the outermost 

foliage are most affected. Trees usually recover 

during the growing season with little residual 

effect.

•Frost or freeze injury occurs in the sprin

when susceptible new foliage or young shoots 

are exposed to sudden freezing temperatures (figs. 259-261). Greatest vulnerability is from bud break to final shoot and leaf elongation. The extent 

of damage depends on the stage of development at the time of frost. New 

leaves and shoots are often killed, especially during earlier expansion. 

Damage frequently occurs on several species in an area.

Figure 260. Frost damage to red oak 

shoots. Photo: Andrew J. Boone, South 

Carolina Forestry Commission, Bugwood.

org.

Figure 261. Frost damage with branch droop. Photo: Petr 

Kapitola, State Phytosanitary Administration, Bugwood.org.

Figure 258. Close-up of red belt damage. 

The outer foliage is usually the most damaged. Photo: Oscar Dooling, USDA Forest 

Service, Bugwood.org.

Figure 259. Frost damage to buds of Douglas-fir. Photo: 

USDA Forest Service, Bugwood.org.
142 USDA Forest Service RMRS-GTR-241. 2010.

A B I O T I C I N J U R I E S A N D M I S C E L L A N E O U S 

Within a day or two of freeze injury, foliage and shoots become limp and 

begin to fade to yellow. Damaged tissues at first appear water soaked. After 

a week or more, tissues become shriveled; foliage turns reddish brown, dark 

brown, or black, depending on the species, and can droop; and young shoots 

turn brown and can droop. Dead buds become dark brown in the interior. 

Dead leaves and shoots eventually break off or abscise during the next few 

weeks. New shoots and leaves begin to grow almost immediately.

•Road salt is a common type of chemical damage (fig. 262). Salt can affec

the foliage indirectly through root absorption or directly when salt spray 

settles on trees near high-speed roads. Damage can occur wherever chlorides (sodium, calcium, or magnesium) are applied for deicing in the winter 

or dust abatement in the summer. Most of the time, symptoms are insignificant and injury is very close to a road. Symptoms may be severe along highways where large quantities of salt are used. Symptoms can also occur in 

low areas where salts accumulate after runoff. Plant tissue can be analyzed 

in a laboratory to determine the amount of salt present. Symptoms can differ 

between the two entry methods.

Root uptake damage is evident after spring thaw in conifers in second-year 

needles and soon after budbreak in hardwoods. Burn of needle tips, with the 

bases of needles remaining green, is common in conifers, and browning of leaf 

margins is common in hardwoods. With higher salt concentrations, buds will 

break with reduced foliage, more pronounced tip burn or brown leaf margins 

develop, and trees have reduced growth. Symptoms can intensify through the 

season with 1-year-old needles showing symptoms mid-winter and premature 

leaf drop of the 2- and 3-year-old needles in conifers. Symptoms of severe 

damage include delayed or lack of bud break and branch or tree mortality.

Figure 262. Salt damage is evident only near the road. Photo: William M. Ciesla, Forest Health 

Management International, Bugwood.org.
USDA Forest Service RMRS-GTR-241. 2010. 143

A B I O T I C I N J U R I E S A N D M I S C E L L A N E O U S

On conifers, tip browning from salt spray is common, and browning can 

progress toward the base of needles throughout the growing season. Salt spray in 

hardwoods often results in mortality of terminal buds. This mortality can result 

in bud break of several lateral buds, causing brooms. Unlike damage caused by 

root uptake, marginal browning rarely occurs with salt spray on hardwoods.

•Herbicide damage is another type of chemical damage (figs. 263-265). Herbicide

can affect foliage directly or indirectly through root uptake. Herbicide damage 

often follows a pattern consistent with application of the chemical, such as a 

swath about the width of a spray boom where weeds were sprayed. Weed treatment areas include: along roads, rights-of-way, fuelbreaks, or around dwellings. 

In many situations, plant samples can be submitted to a laboratory to determine 

the amount and type of chemical in the tissue.

Figure 263. Herbicide damage on pine. Photo: 

Susan K. Hagle, USDA Forest Service, Bugwood.

org.

Figure 264. Distorted shoots due to herbicide damage on 

pine. Photo: Minnesota Department of Natural Resources, 

Bugwood.org.

Figure 265. Herbicide damage on hardwood species. Photo: Joseph O’Brien, USDA Forest Service, 

Bugwood.org.
144 USDA Forest Service RMRS-GTR-241. 2010.

A B I O T I C I N J U R I E S A N D M I S C E L L A N E O U S 

Symptoms vary by chemical. Preemergence: photosynthetic inhibitors (simazine) can cause leaf yellowing. Postemergence: growth regulators (Glyphosate) can 

cause abnormal leaf development, tip yellowing, and dieback; Picloram (2,4-D) 

symptoms are similar to glyphosate and also include twisted petioles and shoots. 

Contact (Paraquat) injury results from drift, causing small dead spots on foliage. 

High concentrations of any of these herbicides can cause mortality.

Impact—Losses in the Region from abiotic foliage damage are usually minor 

compared to those caused by diseases and insects, with the exception of fire. 

Most biotic diseases and insects affect a single tree species or group of species. 

Abiotic agents often affect various tree, shrub, and herbaceous species in an area. 

However, some tree species are more resilient to some of the damage agents.

Extensive damage from any of these agents can result in growth loss or stunting. However, abiotic foliage damage rarely causes direct mortality, with the 

exception of fire and, in extreme cases, herbicide damage. Stress associated with 

these damages can make trees more susceptible to some diseases (especially cankers and root diseases) and insects (such as wood borers and bark beetles), which 

can result in mortality.

Management—There are few management options for many of the abiotic foliage damages, with the exception of fire, drought, salt, and herbicide damage.

Fire management should include elements of fire prevention and fire suppression. Because many fires are caused by people, educating the public is the 

best option. Methods that reduce the spread of fires such as reducing fuel loads, 

thinning stands, and creating firebreaks will reduce the number of trees damaged. 

After fire, prompt salvage cuts can be used to remove trees of value before they 

are colonized by decay fungi and wood borers, if consistent with management 

objectives.

For drought, management of competing vegetation, stand thinning, planting 

techniques that reduce drought, and selection of a tree species compatible with a 

site will reduce losses.

For road salt, prudent use will reduce losses. To reduce over-application, 

machines should be calibrated to apply the optimum level of salt for reducing 

surface ice or dust in order to not apply excessive levels. Mortality can be high 

when salts are applied for dust abatement before heavy rainstorms, so this should 

be avoided. In areas where trees are planted along roads, salt-tolerant species 

could be used. The amount of salt damage is directly related to the distance from 

a road, with the exception of salt runoff in limited downhill areas. Therefore, 

planting trees farther from the road or cutting trees near a road will reduce salt 

damage and also improve visibility.

To reduce herbicide damage, read the label and apply the chemical as directed. Also, check and properly calibrate equipment. Select herbicides that will do 

the job but pose the least risk to trees in the area. Tree species vary in tolerance 

to some herbicides.

References: 17, 18, 19, 38, 40, 83, 121, 159
USDA Forest Service RMRS-GTR-241. 2010. 145

A B I O T I C I N J U R I E S A N D M I S C E L L A N E O U S

Abiotic Stem Damage

Injury or damage from non-living agents

Cause—Common causes of injury or damage to stems by non-living agents include mechanical abrasion, physical constriction, fire, lightning strikes, hot and 

cold extremes, and high winds.

Hosts—All tree species throughout the Rocky Mountain Region can be damaged. Species with thin bark and most young trees are more susceptible to mechanical damage, fire, freeze injury/sunscald, solar heat injury, frost cracks, and 

hail.

Signs and Symptoms—Common damages in the 

Region:

•Mechanical damage is caused by impacts or rubbing from various sources such as vehicles (skidders, cars, etc.), falling trees or branches hittin

other trees, people chopping with axes and hatchets, or other contact sources (fig. 266). Mechanical injury may lead to resin production in conifers, 

and callus ridges eventually form at the edges of 

the damage. Initially, scarring or deformation of 

the tree results. Depending on the wound size, the 

callus can form new bark over time and the wound 

can heal. The wounds allow entry of wood-decay 

fungi. If trees are damaged completely around 

the stem (girdled), the part 

above the girdle dies.

•Fire injury can be fro

direct burning or from radiant heat (figs. 267-268). Initially, fire injury is obvious 

and mortality is common. 

Fire scars can be superficial 

or deep and are often resinous 

on conifers that are not killed. 

Callus ridges eventually form 

at the edges of the damage on 

trees that survive the initial 

fire. Over time, some evidence of char usually can be 

found on the bark and/or on 

exposed wood at the base of 

trees. Trees that survive the 

initial fire injury are often 

Figure 266. A mechanical wound 

caused during logging. Photo: Susan 

K. Hagle, USDA Forest Service, 

Bugwood.org.

Figure 267. Old fire scar 

with intact bark covering 

most of the injury. Photo: 

Susan K. Hagle, USDA 

Forest Service.

Figure 268. Charred bark and resin 

flow are common symptoms of fire 

damage. Photo: Kurt K. Allen, USDA 

Forest Service.
146 USDA Forest Service RMRS-GTR-241. 2010.

A B I O T I C I N J U R I E S A N D M I S C E L L A N E O U S 

attacked by diseases such as root, butt, or stem decays 

and/or by insects such as bark beetles.

•Lightning can result in direct mortality. However

in most cases, trees are only slightly damaged (figs. 269-270). Trees can be 

blown apart at impact, or lightning can cause fires that result in fire injury. 

More often, damage is in the form of a long, narrow furrow in the bark with 

thin layers of wood blown out. The furrow often extends down to the soil and 

may spiral to some extent. Bark beetles occasionally colonize trees soon after 

lightning strikes, and the wounds can allow entry of diseases, especially wood 

decays.

•Freeze injury/sunscald is caused by a sudden drop in temperature in the winter (i.e., sudden freezing temperatures) that damages sun-warmed and, therefore, non-hardened cambium (fig. 271). Symptoms are seen on the south t

southwest side of stems. At first, the bark is discolored; if the injury is severe 

enough the bark will become rough and dead bark will flake. If the cambium 

is killed, the scar extends to the sapwood and bark is eventually sloughed off. 

Little or no resinosis results. A ridge of callus will form between live and dead 

cambium.

Figure 269. Lightning-struck 

trees are often attacked by 

bark beetles or decay fungi. 

Photo: James T. Blodgett, 

USDA Forest Service.

Figure 270. Lightning injury to an elm. Photo: 

William Jacobi, Colorado State University, 

Bugwood.org.

Figure 271. Sunscald 

on the south side of thinbarked trees. Photo: Susan 

K. Hagle, USDA Forest 

Service, Bugwood.org.
USDA Forest Service RMRS-GTR-241. 2010. 147

A B I O T I C I N J U R I E S A N D M I S C E L L A N E O U S

•Solar heat injury looks similar to freeze injury/sunscal

but occurs when bark is suddenly exposed to intense sun 

(heat), which can result from pruning or stand thinning. 

This does not occur if the bark developed in the sun or 

when the bark is slowly exposed to the sun, and it only occurs in thin-barked species. As with freeze injury/sunscald, 

solar heat injury occurs on the south to southwest side of 

stems and produces the same symptoms. Damage to stems 

from solar heat is not well documented and is occasionally 

referred to as sunscald.

•Frost cracks are caused by an extreme drop in temperature during the winter when trees are dormant (fig. 272)

Shrinking of the bark or outer wood as it cools quickly, 

compared to warmer inner wood, causes the bark or wood 

to separate or split. This can cause short or long cracks that 

are usually vertical, but long cracks can spiral somewhat 

on the stem. Unlike lightning damage, frost cracks usually have little bark loss. Cracks can form deep into the 

wood. Frost cracks seldom result in mortality. However, 

this damage often leads to extensive heartrot when cracks 

extend deep into the stem. Callus ridges form on both sides 

of the crack.

•Mechanical girdling is caused most often by peopl

wrapping wire, rope, or other objects partially or completely around trees (figs. 273-274). Vines or other 

Figure 272. Long, vertical 

frost crack. Photo: James 

Solomon, USDA Forest 

Service, Bugwood.org.

Figure 273. Girdling by barbed wire. 

Photo: Robert L. Anderson, USDA 

Forest Service, Bug-wood.org.

Figure 274.

Girdling by a 

wire. Photo: 

Francis 

Gwyn Jones, 

Bugwood.org.
148 USDA Forest Service RMRS-GTR-241. 2010.

A B I O T I C I N J U R I E S A N D M I S C E L L A N E O U S 

natural objects occasionally cause mechanical girdling. Because trees continually grow in diameter, mechanical 

girdling can eventually result in tree 

mortality or mortality of the part of the 

tree exposed to the girdling object.

•Hail damage is a type of mechanica

damage caused by hail impacts (figs. 

275-276). This damage often affects 

trees in a larger patchy area, is directional (on the upper side of horizontal 

branches), and does not girdle stems or 

branches. Therefore, stem and branch 

mortality does not occur unless the stems 

or branches are snapped by the impacts. 

Initial scarring results, but if trees are not 

infected by diseases such as pine shoot 

blight or other canker diseases, callus 

ridges eventually form and wounds heal.

•Wind damage from very strong wind

can result in snapping of the main stem 

(fig. 277). This type of damage is not 

Figure 277. Damage from strong winds. Photo: 

Gil Wojciech, Polish Forest Research Institute, 

Bugwood.org.

Figure 275. Directional damage caused by hail. Photo: James T. 

Blodgett, USDA Forest Service.

Figure 276. Close-up of hail damage 

on an aspen. Photo: Steven Katovich, 

USDA Forest Service, Bugwood.org.
USDA Forest Service RMRS-GTR-241. 2010. 149

A B I O T I C I N J U R I E S A N D M I S C E L L A N E O U S

common with healthy stems in the Rocky Mountain Region but has been reported. Snapping or breaking of stems is common when associated with decay. 

Snapping associated with stem decay or root and butt rot can occur in conditions from no wind to high winds. Snapping of solid (non-decayed) stems 

requires very high winds and usually results in many damaged trees in an area 

in a short time. Wind-broken wood has a more jagged appearance than breaks 

associated with decay. Winds can also shake stems, causing separation of annual rings. Resin deposition in wood at the damage site and/or wood separation 

can result in significant log defects.

Impact—Direct losses from abiotic stem damage are usually minor compared to 

those caused by diseases and insects, with the exception of fire. In some situations, many trees and/or species in an area are affected (such is the case from fire, 

high wind, and hail), and in other situations, only individual trees are damaged. 

Extensive damage from any of these agents can result in growth loss, stunting, or 

mortality. Wounds are important as points of entry for diseases (especially cankers and wood decays) and insects. Callus ridges are the result of natural healing 

of trees as new bark forms around damaged bark. In most cases, callus will form 

and trees will recover if not attacked by diseases or insects.

Management—There are few management options for most of the abiotic stem 

damages, with the exception of mechanical damage and mechanical girdling 

caused by people and fire damage. Care should be exercised in pruning and thinning to avoid sudden exposure that could lead to freeze injury/sunscald, solar 

heat injury, or frost cracks.

For mechanical damage and mechanical girdling caused by people, public 

education is the best option. For mechanical damage associated with logging, 

options include: minimize stand entry during management, keep vehicles away 

from leave trees, use bumper trees, and include fines or value reductions in contracts for the number of trees damaged.

Fire management should include elements of fire prevention and fire suppression. Because many fires are caused by people, public education is the best 

option. Methods that reduce the spread of fires such as reducing fuel loads, thinning stands, and creating firebreaks will reduce the number of trees damaged. 

After fire, prompt salvage cuts can be used to remove trees of value before they 

are colonized by decay fungi and wood borers. Sanitation cuts can be used to 

remove damaged trees, if consistent with management options.

References: 19, 159

Wetwood

Watersoaked, discolored, and often smelly 

wood in living trees

Cause—Wetwood is not a disease. It results from a physiological process that 

occurs when the living cells in the wood die. Although it is sometimes called 
150 USDA Forest Service RMRS-GTR-241. 2010.

A B I O T I C I N J U R I E S A N D M I S C E L L A N E O U S 

“bacterial” wetwood, this is 

a misnomer because it is not 

caused by bacteria. However, various bacteria colonize 

wetwood and, in some cases, cause additional damaging effects.

For many years, wetwood was thought to be a 

bacterial disease. However, 

later experiments showed 

that wetwood can be formed following wounding under conditions that preclude 

bacterial growth. During death of the parenchyma in wood, calcium, potassium, 

and magnesium salts are mobilized into the area, lowering the osmotic potential. 

Moisture accumulates in response to the osmotic gradient, and a drier transition 

zone with living parenchyma separates sapwood from wetwood.

Hosts—Wetwood occurs in many hardwoods and in some conifers. In this Region, wetwood often occurs in elm, maple, ash, cottonwood, aspen, and white fir.

Signs and Symptoms—Wetwood is wood that does not conduct water but has 

a watersoaked, dark appearance (figs. 278-280) and a fetid, fermentative odor. 

It is the normal condition of the heartwood in some species but may occur uncommonly in others. It may also appear in sapwood in response to wounds or 

biological attacks.

Because of bacterial fermentation, the wetwood liquid may be under pressure. 

In some areas where white fir is harvested, squirting of the foul liquid under 

pressure onto loggers has led to the appellation “piss-fir.” Externally, especially 

in elm, the fermented liquid may be exuded through cracks or branch stubs, leaving streaks of bleached or discolored bark (figs. 281-282). The exudate is often 

colonized by additional micro-organisms when it reaches the surface, leading 

to a thickened consistency and additional odors. Such liquid is called wetwood 

slime and the condition is called slime flux.

Figure 280. Wetwood in radial section of white fir. Note the narrow, 

dry transition zone between sapwood on left and wetwood. Photo: 

Jim Worrall, USDA Forest Service.

Figure 278. Wetwood in cross section of narrowleaf cottonwood. Photo: Jim Worrall, USDA Forest Service.

Figure 279. Wetwood in cross section of white fir. 

Photo: Jim Worrall, USDA Forest Service.
USDA Forest Service RMRS-GTR-241. 2010. 151

A B I O T I C I N J U R I E S A N D M I S C E L L A N E O U S

Impact—Wetwood inhibits wooddecay fungi. Wood that is wet 

has very low O2 availability, and 

the bacteria present in wetwood 

further reduce the O2 content to 

a level too low to support fungal 

growth. Bacteria also produce organic acids that strongly inhibit 

fungal growth. Thus, the bacteria 

generally seem to function more 

as beneficial symbionts, aiding 

the tree’s defense, rather than as 

pathogens.

In some species, most notably 

elm, wetwood fluid may develop 

pressure due to gasses produced 

by the bacteria. When this happens, the liquid may be forced 

into and kill adjacent sapwood and 

cambium. In severe cases foliar 

symptoms may result, including 

scorch, wilt, yellowing, and defoliation. The fermented liquid may 

seep out through branch stubs and 

cracks and discolor the bark surface as it flows down.

Wetwood is associated with a 

variety of problems when making wood products:

•Wood is more difficult and requires more energy to dr

•Wood dries unevenly and may warp and twis

•During kiln drying, vapors of the volatile organic acids (acetic, propionic

butyric) cause kiln corrosion.

•Wetwood is associated with ring shake and honeycomb, two lumber defects

Ring shake in elm has led to the term “onion elm” in the lumber trade.

References: 30, 158, 195, 196, 197

Figure 281. Bark bleaching 

and discoloration from 

wetwood fluid flowing from 

branch stub in elm. Photo: 

William Jacobi, Colorado State 

University, Bugwood.org.

Figure 282. Bark bleaching 

and discoloration from 

wetwood fluid flowing from 

crack in elm. Photo: Mike 

Schomaker, Colorado State 

Forest Service, Bugwood.org.
152 USDA Forest Service RMRS-GTR-241. 2010.
USDA Forest Service RMRS-GTR-241. 2010. 153
154 USDA Forest Service RMRS-GTR-241. 2010.

B A R K B E E T L E S 

Introduction to Bark Beetles

Bark beetles are the most destructive insects in western coniferous forests. It 

has been estimated that 90% of insect-caused tree mortality and more than 60% 

of the total insect-caused loss of wood growth in the United States is due to bark 

beetles.

The Rocky Mountain 

Region has a large complex of bark beetles 

composed of many genera 

and species. Frequently, 

several species are found 

attacking the same host 

tree, and therefore, it may 

be difficult to discern 

what species initiated the 

attack. Although species 

of Dendroctonus are the 

most significant tree killers in the western United 

States, other bark beetle species also play important roles in the conifer forests of 

the Rocky Mountain Region (fig. 283).

General Features—Bark beetles derive their name because they live and develop in the bark and wood of trees and shrubs. Adults excavate egg galleries in 

living bark (phloem). All bark beetle life stages are spent in the phloem, inner 

bark, and bark, except when adults leave the tree they developed in to fly to new 

host trees. Bark beetles feed on the phloem of their host trees during adult and 

larval stages.

Crowns of successfully attacked trees turn from green to yellow to reddish 

brown. This color change, an indication of a dying tree, may occur from a month 

to more than 2 years after successful attack, depending on the temperature, moisture conditions, and density of beetles in the tree. Close inspection of infested 

tree trunks will show either small globules of resin (pitch tubes), small holes 

through the bark, or reddish boring dust in bark crevices and around the tree 

base. The removal of bark from infested trees will reveal two types of galleries: 

egg and larval (fig. 284). Egg galleries constructed by adult beetles are rather 

uniform in width. Larval galleries depart at right angles from egg galleries and 

increase in size as the young grow.

Life Cycle—Bark beetles have four stages of development during their life: egg, 

larva, pupa, and adult. All stages are found under the bark. The small, white eggs 

are in niches along the sides of the egg gallery. Larvae are small, white grubs 

with distinct heads and no legs (fig. 285a). They are C-shaped and are found in 

the feeding galleries off the sides of the egg gallery. The pupae are found in small 

chambers at the end of the feeding galleries.

Figure 283. Adult bark beetles (from Hagle and others 2003).
USDA Forest Service RMRS-GTR-241. 2010. 155

Many bark beetles prefer 

weakened host trees, however, during environmental 

conditions favorable for beetle 

development, populations 

may build up rapidly and 

successfully attack healthy 

trees. Most bark beetles have 

a symbiotic relationship with 

blue-stain fungi. The blue 

stain fungi can completely 

penetrate the sapwood within 

a year. The fungi invade the 

living tissues in sapwood—the 

Figure 284. Bark 

beetle gallery patterns 

(from Hagle and others 2003).

Figure 285. Larval bark beetle compared to larvae of other common boring insects (from Hagle and others 2003).

B A R K B E E T L E S
156 USDA Forest Service RMRS-GTR-241. 2010.

B A R K B E E T L E S 

ray parenchyma and epithelial cells of resin canals—and cause the death of the sapwood. This action, plus the bark beetle feeding, causes the death of a host tree.

Many bark beetles produce chemical compounds called pheromones that are 

used to communicate with other beetles. Aggregation pheromones cause beetles 

to congregate in certain areas and mass-attack trees. Anti-aggregation pheromones cause beetles to disperse to neighboring trees or other areas.

Reference: 60

Ash Bark Beetles

Causes branch dieback on ash

Name and Description—Hylesinus spp. [Coleoptera: Curculionidae: Scolytinae]

Larvae of ash bark beetles are white, C-shaped grubs. Adults are small, gray 

beetles about 1/13-1/6 inch (2-4 mm) long with black markings (fig. 286).

Hosts—Ash

Life Cycle—Adults overwinter in tunnels under the bark of infested branches. 

In the spring, females begin constructing egg galleries in trees that are typically recently felled or weakened host 

trees. The egg galleries run perpendicular to the branch or trunk (fig. 287). Eggs 

are laid along the sides of the galleries. 

Larvae tunnel between the bark and the 

wood throughout the summer, feeding 

away from the egg gallery. There can 

be one to three generations per year, depending on location.

Figure 288. Ash bark beetle damage. Photo: James Solomon, 

USDA Forest Service, Bugwood.org.

Figure 286. Adult ash bark beetle. Photo: David 

Cappaert, Michigan State University, Bugwood.org.

Figure 287. Ash bark beetle galleries. 

Photo: James Solomon, USDA Forest 

Service, Bugwood.org.
USDA Forest Service RMRS-GTR-241. 2010. 157

Damage—Generally, the favored breeding material is recently cut or broken trees. 

Living trees weakened by mechanical damage or disease may also be attacked. 

Entrance, exit, and breathing holes can be found on the outside of infested trees 

(fig. 288). In July or August, the leaves on branches that have been girdled will turn 

yellow and then brown as the branch dies.

Management—Management is generally not warranted. As these beetles generally attack much-stressed trees, maintaining tree vigor will reduce impact. Infested branches can be removed prior to beetle emergence if necessary.

References: 39, 174

Aspen bark beetles

Attack stressed aspen

Name and Description—Trypophloeus populi Hopkins and Procryphalus mucronatus (LeConte) [Coleoptera: Curculionidae: Scolytinae]

Two closely related scolytid beetles (fig. 289) attack stressed aspen, and both 

are members of the tribe Cryphalini. 

Difficulty in distinguishing the two arises 

from their close taxonomic relationship 

and from their very small size. Both species of beetles are less than 1/10 inch 

(1.7-2.2 mm) long. While a number of 

characteristics can help determine probable species identity in the field and aid 

in identification when suitable specimens 

are unavailable (i.e., post-emergence), the 

only certain means of identification is collecting specimens for later examination in 

the laboratory.

Host—Aspen

Life Cycle—The natural history of these 

beetles has been described in detail in ref. 

134. This description includes information 

regarding the insects’ mating behaviors, 

gallery construction methods, and phenologies. For both of these species, the female 

initiates gallery construction and is soon joined by a male. Both beetles are monogamous species, and the male goes to some lengths to exclude rival males from the 

gallery. Following gallery construction and mating, a series of eggs are deposited 

in the gallery. Larvae feed upon phloem tissues in galleries extending from the egg 

galleries. The sudden appearance of the male at female initiated galleries indicates 

that some sort of aggregating pheromone may be utilized by these beetles. The exact 

nature of the pheromones and whether they serve as further isolating mechanisms is 

Figure 289. Adult Trypophloeus populi. Size 

and appearance of Procryphalus mucronatus is 

similar; however, note the distinct basal margin of 

the pronotum on this insect. The basal pronotum 

margin in P. mucronatus is rounded. Also, there is 

a lack of setae on the basal half of the pronotum of 

Trypophloeus populi, while Procryphalus mucronatus has setae covering the entire pronotum. Photo: 

Tom Eager, USDA Forest Service.

B A R K B E E T L E S
158 USDA Forest Service RMRS-GTR-241. 2010.

B A R K B E E T L E S 

Table 14. Comparison of aspen bark beetle phenology.

Overwinter Late May to mid-June Mid-June to Autumn

Procryphalus mucronatus Larvae, pupae, and Emergence, attack of Egg hatch and larval to

pre-emergent adults new host, and pupal stages with some

oviposition pre-emergent adults

Trypophloeus populi Larvae Completion of Adult emergence,

metamorphosis to oviposition, and egg hatch

pre-emergent adults to early larval instars

unknown. Despite the many similarities in the life history of these beetles, they do 

differ in phenology. These differences can be seen in table 14.

Damage—As indicated above, these two species of beetles attack aspens that are 

under stress. When discussing insects that attack trees, it is common to refer to insects that attack otherwise healthy trees as primary insects. Insects that specialize 

in attacking trees under stress or that are close to death are referred to as secondaries. Insects that feed upon completely dead trees can be considered tertiary insects. 

However, it has been noted that Trypophloeus populi attacks trees that still have a 

large component of “green bark,” while Procryphalus mucronatus is found in trees 

on which the bark is almost entirely dead. Thus, even though both of these species 

of insects are considered secondaries, there is still a degree of host selection discrimination based on tree vigor.

Other aspects of the beetles’ biologies fit into this construct that Trypophloeus 

populi attacks earlier in the process of host decline than does Procryphalus mucronatus. The galleries of Trypophloeus populi are typically stained black (fig. 290), 

which indicates that they carry a symbiotic fungus that helps them overcome the host 

tree defense mechanisms. The eggs of T. populi are laid in clusters, and the larvae 

hatch out to feed upon the fungal-infused host material. In contrast, Procryphalus 

mucronatus larvae are more spread 

out in the presumably nutrient deficient dead host tissues. A comparison 

of several of the key aspects of qualitative differences in the two species 

of beetles can be seen in table 15.

In addition to the characteristics 

that can be observed in the field, a 

number of morphological characteristics can be observed under a 

high-powered microscope. These 

distinguishing traits are summarized 

in table 16.

Management—The status of these two beetles as secondary insects indicates that 

the best strategy to reduce their impact is to maintain host vigor. The sudden prominence of these two previously obscure beetles is indicative of the relationship between recent drought and aspen health.

References: 134, 190, 191

Figure 290. Dark, fungal staining associated with 

Trypophloeus populi galleries. Photo: Jim Worrall, USDA 

Forest Service.
USDA Forest Service RMRS-GTR-241. 2010. 159

Blue Spruce Engraver

An important pest of ornamental blue spruce

Name and Description—Ips hunteri Swaine [Coleoptera: Curculionidae: Scolytinae]

The blue spruce engraver is a common bark beetle of Colorado blue spruce. 

The insect has become a problem in ornamental plantings of Colorado blue 

spruce along the Front Range of Colorado and southeastern Wyoming. In some 

forested situations, the blue spruce engraver can be found infesting the tops of 

Colorado blue spruce trees killed by the spruce beetle, Dendroctonus rufipennis. 

The beetle is approximately 1/8-1/4 inch (3-4 mm) long. The larva is a C-shaped, 

legless grub.

Host—Colorado blue spruce, Picea pungens, appears to be the only confirmed 

host for Ips hunteri, although Engelmann spruce, Picea engelmannii, is also 

listed as a host. This bark beetle has been recorded in Arizona, Colorado, Utah, 

and Wyoming.

Life Cycle—The blue spruce engraver has at least two generations per year. The 

biology of the blue spruce engraver is not known in any detail. Beetle flight 

Table 16. Morphological characteristics used to determine aspen bark beetle species in laboratory.

Funicle structure 

(segments of 

antennae below Antennal club

club) structure Pronotal setae Pronotal margins

Procryphalus mucronatus Four segmented Pointed club Pronounced No distinct, raised line

funicle setate only on on basal and lateral

anterior portion margins of pronotum

of pronotum

Trypophloeus populi Five segmented Elongate, oval Pronounced Distinct, raised line on

funicle club setae on entire basal and lateral

surface of margins of pronotum

pronotum

Table 15. Qualitative characteristics to distinguish two species of aspen bark beetles.

Gallery Egg Gallery Host Post-hatching

staining distribution construction condition bark surface

Procryphalus mucronatus No Singly along Generally deeper Tree either dead Bark remains

length of in wood; entrance or very close intact above

gallery nearly perpen- to death galleries

 dicular to bark 

 surface

Trypophloeus populi Black In clusters in Closer to surface Stressed tree Bark tends to

staining egg niches of bark; entrance but maintaining crack open

surrounding angled into bark green bark above galleries

galleries

B A R K B E E T L E S
160 USDA Forest Service RMRS-GTR-241. 2010.

B A R K B E E T L E S 

is thought to begin in early spring along 

Colorado’s Front Range.

Damage—The blue spruce engraver attacks weakened trees and windthrown 

trees. In urban plantings, however, 

the beetle infests the tops of drought 

stressed trees and progressively kills 

the trees over several years and multiple generations (fig. 291). The egg gallery (fig. 292) generally consists of two 

branches extending in opposite directions, and it may be diagonal, transverse, 

longitudinal, or curved. The beetle is occasionally seen killing Colorado blue 

spruce in forested situations (fig. 293).

Management—Homeowners should 

maintain adequate soil moisture levels 

for their Colorado blue spruce throughout the entire year with a regular watering schedule. Injury and additional stresses, such as construction activities that 

damage the root systems of the trees, should be avoided.

Figure 293. Aerial view 

of blue spruce engraver 

mortality. Photo: William 

M. Ciesla, Forest Health 

Management International, 

Bugwood.org.

Figure 292. Egg galleries of the blue spruce engraver. Photo: Whitney Cranshaw, Colorado State University, 

Bugwood.org.

Figure 291. Blue spruce that has been top-killed 

by the blue spruce engraver, Ips hunteri. Photo: 

William M. Ciesla, Forest Health Management 

International, Bugwood.org.
USDA Forest Service RMRS-GTR-241. 2010. 161

Ornamental plantings of Colorado blue spruce that have been top-killed by 

the blue spruce engraver should be removed because the trees generally decline 

quickly and contribute to the abundance of beetles in the area. In addition, stand 

sanitation and removal of windthrown branches can circumvent an increase in 

beetle populations. Protective insecticide sprays have been used successfully to 

prevent infestation; treatment should be completed by April along Colorado’s 

Front Range.

References: 35, 190

Cedar Bark Beetles

Can kill twigs, branches, or entire trees

Name and Description—Phloeosinus spp. [Coleoptera: Curculionidae: Scolytinae]

Cedar bark beetles are small, reddish brown to black beetles that are approximately 1/8 inch (3 mm) long. Larvae are small, cream-colored, legless grubs with 

brown head capsules similar to other bark beetle larvae. Cedar bark beetles are 

not known as aggressive tree killers but can kill twigs, branches, or entire trees.

Hosts—Rocky Mountain, Utah, and one-seed junipers, as well as ornamental 

and windbreak plantings of eastern red cedar.

Life Cycle—There is one generation per year. Beetles overwinter beneath the bark as larvae. Adults 

mature and fly to colonize new hosts from July 

through September. Timing depends on the particular beetle species and local weather conditions.

Damage—Cedar bark beetles typically colonize 

broken branches and trees stressed by drought, soil 

compaction, stem breakage, animal damage, and 

other similar factors. Beetles feed on twigs prior 

to brood production. Twigs are hollowed out and 

their tips die as a result. Dead twig/branch tips (often called flagging) may be scattered throughout a 

tree’s crown. This impact may or may not be significant, depending on factors such as number of 

flagged branches, tree size, and overall tree health. 

Cedar bark beetles’ most significant impact is when 

they colonize a tree’s branches and trunk to produce 

brood beneath the bark. Death of major branches or 

of the entire tree may result.

Evidence of beetle colonization, though often 

difficult to see, includes the presence of very fine 

boring dust in bark crevices and around the base of 

the trunk. If boring dust is present, distinct galleries 

Figure 294. Galleries of cedar 

bark beetles that are etched into 

the face of sapwood. Photo: 

William M. Ciesla, Forest Health 

Management International, 

Bugwood.org.

B A R K B E E T L E S
162 USDA Forest Service RMRS-GTR-241. 2010.

B A R K B E E T L E S 

can likely be detected by removing a small section of bark. The gallery pattern 

will be etched into the face of the sapwood (fig. 294). Crowns of infested trees 

will fade from green to yellow and eventually to red. By the time the entire tree 

crown is red, beetles have likely matured and are gone, having flown to other 

host trees. If so, many tiny, dust-free holes should be evident on the surface of 

the bark.

Management—Management of cedar bark beetles is seldom necessary in general forest settings. Preventive insecticide sprays, applied in late spring or early 

summer, have been used to protect individual high-value trees in recreational and 

historic settings and within windbreaks.

References: 50, 113

Douglas-Fir Beetle

Attacks and kills Douglas-fir trees

Name and Description—Dendroctonus pseudotsugae Hopkins [Coleoptera: 

Curculionidae: Scolytinae]

The Douglas-fir beetle is a common bark beetle that kills Douglas-fir trees. 

Adult beetles are cylindrically-shaped and about a 1/4 inch (6 mm) long. The 

head and thorax are black, and wing covers are reddish brown. Eggs are white 

and very small (1/25 inch [1 mm] long). Larvae are legless and white with light 

brown heads. Larvae can grow up to 1/4 inch (6 mm) long. Pupae are white, and 

some adult features are often present.

Hosts—Douglas-fir

Life Cycle—Douglas-fir beetles have a 

1-year life cycle and overwinter as adults or 

larvae. Beetles usually emerge mid to late 

spring, when the temperature is 60° F and 

above. However, a small portion of beetles 

emerge later in midsummer. Some adults 

that make early spring attacks can reemerge 

and make a second attack from late June to 

August. Distinctive vertical egg galleries 

(5-12 inches [13-30 cm] long) are constructed 

by the female in the phloem layer (fig. 295). 

Eggs are laid in groups, alternating along opposite sides of the gallery. Eggs hatch in 1-3 

weeks, and newly hatched larvae mine out at 

right angles from the egg gallery. Mature larvae construct a pupal chamber at the end of 

their mines. Figure 295. Douglas-fir beetle egg and larval 

galleries. Photo: Kenneth Gibson, USDA 

Forest Service, Bugwood.org.
USDA Forest Service RMRS-GTR-241. 2010. 163

Damage—The larvae feed under the bark in 

the phloem layer, introducing fungi, yeasts, and 

other organisms, and lead to tree death. The first 

sign of attack is reddish orange frass in bark 

crevices that is expelled by attacking beetles (fig. 

296). However, frass can wash away and attacks 

may be above eye-level, making it difficult to 

locate attacked trees. Pitch-tubes are not usually 

present, but many trees will have pitch streaming 

(clear resin) down the tree bole from the top of 

the beetle-colonized area. Tree foliage discolors 

several months to a year later, transitioning from 

green to reddish brown in that time.

Douglas-fir beetles prefer to attack trees 

that are injured by fire scorch, defoliation, 

windthrown, or root disease. When low beetle 

populations are present, individual or small 

groups of trees will be attacked. Once populations build up, large outbreaks can occur that kill 

thousands of trees. Stand conditions and weather 

can also strongly influence Douglas-fir beetle 

populations.

Management—The best management is to promote stand vigor by thinning. 

Prompt removal of windthrown, severely fire-damaged trees or trees damaged 

by other stand disturbances is also recommended. Because Douglas-fir beetles 

preferentially attack burned trees, removing fuels from beneath large-diameter 

Douglas-fir trees before a prescribed burn can reduce tree scorch and, consequently, the tree’s susceptibility to attack by Douglas-fir beetle. Attacks are most 

severe in unmanaged stands, on trees that are largest in diameter, and in dense 

stands. If direct control is deemed necessary, trees can be protected using the anti-aggregation pheromone methylcyclohexanone (MCH), which disrupts beetle 

aggregation. Combining MCH with salvage of infested trees has been successful 

at reducing subsequent tree mortality. However, under condition of intense or 

long-lived outbreaks, even MCH has sometimes failed to protect trees. Direct 

control is usually implemented in small, high-value areas.

References: 50, 150

Douglas-Fir Pole and Engraver Beetles

Attack small Douglas-fir trees

Name and Description—Douglas-fir pole beetle—Pseudohylesinus nebulosus

(LeConte)

Douglas-fir engraver beetle—Scolytus unispinosus LeConte

Figure 296. Reddish orange frass from 

Douglas-fir beetle attack. Photo: Sandy 

Kegley, USDA Forest Service, Bugwood.

org.

B A R K B E E T L E S
164 USDA Forest Service RMRS-GTR-241. 2010.

B A R K B E E T L E S 

An engraver beetle—Scolytus monticolae (Swaine) (= S. tsugae [Swaine])

[Coleoptera: Curculionidae: Scolytinae]

Douglas-fir pole and engraver beetles attack small-diameter Douglas-fir trees 

and tops of larger trees. They are commonly active during droughty periods. The 

Douglas-fir pole beetle adults are brown, slender, about 1/8 inch (3 mm) long, 

appear dull due to the dense covering of scales, and have a round posterior. The 

engraver beetles average less than 1/8 inch (3 mm) long and have a “sawed-off” 

posterior.

Hosts—Douglas-fir is the principal host.

Life Cycle—Depending on the location, 

Douglas-fir pole beetles and engraver beetles have one to two generations per year. 

Beetles usually emerge and attack in the 

spring. A short (1-3 inches [2.5-7.6 cm]), 

longitudinal egg gallery is constructed in 

the cambium layer, often with two branches—one up and one down the trunk—originating from the central entrance tunnel. 

The Douglas-fir engraver beetle gallery 

can be unbranched, extending in one direction from an enlarged chamber or notch. The galleries of the Douglas-fir engraver beetles (fig. 297) 

can be distinguished from the galleries of Douglasfir pole beetle (fig. 298) by the well-defined nuptial 

chamber made by engraver beetles. Larval galleries 

tend to turn upward and downward depending on if 

they are above or below the notch. Douglas-fir pole 

beetle adults overwinter in niches cut into the bark. 

Douglas-fir engraver beetles overwinter as larvae.

Damage—These beetles cause mortality in smaller 

trees and top-kill or branch-kill in larger trees, with 

occasional mortality in larger trees. They also commonly attack thin-barked portions of logging slash. 

The larvae feed under the bark in the phloem layer. 

They can be one of several agents that kill a tree. The 

size of the emergence holes and distinctive galleries distinguish the species from other beetles such as 

Douglas-fir beetle.

Douglas-fir pole beetles and engraver beetles 

prefer to attack trees that are injured by fire scorch, 

defoliation, blowdown, or root disease. Stand conditions and weather can also 

strongly influence beetle populations. Under drought conditions, they have been 

known to attack and kill Douglas-fir as large as 12 inches (30 cm) in diameter.

Figure 297. Galleries of Douglas-fir engraver 

beetle, Scolytus unispinosus. Photo: Wayne 

Brewer, Auburn University, Bugwood.org.

Figure 298. Galleries of 

Douglas-fir pole beetle, 

Pseudohylesinus nebulosus. 

Photo: Karen Ripley, Washington 

Department of Natural Resources.
USDA Forest Service RMRS-GTR-241. 2010. 165

Management—Because Douglas-fir pole beetles and engraver beetles are secondary insects associated with trees under stress, enhancing tree/stand quality 

will help to prevent attacks. The best management approach is to promote stand 

vigor by thinning and promptly removing windthrown trees or trees damaged by 

other stand disturbances.

References: 35, 50

Elm Bark Beetles

Native and introduced bark beetles of elm

Name and Description—Native elm 

bark beetle—Hylurgopinus rufipes

Eichhoff

Smaller European elm bark beetle—Scolytus multistriatus (Marsham)

Banded elm bark beetle—S. 

schevyrewi Semenov [Coleoptera: 

Curculionidae: Scolytinae]

Three species of bark beetles are 

associated with elms in the United 

States: (1) the native elm bark beetle 

(fig. 299) occurs in Canada and south 

through the Lake States to Alabama 

and Mississippi, including Kansas and 

Nebraska; (2) the introduced smaller 

European elm bark beetle (fig. 300) occurs throughout the United States; and 

(3) the introduced banded elm bark 

beetle (fig. 301) is common in western 

states and is spreading into states east of 

the Mississippi River. Both the smaller European elm bark beetle and the 

banded elm bark beetle were introduced 

into the United States from Europe 

and Asia, respectively. Hylurgopinus 

rufipes adults are approximately 1/12-

1/10 inch (2.2-2.5 mm) long; Scolytus 

multistriatus adults are approximately 

1/13-1/8 inch (1.9-3.1 mm) long; and S. 

schevyrewi adults are approximately 

1/8-1/6 inch (3-4 mm) long. The larvae 

are white, legless grubs.

Figure 299. Native elm bark beetle. Photo: J.R. 

Baker and S.B. Bambara, North Carolina State 

University, Bugwood.org.

Figure 300. Smaller European elm bark 

beetle. Photo: J.R. Baker and S.B. Bambara, North 

Carolina State University, Bugwood.org.

Figure 301. Banded elm bark beetle. Photo: Jim 

LaBonte, Oregon Department of Agriculture.

B A R K B E E T L E S
166 USDA Forest Service RMRS-GTR-241. 2010.

B A R K B E E T L E S 

Hosts—Hosts for the native elm bark beetle include the various native elm species in the United States and Canada, while the introduced elm bark beetles also 

infest introduced species of elms, such as English, Japanese, and Siberian elms. 

American elm is the primary host tree for the native elm bark beetle. Siberian 

elm is the native host tree species for the banded elm bark beetle in Asia. Siberian 

elm is a host for the smaller European elm bark beetle in the United States but 

not in Europe because the tree species’ range does not overlap the beetle’s native 

range in Europe.

Life Cycle—The native elm bark beetle has a variable life cycle depending on 

latitude, with two generations in the southern portion of its range and a 1-year 

life cycle in the northern portion of its range. The egg gallery for this species has 

two branches and barely etches the wood (fig. 302). It is not known whether the 

native elm bark beetle utilizes aggregation pheromones.

The smaller European elm bark beetle and the banded elm bark beetle have 

two generations per year in most locations and may have a third generation in 

some warmer locations. Beetles fly in the spring and infest dying elms. The egg 

galleries have a single branch and generally score the wood surface (figs. 303-

304). Larvae develop through the summer months and overwinter as larvae in 

brood trees. The smaller European elm bark beetle produces an aggregating 

pheromone, but the banded elm bark beetle apparently does not produce one. 

Host odors play an important role in both of these introduced species.

Damage—The first sign of attack is the accumulation of boring dust around 

the base of the tree. Trees infected with Dutch elm disease, caused by the 

Figure 302. Native elm bark beetle 

egg gallery. Photo: Roland J. Stipes, 

Virginia Polytechnic Institute and 

State University, Bugwood.org.

Figure 303. Smaller European 

elm bark beetle egg gallery. Photo: 

William M. Ciesla, Forest Health 

Management International, Bugwood.

org.

Figure 304. Banded elm bark 

beetle egg gallery. Photo: Jose 

Negron, USDA Forest Service.
USDA Forest Service RMRS-GTR-241. 2010. 167

exotic fungal pathogen, Ophiostoma novo-ulmi, are highly 

attractive to elm bark beetles 

(fig. 305). The native elm bark 

beetle undergoes a period of 

feeding on healthy elms before 

seeking a dying tree for gallery 

construction and brood production, which provides an opportunity for adults to transmit the 

Dutch elm disease pathogen to 

uninfected elms. The smaller 

European elm bark beetle is 

an efficient vector of Dutch 

elm disease in American elms 

because newly emerged adults 

undergo a period of maturation 

feeding in tunnels on branches 

of healthy elm canopies prior 

to dispersing to dying elms for 

gallery construction, thereby 

creating an opportunity to transmit the fungal pathogen to healthy trees. The 

banded elm bark beetle appears to have the same behavior and is known to carry 

the fungal pathogen when brood beetles leave diseased trees, but it has not yet 

been confirmed as a vector of Dutch elm disease.

Management—The native elm bark beetle is not an aggressive species and was 

not a major pest problem prior to the introduction of Dutch elm disease. The 

smaller European elm bark beetle and the banded elm bark beetle are serious 

pests of native and introduced elms. The banded elm bark beetle is more aggressive than the smaller European elm bark beetle and appears to be displacing this 

species in western states. Drought stress predisposes exotic elm species to infestation by these two introduced elm bark beetle species. In urban settings, rapid 

removal of Dutch elm disease-infected American elms followed by destruction 

of the host material is the key to successfully managing these two species.

Reference: 190

Fir Engraver

Horizontal egg galleries

Name and Description—Scolytus ventralis LeConte [Coleoptera: Curculionidae: Scolytinae]

The fir engraver is an important bark beetle of true firs. In the central 

Rocky Mountains, this insect is most frequently observed in white fir, Abies 

Figure 305. American elm dying from Dutch elm disease. 

Photo: Jose Negron, USDA Forest Service.

B A R K B E E T L E S
168 USDA Forest Service RMRS-GTR-241. 2010.

B A R K B E E T L E S 

concolor. Epidemics of fir engraver are 

observed most frequently in the Pacific 

Northwest and California and are often 

associated with periods of drought. The 

adults are 1/10-1/7 inch (2.5-3.8 mm) 

long (fig. 306). The larvae are small, 

white, legless grubs.

Egg galleries are transverse, often 

with a visible nuptial chamber at the 

center of the two arms of the gallery. 

The female deposits eggs in niches on 

both sides of the gallery. The larval galleries are longitudinal, and both the egg 

and larval galleries score the wood deeply 

(fig. 307). Pupation occurs in the inner 

bark at the end of the larval galleries.

Hosts—In the southern Rocky Mountains, 

this insect is most commonly found in 

white fir. Throughout the West, the fir engraver may be found in a variety of true fir 

species, including white fir, grand fir, and 

California red fir and may occasionally be 

found infesting other species, including 

Douglas-fir. The range of this bark beetle 

includes British Columbia south through 

the Pacific Northwest and Rocky Mountains into New Mexico and Arizona.

Life Cycle—The fir engraver has a 1-year 

life cycle except in cooler portions of its 

range where complete development takes 

2 years. In Colorado, flight may occur any 

time from early spring until early fall. The 

fir engraver is monogamous, and females 

initiate attack on host trees. Apparently, 

this species does not utilize aggregating pheromones during its attack, and the 

dynamics of attack appear to be associated with primary host volatiles alone. The 

fir engraver transports a brown-staining fungus that is important for successful 

development of the brood larvae.

Damage—The fir engraver infests boles, large branches, slash, and windthrown 

trees larger than 4 inches (10 cm) in diameter. Trees infected with root disease or 

defoliated by Douglas-fir tussock moth or western spruce budworm are especially subject to attack. Trees may be killed outright (fig. 308) or attacked repeatedly 

for multiple years, leading to patches of dead bark. The beetles also may attack 

the tops of trees, causing top-kill.

Figure 306. Fir engraver adult. Photo: Don Owen, 

California Department of Forestry and Fire Protection, 

Bugwood.org.

Figure 307. Fir engraver egg and larval galleries. Photo: Intermountain Region, USDA Forest 

Service, Bugwood.org.
USDA Forest Service RMRS-GTR-241. 2010. 169

Management—Maintaining good tree vigor 

in ornamental settings is 

important for preventing losses to fir engraver. 

Also, care during construction and soil disturbing activities should 

be practiced to limit root 

and lower stem damage. 

Natural controls include 

predators and parasitoids. Direct control 

methods are considered 

impractical due to the 

tendency of beetles to be common in portions of living trees that are out of 

view from the ground. Insecticides labeled for bark beetles will provide protection from fir engraver attack.

Reference: 190

Limber Pine Engraver

Attacks branches and boles

Name and Description—Ips woodi Thatcher [Coleoptera: Curculionidae: 

Scolytinae]

The limber pine engraver is a common bark beetle in the central Rocky 

Mountains. The beetle is found across the range of limber pine, including in the 

states of Arizona, California, Colorado, Idaho, Montana, New Mexico, Utah, and 

Wyoming and in the province of Alberta, Canada. 

The beetle is approximately 1/8-1/5 inch (3.5-4.7 mm) 

long (fig. 309). The larva is a C-shaped, legless grub.

Hosts—Host trees for I. woodi include limber pine 

(fig. 310) and southwestern white pine. It seems 

likely that I. woodi also will infest Rocky Mountain 

bristlecone pine, which often occurs with limber 

pine at high elevations in the central Rocky Mountains, although this species is not listed as a host tree.

Life Cycle—There is no available information concerning the biology of I. 

woodi. Beetles have been observed attacking white pine blister rust-infected limber pine in spring, and brood beetles have been observed under the bark in late 

summer.

Damage—Ips woodi infests large limbs and boles of weakened or droughtstressed limber pines and fallen trees. The beetle is common in limber pines 

Figure 309. Ips spp. beetle adult. 

Image: Whitney Cranshaw and 

David Leatherman, Colorado State 

University.

Figure 308. White fir mortality caused by fir engraver on the Rio 

Grande National Forest. Photo: William M. Ciesla, Forest Health 

Management International, Bugwood.org.

B A R K B E E T L E S
170 USDA Forest Service RMRS-GTR-241. 2010.

B A R K B E E T L E S 

severely infected with white pine blister rust. The beetle also can be found 

in the upper crowns of limber pines 

attacked by the mountain pine beetle, 

Dendroctonus ponderosae. The galleries are typical Ips spp. galleries—longitudinal in orientation, resembling a narrow tuning fork.

Management—Although this bark beetle has not received much attention by 

entomologists, it is of some importance with regard to managing limber pines 

(fig. 311) for potential genetic resistance to white pine blister rust. Several limber pines have been identified with significant levels of resistance to this exotic 

fungal pathogen. Maintaining these trees in a healthy, cone-producing condition 

will be important in characterizing the source of this resistance.

Reference: 190

Lodgepole Pine Beetle

An uncommon lodgepole pine bark beetle

Name and Description—Dendroctonus murrayanae Hopkins [Coleoptera: 

Curculionidae: Scolytinae]

The lodgepole pine beetle (fig. 312) is a rarely encountered species in the 

genus. This bark beetle is not thought to be an aggressive species, although it is 

sometimes associated with tree mortality. The lodgepole pine beetle resembles 

Figure 310. Range of limber pine in western 

North America. Image: Anna Schoettle, USDA 

Forest Service.

Figure 311. Limber pine tree in Colorado. Photo: Anna 

Schoettle, USDA Forest Service.
USDA Forest Service RMRS-GTR-241. 2010. 171

the more common spruce beetle, 

D. rufipennis, based on external 

features and coloration of the adult 

and the general shape of the egg 

galleries. The beetles are most 

commonly encountered in the 

lower 2 ft (61 cm) of the bole of 

lodgepole pine trees and may be 

associated with trees attacked 

by the mountain pine beetle, D. 

ponderosae. The adults are approximately 1/4 inch (5.0-7.3 mm) 

long and are dark brown with reddish 

brown wing covers. The larvae are 

white, legless grubs.

Hosts—Hosts for the lodgepole pine 

beetle include lodgepole pine, jack 

pine, and eastern white pine. In the 

central Rocky Mountains, the beetle 

has been collected on the Pike National Forest in Jefferson County, 

Colorado, and on the Medicine Bow, 

Bighorn, and Shoshone National Forests in Wyoming.

Life Cycle—The biology of this 

species is not well-documented. 

The life cycle is thought to be approximately one generation per year. The 

adults disperse in June or early July through early September, with initial 

attacks observed by mid-July. The egg galleries (fig. 313) are vertical and 

approximately 5 inches (13 cm) long, and the eggs are deposited in groups of 

20-50 in broad niches on either side of the egg gallery. The larvae feed together 

collectively, beginning perpendicular to the egg gallery and then turning up or 

down. Larvae may finish feeding in separate galleries and prepare a chamber 

for pupation, or they may pupate within the frass of the main larval feeding 

area.

Damage—The beetle has been found infesting stumps, windfalls (where they 

prefer the underside of the stem), and the lower 2 ft (61 cm) of weakened trees 

larger than 8 inches (20 cm) DBH (fig. 314).

Management—Although the lodgepole pine beetle has been associated with 

epidemics of other bark beetles, its relative significance in these events is 

not fully understood. In most situations, this species attacks and kills trees 

that are severely damaged or are dying from other causes. In managed 

stands, the beetle is found infesting the stumps and larger roots of cut trees. 

Figure 312. Lodgepole pine beetle adult. Photo: Dan Jensen, 

University of Alberta.

Figure 313. Egg galleries of the lodgepole pine beetle. 

Photo: Brian Howell, USDA Forest Service.

B A R K B E E T L E S
172 USDA Forest Service RMRS-GTR-241. 2010.

B A R K B E E T L E S 

The lodgepole pine beetle has 

never been associated with significant economic losses. As a result, 

management options for this bark 

beetle have never been developed.

Significant controversy surrounds this beetle because of its 

similarity to the spruce beetle and 

because of reports of spruce beetle 

killing lodgepole pine during the 

Flat Tops spruce beetle epidemic 

in Colorado in the late 1940s and 

early 1950s. Although the lodgepole pine mortality was attributed 

to the spruce beetle, specimens that 

were collected during the epidemic 

and placed in museums all proved 

to be the lodgepole pine beetle. In 

a recent spruce beetle epidemic in 

the Medicine Bow Mountains of 

Wyoming, spruce beetles were observed attacking lodgepole pine trees where 

this species was mixed with Engelmann spruce. Surprisingly, only trees that 

were also attacked by the lodgepole pine beetle died. In the absence of lodgepole pine beetle, spruce beetle-attacked lodgepole pine trees survived and 

spruce beetles died inside their galleries, exited their galleries and died at the 

base of the tree, or abandoned their galleries.

References: 51, 190

Mountain Pine Beetle

Aggressive bark beetle of western pines

Name and Description—Dendroctonus ponderosae Hopkins [Coleoptera: Curculionidae: Scolytinae]

The mountain pine beetle is the most aggressive, persistent, and destructive 

bark beetle in the western United States and Canada. Adult beetles are dark 

brown to black, cylindrically-shaped, and 1/4 inch (4-7.5 mm) long (fig. 315). 

Larvae are small, white grubs with tan head capsules.

Hosts—Most native and introduced species of pines are hosts for mountain pine 

beetle. In the Rocky Mountain Region, ponderosa, lodgepole, whitebark, limber, 

and bristlecone pines are all attacked. During large outbreaks, Engelmann and 

blue spruce have been attacked and successfully colonized.

Life Cycle—Mountain pine beetles overwinter mostly as larvae beneath (or within) the inner bark of host trees. Occasionally, pupae and callow adults may also 

overwinter. In most lodgepole and ponderosa pine stands, larvae pupate at the ends 

Figure 314. Lodgepole pine trees attacked at the base 

by the lodgepole pine beetle. Photo: Brian Howell, USDA 

Forest Service.
USDA Forest Service RMRS-GTR-241. 2010. 173

of their feeding galleries in late spring. 

Adults emerge and attack from about 

early July through August, depending 

on elevation and temperature (fig. 316). 

Egg galleries are more or less straight 

and vertical and may be up to 30 inches 

(76 cm) long. Eggs are laid along each 

side of the gallery in individual niches. 

Both niches and egg galleries are tightly 

packed with frass. Eggs hatch and larvae 

feed until freezing temperatures cause 

dormancy. 

Extremely 

cold temperatures 

(prolonged 

p e r i o d s 

b e l o w 

-30°F) can 

cause significant mortality and can lead to an outbreak reduction or end.

Damage—This is one of the few bark beetles that usually make obvious pitch tubes 

on bark surface at the attack site (fig. 317). 

Pitch tubes are masses of red, amorphous 

resin mixed with bark and wood borings. 

Boring dust is evident in bark crevices and 

around the base of infested trees. Under 

bark, look for straight, vertical egg galleries with a crook or J shape at the beginning, 

which can extend upward 30 inches (76 cm) 

or more (fig. 318). Galleries are packed 

tightly with boring dust. Larvae (grubs) are 

present during fall and winter. Most pupate 

in late spring, and adults emerge from the 

bark in midsummer to attack new trees. Infested trees fade within a year from 

yellow-green to red-brown. Hosts may have their bark removed by woodpeckers 

searching for larvae.

Management—Outbreaks usually develop in mature to over-mature forests. 

Large reserves of these forests pose a constant hazard in areas climatically favorable for the mountain pine beetle. In addition, management plans for reserved 

areas such as parks and wildernesses should consider the need for protection 

against destructive outbreaks. Management should focus on forests and not on 

the beetle by altering stand conditions that favor buildup of beetle populations. 

There are two basic approaches to reducing losses from mountain pine beetle 

Figure 315. Adult mountain pine beetle. Photo: 

Erich G. Vallery, USDA Forest Service, Bugwood.

org.

Figure 316. Mountain pine beetle life cycle (from Johnson 1982).

Figure 317. Pitch tubes and boring dust at site 

of mountain pine beetle attack. Photo: Kenneth 

E. Gibson, USDA Forest Service, Bugwood.org.

B A R K B E E T L E S
174 USDA Forest Service RMRS-GTR-241. 2010.

B A R K B E E T L E S 

in pine forests: (1) long-term (preventive) forest management, and (2) direct 

control.

The strategy of preventive management is to keep beetle populations 

below injurious levels by limiting the 

beetles’ food supply through forestry 

practices designed to maintain or increase tree/stand resistance. Preventive 

management addresses the basic cause 

of epidemics—stand susceptibility—

and is considered the most satisfactory 

long-term solution. It includes a combination of hazard rating, priority setting, 

and silvicultural manipulations. In 

contrast, suppression of beetle populations, (that is, killing them by various 

methods of direct control) treats only 

the problem of too many beetles.

Methods of direct control include 

felling and burning, debarking, or solarizing infested trees. Effects of direct 

control are only temporary, so the treatment must be implemented yearly as 

long as beetle infestations exist. When properly used, direct control might be 

effective both in reducing the rate of the spread and the intensification of infestations; but it should be considered only a holding action until susceptible stands 

can be altered silviculturally. Individual trees can also be protected from fatal 

attacks through the use of chemicals applied prior to tree infestation (preventive 

sprays). This treatment must be repeated as long as beetle infestations exist in the 

area. Preventive sprays have been used in combination with treatments such as 

sanitation (removing beetle-infested trees) and thinning to increase overall stand 

health while protecting individual high-value trees. Use of preventive spray 

treatments in Rocky Mountain Region recreation areas should be supported by a 

site-specific vegetation management plan.

References: 3, 50, 93

Pine Ips Species (Engraver Beetles)

Attracted to green slash

Name and Description—Ips spp. [Coleoptera: Curculionidae: Scolytinae]

Adult beetles are cylindrical, dark red-brown to black, and typically 1/8-3/16

inch (3-5 mm) long. Some species may be as long as 1/4 inch (6 mm). They have 

Figure 318. Mountain pine beetle gallery has a 

crook at the end. Larvae are in feeding galleries. 

Photo: Ladd Livingston, Idaho Department of Lands, 

Bugwood.org.
USDA Forest Service RMRS-GTR-241. 2010. 175

a dish-shaped depression on the end of their abdomens with spines along each 

side (fig. 319). Larvae are typical C-shaped and are indistinguishable from other 

bark beetle species larvae.

Hosts—All pine species are attacked.

Life Cycle—Normally, there are two generations of the beetle each year (fig. 320). 

In dry years, three or even four generations may occur. Winter is passed primarily in 

the adult stage beneath the duff on the forest floor or within infested material. Adults 

become active early in the spring, infesting fresh slash or winter-damaged trees. Initial flights vary with weather but probably occur most often in late April to early 

May. This brood develops into adults after 40-55 days, and they attack slash and 

standing trees by August. Attacks are initiated by male beetles, which construct 

nuptial chambers beneath the bark. 

Each one then attracts several females, 

which, after mating, construct egg galleries radiating from the nuptial chamber 

(fig. 321). 

Egg galleries are 

kept free 

of boring 

dust and 

frass, unlike those 

of many 

other bark beetles. Beetles prefer fresh debris 

from logging, construction activity, or natural 

events, but living trees may be attacked during outbreaks.

Damage—In standing trees, fading tops of 

large trees or whole crowns in small trees 

can be indicators of Ips spp. infestation (fig. 

322). Other external evidence consists of 

accumulations of boring dust in bark crevices and at the base of the tree (fig. 323). 

Occasionally, pitch tubes can be found on 

the trunk. Characteristic egg galleries may 

be found under the bark, slightly engraving 

Figure 319. Adult Ips pini beetle with depression 

and spines on end of elytra. Photo: Natasha Wright, 

Florida Department of Agriculture and Consumer 

Services, Bugwood.org.

Figure 320. Life cycle of pine engraver beetles (from Johnson 1982).

Figure 321. Egg galleries of Ips sp. 

radiating from nuptial chamber. Photo: 

Jerald E. Dewey, USDA Forest Service, 

Bugwood.org.

B A R K B E E T L E S
176 USDA Forest Service RMRS-GTR-241. 2010.

B A R K B E E T L E S 

the sapwood, hence the common name, engraver beetle. In slash, look for boring 

dust and galleries.

Management—Most pine engraver problems are associated with disturbances 

such as windthrow and ice breakage, drought in spring and early summer, thinning, logging, fires, road construction, housing development, or tops of trees 

being weakened or killed by other agents. Pine engraver beetles overwinter in 

the adult stage and normally infest green slash only in the spring. Logging slash 

created from December through June can be especially hazardous because it 

provides large amounts of breeding material. Slash should not be created during 

this time period unless it can be treated prior to beetle emergence. During years 

of extremely low spring soil moisture, overwintering beetles have been known 

to attack and kill living trees. Silvicultural strategies are effective, particularly 

thinning stands to maintain tree resistance to attack. Thinned, vigorous stands of 

ponderosa pine are less attractive to pine engraver beetles. During drought years, 

stand vigor is even more important. Stands in which basal area has been reduced 

to 80-100 square ft per acre (18-23 square m per hectare) have been found to 

be less susceptible to beetle attack. Recently thinned stands may temporarily 

be more attractive because of the presence of fresh slash or logging damage to 

residual trees.

References: 50, 93, 103

Figure 322. Top-kill from Ips pini attack. Photo: 

William M. Ciesla, Forest Health Management 

International, Bugwood.org.

Figure 323. Boring dust on bark of pine attacked 

by pine engraver beetles. Photo: Kenneth E. 

Gibson, USDA Forest Service, Bugwood.org.
USDA Forest Service RMRS-GTR-241. 2010. 177

Pinyon Ips

Causes death of mature pinyon pines

Name and Description—Ips confusus (LeConte) [Coleoptera: Curculionidae: 

Scolytinae]

The pinyon ips (fig. 324) is a small, 

brown, cylindrical bark beetle with spines 

on the distal portion of the abdomen, 

which is typical for this genus of bark 

beetles. Pinyon ips has five such spines. 

The adult’s length ranges between 1/8 and 

1/4 inch (3 and 6 mm).

Host—The pinyon ips is recognized as 

a primary cause of mortality for mature 

pinyon pines. Within Colorado, it attacks 

two-leaf pinyon (Pinus edulis), but other 

pinyon pine species serve as hosts to this 

insect in other areas.

Life Cycle—Adult beetles attack potential hosts in the warmer months of the 

year, typically from March until the end of October. There are two and one-half 

to three generations per year, although overlapping broods are quite common. 

The adult male initiates attacks in the bole and larger branches of the host and 

constructs a central nuptial chamber. Males release pheromones that attract two 

to four responding females who then mate 

with the male and construct individual egg 

galleries radiating away from the central 

nuptial chamber. The resulting gallery pattern frequently appears to have a Y shape 

to it (fig. 325). The females lay numerous 

eggs in the galleries, and the resulting larvae feed beneath the bark until they are 

ready to pupate. The mature larvae create 

individual pupal chambers, from which 

they emerge through the bark to seek out 

new hosts. During the winter months, large 

numbers of adult beetles seek overwintering sites in the lower boles of host trees. 

While no brood is produced from these 

“feeding galleries,” (fig. 326) large numbers of overwintering beetles can effectively girdle and kill the host tree.

Damage—Trees that have been attacked 

are fairly conspicuous. The initial phases 

Figure 325. Typical Y-shaped gallery of pinyon 

ips. Photo: Tom Eager, USDA Forest Service.

Figure 324. Pinyon ips adult (Ips confusus). Photo: 

William M. Ciesla, Forest Health Management 

International.

B A R K B E E T L E S
178 USDA Forest Service RMRS-GTR-241. 2010.

B A R K B E E T L E S 

of attack are notable for 

the large amounts of resin 

or “pitch” that readily flow 

from the attack site. This 

pitch flow constitutes a major component of the host tree’s defense system, and 

the phenomenon of mass attack that is driven by pheromone release serves to attract enough beetles to exhaust this defense. Periods of low moisture availability 

mean that less resin is produced by the tree, thus decreasing the tree’s defenses.

Like other bark beetles, pinyon ips can operate in either an endemic or epidemic fashion. Most of the time, populations of this insect are sparse, and the 

insects persist in pinyon stands by attacking damaged or stressed host trees. 

Mechanical damage, fire injury, drought, and stress created by other damaging 

agents (particularly black stain root disease) often increase host tree susceptibility. Human activity that results in tree damage to any portion of the trees, 

including roots and branches, can create habitat suitable for the beetles. In addition to the stress created by damage to host trees, mechanical wounding of trees 

releases volatile compounds found in tree resin that are particularly attractive 

to Ips spp. beetles. Small outbreaks of the beetle are often initiated by thinning, 

road or structural construction, or other similar activities.

When large numbers of suitably stressed host trees become available, pinyon ips populations can increase rapidly and kill large numbers of pinyon trees. 

During periods of outbreak conditions, beetle populations can increase in susceptible stands and then spread to adjacent, unsusceptible stands, killing large 

numbers of host trees. Vast acreages of pinyon trees can be affected at times; a 

noteworthy outbreak from 2002 to 2004 killed millions of pinyon pines over an 

area that included portions of six southwestern states.

Management—Over the long run, the most economical and efficient means of 

protection is maintenance of pinyon trees and stands in as thrifty a condition as 

possible. Reduced stocking and sanitation of damaged or diseased trees will reduce the chance that pinyon ips can build up in susceptible hosts and then emerge 

to attack additional trees. High-value trees in landscaping and recreational settings can be protected by using a chemical protective spray. Care must be taken 

to ensure complete coverage of the tree, as insufficient treatments will result in 

the death of the tree. During periods of drought, supplemental watering may also 

provide a degree of protection to stressed trees.

References: 42, 46, 50, 123

Figure 326. Overwintering or 

“feeding galleries” of pinyon ips. 

Photo: Tom Eager, USDA Forest 

Service.
USDA Forest Service RMRS-GTR-241. 2010. 179

Pinyon Twig Beetles

Often overlooked but important branch and tree killer

Name and Description—Pityophthorus spp. [Coleoptera: Curculionidae: 

Scolytinae]

A number of different bark beetles 

will attack the branches and twigs of 

pinyon pine trees, but it is the members 

of the genus Pityophthorus that are most 

commonly referred to as pinyon twig 

beetles (fig. 327). Despite their small 

size, these scolytids behave in a manner 

similar to larger bark beetles that attack the boles and large branches of host trees.

Pityophthorus is the largest genus of bark beetles in North America with over 

120 different species. The very small size of these insects (less than 1/10 inch 

[1.5-2 mm] long), combined with the large degree of diversity within the genus 

caused the authors of ref. 50 to advise their readers that “Identification is for experts and students of taxonomy.” However, the similarity of behavior within this 

group allows the practitioner to treat these beetles as if they were all the same.

Host—Pinyon

Life Cycle—Twig beetles become active in the early spring, and, depending on 

local temperature regimes, this can be as early as mid-March. The adult beetles 

attack the branches at varying distances from the branch tip. If large numbers of 

beetles are present, they will utilize sections of the branch further away from the 

tips. The beetles will bore through the thin bark of the branches, creating small 

galleries in the phloem. Several females will mate with a single male, but the 

confined spaces of the branch tips result in a non-descript gallery pattern. Pheromones are used by twig beetles to coordinate attacks and recruit large numbers 

of beetles to specific hosts, but the exact constituents of the pheromone system are generally unknown. Attack sites are notable by the presence of 

tiny pitch tubes (fig. 328) in the generally pitchy 

branches of pinyon pine. Tiny eggs are laid within 

the branches that hatch quickly. Beetle larvae feed 

on the phloem within the branch, finally pupating within the bark. The emerging adult beetle 

will feed for a short time within the branch before emerging to attack new host sites. Generally, 

there are several generations over the course of 

the warmer months, but the length and number of 

generations are highly dependent on local weather 

conditions. In general, there are between two and 

four generations per year.

Figure 327. Pityophthorus sp. Photo: E. Richard 

Hoebeke, Cornell University, Bugwood.org.

Figure 328. Pitch tubes on pinyon 

pine from attack by Pityophthorus

spp. Photo: Tom Eager, USDA Forest 

Service.

B A R K B E E T L E S
180 USDA Forest Service RMRS-GTR-241. 2010.

B A R K B E E T L E S 

The quick generation time of twig beetles is matched by rapid changes in 

the host’s appearance. As noted above, the first indication of twig beetle activity 

is the appearance of tiny pitch tubes at each attack site. As time passes and the 

beetles consume more of the host tissues, the needles of an affected branch will 

start to dry out and turn to a golden yellow. As feeding continues, the branch tip 

will die, and individual needles will start to drop off of the branches. About the 

time the new adult beetles are ready to leave the infested material, the affected 

branches will consist of a light husk of bark, with the phloem having been completely consumed and a wooden pith remaining. Tearing away the very thin bark 

will reveal some very fine powder (frass) that has been left behind by the feeding 

beetles. Though twig beetle activity is sometimes confused with the attacks of 

the pinyon ips (Ips confusus), with some practice, it is easy to distinguish the 

two, even from a distance. Trees attacked and killed by pinyon ips have numerous pitch tubes and leave boring dust on the bole of the tree. The coloration of the 

foliage of a tree killed by pinyon ips quickly fades from green to light brown to 

a distinctive reddish brown. In the case of pinyon twig beetles, the fact that these 

beetles typically do not attack portions of the tree with thick bark means that only 

the smaller twigs and branches 

will be affected, leaving a portion of the foliage unaffected. 

The distal tips of the branches 

will be attacked, and the crown 

of the tree will have a frosted 

appearance with the overall 

color being more yellow to tan 

rather than red (fig. 329). Final 

diagnosis can be determined by 

examining the branch tips for 

the previously mentioned signs 

of beetle activity. During epidemics, all of the branches on a 

tree may be attacked, ultimately 

killing the entire tree.

Damage—In general, pinyon twig beetles do not cause much of a problem in 

pinyon stands, but they can achieve pest status under certain conditions. During 

periods of drought, their numbers can become large as they are quick to take 

advantage of drought-stricken hosts. The close relationship between moisture 

availability and the host tree’s ability to produce pitch means that the ability 

of pinyon trees to defend themselves declines under drought conditions. Twig 

beetles can also cause problems in the wake of human activity. Cutting, pruning, 

or disrupting the root system of pinyon trees can weaken them to the point that 

they become more susceptible to the twig beetle. In addition, the volatile odors 

that are created by host wounding can attract and concentrate twig beetles that 

then attack and damage the residual trees.

Figure 329. Pinyon tree under attack by twig beetles. Note 

the “frosted” appearance, with outer twigs affected and inner 

branches still green. Photo: Tom Eager, USDA Forest Service.
USDA Forest Service RMRS-GTR-241. 2010. 181

Management—Management techniques that maintain pinyon trees and stands 

in a healthy condition are probably the best defense against twig beetles. Proper 

techniques should be exercised if tree cutting or disturbances are unavoidable 

within a pinyon stand. These techniques include limiting disturbance to the 

colder months of the year when the beetles are inactive, removing slash that 

would otherwise attract beetles, and monitoring affected stands for signs of beetle attack for a period following such activities. Supplemental watering during 

drought periods helps the trees avoid stress. However, if beetle numbers appear 

to be increasing in a general area, this additional care may not be enough to prevent attack. In such cases, preventive sprays may be used to protect individual 

trees. Such sprays are also suitable for protecting against pinyon ips.

Reference: 50

Red Turpentine Beetle

Large pitch tubes low on tree trunks

Name and Description—Dendroctonus valens LeConte [Coleoptera: Curculionidae: Scolytinae]

The red turpentine beetle is the largest (1/4-3/8 inch [6-10 mm] long) and most 

widely distributed bark beetle in North 

America. It has a distinctively red-brown 

color (fig. 330) and is a common pest 

of forest, shade, and park trees 8 inches 

(20 cm) or larger in diameter.

Hosts—Can infest any pine species and 

is infrequently found in other conifers

Life Cycle—Red turpentine beetle peak 

flight and attack activity usually occurs in the spring to early summer (fig. 331). 

Beetles emerge from recently cut stumps and dying trees and attack trees, exposed roots, or freshly cut stumps. In summer, eggs hatch in 1-3 weeks. A unique 

feature of red turpentine beetle is that the small larvae feed gregariously, whereas 

most other bark beetle larvae maintain separate feeding tunnels. Red turpentine 

beetle larvae tunnel away from the adult gallery. As they grow, they feed more 

extensively and make an irregularly margined, fan-shaped gallery (fig. 332). The 

beetles overwinter as larvae, new adults, or parent adults.

Figure 330. Red turpentine beetle adult. Photo: 

Ladd Livingston, Idaho Department of Lands, 

Bugwood.org.

Figure 331. Life cycle of the red turpentine beetle (from Johnson 1982).

B A R K B E E T L E S
182 USDA Forest Service RMRS-GTR-241. 2010.

B A R K B E E T L E S 

Damage—Red turpentine beetle attacks generally start near ground level and 

rarely occur above 8 ft (2.4 m) (fig. 333). Attacks are often accompanied by the 

presence of light pink to reddish brown pitch tubes around the base of the tree 

and/or white granular material on the ground. On pines, red turpentine beetle 

pitch tubes may be as large as 2 inches (5 cm) in diameter, much larger than the 

pitch tubes of other pine infesting bark beetles. The large pitch tubes, galleries, 

and beetle size distinguish red turpentine beetle from other bark beetles. Trees 

that have been scorched by fire or stressed by drought are frequently attacked by 

red turpentine beetles. Attack by red turpentine beetles often predisposes trees to 

attack by other bark beetles.

Management—The most effective way to prevent red turpentine beetle attacks 

is to maintain tree vigor and avoid practices that attract beetles. Red turpentine 

beetle attacks frequently occur on pines that have been damaged by fire, mechanical wounding, or root disease. Damage to stands or individual trees should 

be minimized through improved logging, construction, and management practices. Fresh stumps, slow-dying trees, fire scorched trees, exposed roots of live 

trees, and trees with compacted soil around them should be treated or removed. 

Certain pesticide formulations containing carbaryl, chlorpyrifos, or permethrin 

have been proven effective at preventing bark beetle attacks when applied to the 

bark of a tree. Pesticide applied to the lower 6-8 ft (2-2.5 m) of the tree trunk can 

be used to prevent red turpentine beetle attacks, but realize that other species of 

bark beetles may pose a threat to the tree.

References: 50, 93, 160

Figure 332. Red turpentine beetle larvae and feeding 

gallery. Photo: Ladd Livingston, Idaho Department of 

Lands, Bugwood.org.

Figure 333. Large pitch tubes showing attack 

sites of red turpentine beetle. Attacks are usually 

confined to the lower 6 ft (1.8 m) of the bole. Photo: 

Kenneth E. Gibson, USDA Forest Service, Bugwood.

org.
USDA Forest Service RMRS-GTR-241. 2010. 183

Roundheaded Pine Beetle

Elongated, L-shaped galleries beneath bark

Name and Description—Dendroctonus adjunctus Blandford [Coleoptera: Curculionidae: Scolytinae]

The roundheaded pine beetle is a bark beetle that kills pine trees throughout 

its range. This insect’s common name should not be confused with the roundheaded borer, which is the common name for the insect family Cerambycidae. 

The roundheaded pine beetle averages about 1/5 inch (5 mm) long and is a shiny, 

dark brown. By comparison, it is slightly larger than the western pine beetle 

(D. brevicomis) and slightly more slender than the mountain pine beetle (D. 

ponderosae).

Identification of the roundheaded 

pine beetle is most easily accomplished 

by recognizing its unique gallery pattern 

(fig. 334). From the entrance site, the 

gallery will proceed horizontally (i.e., 

across the grain, either right or left) for a 

short distance of 1-2 inches (2.5-5 cm). 

The gallery will then wind vertically for 

an average distance of 12 inches (30 cm), 

forming an elongated L shape. Eggs are 

laid in alternating niches along the egg 

gallery. Once the larvae hatch out, they 

will mine vertically across the grain until they reach the third instar, then they 

will turn out into the outer bark where 

they will continue their development.

Hosts—Within Colorado, it primarily attacks ponderosa pine (Pinus ponderosa) but has also been collected from 

limber pine (P. flexilis).

Life Cycle—The beetle is primarily active in the southwestern United States, but 

its range extends south through central Mexico. Because southern Colorado represents the northern limits of its distribution, several aspects of its life cycle differ 

from the life cycles in other portions of its range. There is typically one generation per year, although in some locales the life cycle may take more than 1 year. 

Roundheaded pine beetles are unique in the United States in that they almost 

always overwinter in the egg stage. This means that they also attack quite late 

in the year, frequently in late October and November. However, this schedule is 

different in the southern reaches of the beetle’s range, where there are typically 

two generations per year. Egg hatch occurs in mid to late March. The larvae feed 

in the phloem for three instars before turning out into the bark. Pupation occurs 

Figure 334. Roundheaded pine beetle galleries. 

Note the narrower, more sinuous nature of this 

gallery compared to mountain pine beetle gallery 

to the right. Photo: USDA Forest Service Archive, 

USDA Forest Service, Bugwood.org.

B A R K B E E T L E S
184 USDA Forest Service RMRS-GTR-241. 2010.

B A R K B E E T L E S 

in midsummer, and the adults may remain within the host tree for 2-3 months 

before they emerge to attack new hosts.

Damage—Outbreaks of this beetle tend to be sporadic and short-lived, and they 

will infest trees of a wide range of size classes. They are most often active on 

ridge tops or other sites with dry, shallow soils. They are frequently found in 

“mixed broods” in trees that are simultaneously under attack by several species of bark beetles. See the Western Pine Beetles entry for further discussion of 

mixed broods.

Management—Because outbreaks are often short-lived, management is seldom 

warranted. In areas where management is desired, thinning stands, eliminating 

infested trees, and applying a preventive spray to non-infested trees can help 

protect trees.

Reference: 50

Spruce Beetle

Windthrown trees can set up outbreaks

Name and Description—Dendroctonus rufipennis (Kirby) [Coleoptera: Curculionidae: Scolytinae]

Adult spruce beetles are dark brown 

to black, with reddish brown to black 

wing covers (fig. 335). Beetles are approximately 1/4 inch (6 mm) long. The 

rear margins of their wing covers are 

evenly rounded. Eggs are approximately 1/16 inch (1.5 mm) long and look like 

tiny pearls. Larvae are legless grubs and 

are approximately 1/4 inch (6 mm) long 

at full development. Pupae are creamy 

white, show some body features similar 

to adults, and are found in individual chambers at the end of larval galleries.

Hosts—Engelmann and blue spruce, Picea engelmannii and P. pungens. During 

very large outbreaks, this beetle has also attacked lodgepole pine, though such 

occurrences are not common.

Life Cycle—A 2-year life cycle is most common, but 1- and 3-year generations 

have occurred. Adults emerge from May through July, depending on local factors that influence beetle development such as temperature, aspect, and elevation. The period of attack may last as long as 5-6 weeks. With a 2-year life cycle, 

brood spend their first winter as larvae and their second as adults.

Female beetles bore through the bark of trees (standing or fresh, cut or fallen) 

and deposit eggs on either side of constructed egg galleries. Egg galleries vary 

from a few to 12 inches (30 cm). Galleries are packed with frass. Larvae emerge 

from eggs and feed in phloem.

Figure 335. Spruce beetle adult. Photo: Steve 

Valley, Oregon Department of Agriculture.
USDA Forest Service RMRS-GTR-241. 2010. 185

In the 2-year life cycle, the first winter is spent in the larval stage. Larvae 

develop into pupae in summer (approximately 1 year after initial attack). The 

second winter is spent as adult beetles. Some of these beetles exit and colonize 

the base of trees, where snow insulates them from extremely cold temperatures. 

Beetles emerge and colonize new hosts in spring/summer, 2 years after initial 

attack.

Damage—The spruce beetle, Dendroctonus rufipennis, is the most significant 

natural mortality agent of mature spruce. Outbreaks have occurred in spruce forests from Alaska to Arizona. Following a 1939 windthrow event, a very large 

spruce beetle outbreak that impacted thousands of acres and spanned more than 

a decade occurred on the White River and Grand Mesa National Forests and 

adjoining lands. Outbreaks cause extensive tree mortality (fig. 336) and can alter 

stand structure and composition. Average tree diameter, tree height, and stand 

density are all reduced following large outbreaks. As a result of the 1940s outbreak, many stands once dominated by mature Engelmann spruce are now dominated by subalpine fir.

The earliest sign of infestation is the presence of fine, bark-colored boring 

dust in bark crevices and around the base of standing trees. Pitch tubes may or 

may not be evident. Spruce beetles prefer down spruce to standing trees. On 

down (windthrown and cut) trees, spruce beetles commonly colonize the lower, 

well-shaded surfaces and may colonize the entire length of the trunk, typically 

up to an 8-inch (20-cm) top. In standing trees, beetle activity is most common 

in the lower 30 ft (9 m) or so of the trunk. Strip attacks (attacks that impact only 

a portion of the tree’s circumference) may be common, especially when beetle 

populations are small. Trees may live despite a strip attack, but such trees are 

often colonized again and killed in subsequent years. 

Tree crowns typically remain green for up to a year after attack. By the second year, needles have faded and soon fall from the tree. The aerial “signature” 

of spruce beetle-infested spruce is not as striking or long-lasting as that of pine 

Figure 336. Spruce beetle activity in Engelmann spruce. Photo: A. Steven Munson, USDA Forest 

Service, Bugwood.org.

B A R K B E E T L E S
186 USDA Forest Service RMRS-GTR-241. 2010.

B A R K B E E T L E S 

beetles in pines. Therefore, aerial detection of spruce beetle is extremely difficult. In the winter, infested trees are often easily identified by the abundance of 

bark flakes on the snow, which is evidence of feeding activity by woodpeckers.

Forest stands most susceptible to attack are located along drainage bottoms, 

have an average DBH of 16 inches (40 cm) or more, have a basal area of over 

150 square ft per acre (34 square m per hectare) and have a canopy comprised of 

more than 65% spruce.

Management—Goals should be based on existing and desired forest conditions.

For endemic spruce beetle populations:

•From stand exam data, determine spruce beetle stand hazard ratings. Prioritize stand treatments to reduce susceptibility among moderate to hig

hazard stands.

•During stand treatments, minimize stump heights, maximize utilization o

green spruce, cut green spruce cull logs into short lengths, and lop and scatter slash to maximize exposure to sunlight.

•Remove windthrown spruce before it is colonized by spruce beetles, or remove such trees after beetle colonization and before brood beetles develo

and exit. If beetle response to recent windthrow is not known, conduct systematic sampling following beetle flight periods.

For growing spruce beetle populations:

•Registered insecticides can be applied to high-value trees to prevent beetl

infestation.

•Identify and fell trap trees (large-diameter green spruce) into well-shade

areas. Trap trees cut in the fall before the beetle flight typically serve as 

optimal spruce beetle habitat. Do not remove limbs. Trap trees are often 

deployed in advance of larger-scale harvest operations and in combination with sanitation efforts to absorb beetles that might otherwise colonize 

green, standing trees. Remove infested trap trees in a timely manner.

•Remove spruce beetle infested trees. This can be accomplished in combination with beetle attack susceptibility reduction such as stand basal are

reduction. These activities should be designed in order to minimize future 

windthrow potential.

•Following windthrow events, remove windthrown spruce after beetle colonization and before brood beetles develop and exi

•Beetle-killed spruce may stand for decades, and many trees will remain viable for products such as house logs. Dead spruce can be salvaged to mee

management objectives. Salvage activities alone do not influence spruce 

beetle populations.

For epidemic spruce beetle populations:

•Once spruce beetle populations reach epidemic proportions and impact larg

landscapes, it is not possible to stop such an occurrence with management 

activities. But certain management activities may still be successful at limited scales.

References: 85, 147, 148, 149
USDA Forest Service RMRS-GTR-241. 2010. 187

Spruce Engraver Beetles

Colonizes upper surface of windthrown and 

weakened trees

Name and Description—Ips pilifrons (Swaine), I. borealis (Swaine), I. hunteri

(Swaine), and I. tridens (Mannerheim) [Coleoptera: Curculionidae: Scolytinae]

All of the four most common adult spruce engraver beetles are four-spined 

and have a distinct declivity at the rear of 

their abdomen with varying numbers of 

spines along the margins (fig. 337). Adult 

beetles are shiny, reddish brown to black, 

and up to approximately 1/6 inch (4 mm) 

long, depending on the species.

Ips pilifrons has been reported as the 

most abundant species of spruce engraver 

beetles in Colorado.

Hosts—Engelmann and blue and white 

spruces, depending on the Ips species

Life Cycle—Life cycles vary by species, from one to multiple generations per 

year. Most overwinter beneath the bark or in forest litter as adults. In spring 

or summer, male beetles fly and initiate attacks on new hosts. They construct 

a nuptial chamber beneath the bark and attract 

multiple females. Mated females construct egg 

galleries and place their eggs along them. Egg 

galleries radiate away from the nuptial chamber, commonly forming a Y-shaped pattern (exception: I. hunteri typically has only two egg 

galleries, which often radiate away from the 

nuptial chamber in opposite directions). Adult 

beetles keep the galleries free of frass/boring 

dust, a characteristic that distinguishes them 

from galleries of Dendroctonus species, which 

are tightly packed with frass. Bark beetles in 

the genus Ips are referred to as engravers as 

their gallery patterns are lightly or sometimes 

deeply “engraved” into the face of the tree’s 

sapwood (fig. 338). Larvae feed away from the 

egg galleries.

Damage—These bark beetles are not known as 

aggressive tree killers. Their preferred habitat 

is downed trees and broken branches and tops. 

However, they do cause some tree mortality 

Figure 337. Adult Ips beetle. Note the declivity at 

the rear of the abdomen with spines along each 

side. These are two characteristics that distinguish 

members of this genus from those of the genus 

Dendroctonus. Photo: Darren Blackford, USDA 

Forest Service, Bugwood.org.

Figure 338. Ips galleries beneath the bark of 

spruce. Egg galleries are free of frass. These 

beetles are called engravers because their 

galleries are “engraved” into the face of the 

host’s sapwood. Photo: Whitney Cranshaw, 

Colorado State University, Bugwood.org.

B A R K B E E T L E S
188 USDA Forest Service RMRS-GTR-241. 2010.

B A R K B E E T L E S 

among stressed trees but, more commonly, only kill the tops of such trees. Outbreaks in standing, healthy trees are uncommon and short-lived.

Spruce engraver beetles are usually found colonizing the upper surfaces of 

windthrown trees and broken tops. They are much more tolerant of sun-exposed 

surfaces than are spruce beetles. Fresh piles of boring dust are key indicators of 

recent infestation. Pitch tubes may be found on green, standing trees, but they 

are not always present. If external signs of infestation are present, removing a 

small section of bark will likely reveal the Y-shaped, frass-free galleries in the 

inner bark; the same pattern engraved on the face of the sapwood; and possibly 

various life stages (larvae, pupae, or adults) of the beetles. Once beetles have 

developed and flown, the outer surface of the bark is peppered with many small, 

round exit holes.

Engraver beetles are often found in downed spruce in combination with other 

bark beetles, including spruce beetle, D. rufipennis. Spruce engraver beetles often occupy habitat beneath the bark of fallen spruce that might otherwise be used 

by the more aggressive spruce beetle. This can be significant, as the availability 

of suitable breeding habitat is the major factor contributing to substantial bark 

beetle population increase.

Management—Management of spruce engraver beetles in spruce is typically 

not necessary in general forest settings, although prudent and timely slash management directed at spruce beetle prevention might also serve to reduce spruce 

engraver beetle populations. Spruce engraver beetle activity can be a periodic 

problem in high-use recreation areas where spruce are stressed from soil compaction, wounded by human activity, etc. In such settings, timely removal of 

infested trees (sanitation) is usually sufficient to prevent/avoid other substantial 

tree mortality. In urban settings, preventive sprays can be used to protect ornamentals from ips attack (e.g., Ips hunteri [blue spruce engraver] often colonizes 

and kills the tops of blue spruce).

References: 50, 176

Twig Beetles

Attack small diameters and can kill trees

Name and Description—Pityophthorus spp., P. confertus Swaine, P. confinis LeConte, P. pseudotsugae Swaine, Pityogenes spp., P. knechteli (Swaine), 

P. carinulatus (LeConte) [Coleoptera: Curculionidae: Scolytinae]

Twig beetles are a large group of difficult to identify tiny bark beetles in several genera. Adult beetles are light brown to almost black and less than 1/10 inch 

(<2.5 mm) long. Twig beetles in the genus Pityogenes have posterior spines, and 

those in the genus Pityophthorus have a smooth posterior. Most go unnoticed, 

feeding in shaded branches, weakened trees, newly felled small trees, or broken 

limbs. When trees are weakened by drought or disease, some species of twig 

beetles can kill trees by attacking the bole and/or by killing all the large branches. 
USDA Forest Service RMRS-GTR-241. 2010. 189

Tiny holes, small piles of orange-colored boring dust, and, occasionally, pitch 

tubes mark the beetles’ entrances into the tree (fig. 339). Star-shaped galleries 

score the sapwood and inner bark (fig. 340).

Hosts—Hosts vary with twig beetle species. Pines and Douglas-fir are more affected than spruce and true firs.

Life Cycle—Life cycles vary by species and have not been well-studied. Amman and others (ref. 2) reported that P. confertus has only two larval instars and 

can complete development in eight weeks. Koch (ref. 105) reported one to two 

generations for twig beetles in lodgepole pine, and Furniss and Carolin (ref. 50) 

reported two or more generations per year, varying with species and locality. All 

twig beetles are polygymous, and female egg galleries radiate out from a central 

nuptial chamber just below the bark. Eggs are deposited in individual niches, and 

larvae mine away from the egg gallery in irregular patterns. Pupation occurs in 

the cambium region, and young adults emerge from individual exit holes.

Damage—Twig beetles are generally innocuous on healthy trees but can cause 

top-kill and tree mortality of drought-stressed or diseased trees. Populations of 

twig beetles can build in slash from activities like spacing and pruning and then 

can move into and kill small trees. Similarly, large populations of twig beetles 

can develop in the tops and branches of trees attacked by other bark beetles. The 

twig beetle, P. knechteli, has caused notable mortality in young lodgepole pines 

in Colorado in association with mountain pine beetle epidemics (fig. 341). Twig 

beetles may also create habitat for endemic mountain pine beetle populations 

by attacking the upper portions of small-diameter suppressed or diseased trees. 

This allows mountain pine beetle to attack the basal foot or two of the tree trunk.

Figure 339. Twig beetle frass at entrance holes 

in lodgepole pine; twig beetle activity is associated with a mountain pine beetle outbreak. 

Photo: Robert Cain, USDA Forest Service.

Figure 340. Twig beetle gallery on lodgepole 

pine. Photo: L. Machauchlan, British 

Columbia Ministry of Forests, Kamloops Forest 

Region.

B A R K B E E T L E S
190 USDA Forest Service RMRS-GTR-241. 2010.

B A R K B E E T L E S 

Management—Management techniques that maintain trees and stands in a healthy 

condition are probably the best defense against twig beetles. Strategies suggested 

for pinyon twig beetle may also be effective for other twig beetles.

References: 2, 50, 105

Western Balsam Bark Beetle

Star-shaped egg galleries

Name and Description—Dryocoetes confusus (Swaine) [Coleoptera: Curculionidae: Scolytinae]

Adult western balsam bark beetles are shiny, dark-colored, and approximately 

1/6 inch (3.4-4.3 mm) long. The rear edges of the wing covers are abruptly rounded and do not have spines. Both male and female beetles have patches of “hairs” 

on the fronts of their heads. Females have a dense and distinctive patch of these 

“hairs” (fig. 342).

Hosts—Subalpine fir, Abies lasiocarpa, other true firs, and very rarely, Engelmann spruce and lodgepole pine

Life Cycle—The biology of this beetle is not well known, but a 2-year life cycle 

has been documented in this Region. The first winter is spent beneath the bark 

as larvae and the second as near fully developed adults. Approximately midsummer, male beetles initiate attacks on trees and construct nuptial chambers beneath the bark. They attract several female beetles. After mating, female beetles 

construct egg galleries that radiate from the nuptial chamber, forming a stellate 

gallery pattern (fig. 343). Larvae feed away from the egg galleries. Females are 

capable of producing eggs during the initial summer of attack and in the following spring. The resulting gallery pattern and brood makeup is often confusing, 

hence the name, Dryocoetes confusus. Two flight periods have been observed in 

Figure 341. Twig 

beetles contribute to 

mortality detected in 

younger lodgepole 

pine stands adjacent 

to mature stands killed 

by mountain pine 

beetle. Photo: Brian 

Howell, USDA Forest 

Service.
USDA Forest Service RMRS-GTR-241. 2010. 191

Utah: a large flight in early to midsummer and a smaller 

flight in the fall.

Damage—The western balsam bark beetle is part of 

a group of organisms that contribute to large amounts 

of subalpine fir mortality in the West. Western balsam 

bark beetles frequent trees weakened by root disease, 

drought, wind breakage, and other damaging factors. 

As beetle populations grow, they become more able to 

take advantage of large areas of susceptible host trees. 

However, unlike mountain pine beetle and spruce beetle, resulting tree mortality is not usually uniform across 

the impacted landscape, although cumulative mortality 

over years can result in extensive landscape mortality 

(fig. 344). Large concentrations of subalpine fir mortality are particularly troublesome in high-elevation developments such as ski areas.

External evidence of beetle attack is often difficult to 

find. If beetle attacks are fresh, small amounts of boring 

dust may be visible along the bark and in proximity to entrance holes. Small amounts of sap may stream for about 

an inch below the entrance hole, but this symptom is not 

always present. If a segment of bark can be removed, 

identification of beetle brood, the stellate gallery pattern, and slight etching of the sapwood will confirm the 

Figure 342. Western balsam bark beetle adults; female on the left, male on the right. Note the dense patch 

of “hairs” at front of female’s head. Photo: Rocky Mountain Region, USDA Forest Service, Bugwood.org.

Figure 343. Stellate (starshaped) gallery pattern of 

western balsam bark beetle 

evident on the face of subalpine 

fir sapwood. Photo: Dave 

Leatherman, Gillette Entomology 

Club, Colorado State University.

B A R K B E E T L E S
192 USDA Forest Service RMRS-GTR-241. 2010.

presence of western balsam bark beetle. Beetle-killed firs retain their red needles 

for 3 or more years, creating a “signature” that is evident from both the ground and 

the air. This needle retention characteristic can complicate aerial detection of most 

recent infestations.

Endemic populations are evident among weakened trees and recently broken branches/tops. Little is known about factors contributing to outbreaks, 

but McMillin and others (ref. 120) documented that windthrown subalpine fir 

is readily colonized by western balsam bark beetles. The authors (along with 

Gibson and others, ref. 55) concluded that because western balsam bark beetle 

responds to windthrow, addressing such material may reduce the possibility of 

a future epidemic. Beetle activity is often common among trees with Armillaria 

root disease. This beetle and root disease, along with with some lesser known 

organisms, account for large amounts of mortality among mature subalpine fir 

stands. Western balsam bark beetle activity has been described as chronic in 

some locations with abundant numbers of host trees.

Management—Management directed specifically at subalpine fir forest susceptibility and western balsam bark beetle/Armillaria root disease activity has 

been primarily limited to treatments that favor spruce. Dead and dying subalpine fir have been harvested and sites have been prepared to promote residual 

spruce health and to favor spruce regeneration. Timely salvage of subalpine fir 

windthrow may help to avoid western balsam bark beetle outbreaks in adjoining 

forests. Aggregate and anti-aggregate pheromones are known for these beetles, 

but they are not yet available for operational management purposes.

References: 50, 52, 55, 102, 120

Figure 344. Western balsam bark beetle contributed to widespread subalpine fir mortality. Photo: Mike 

Blakeman, Rio Grande National Forest, USDA Forest Service.

B A R K B E E T L E S
USDA Forest Service RMRS-GTR-241. 2010. 193

Western Pine Beetle

Meandering egg galleries

Name and Description—Dendroctonus brevicomis LeConte [Coleoptera: Curculionidae: Scolytinae]

The western pine beetle attacks and 

kills mature ponderosa pine throughout 

much of its range. It is one of the smaller 

members of the genus Dendroctonus

(only about 1/8 inch [3-5 mm] long), and 

is a fairly non-descript bark beetle; it 

is dark brown and cylindrical in shape 

(fig. 345).

Host—Ponderosa pine; however, this 

beetle is never found east of the Continental Divide, and, within the Rocky Mountain Region, it is active only in a 

narrow band in western Colorado.

Live Cycle—There are several generations per year of western pine beetle. This 

number varies with location, depending upon elevation and latitude. The number 

of generations is tied directly to the length of the “growing season,” so in some 

portions of their distribution, they will have two generations per year, whereas in 

other areas, there may be as many as four generations per year.

Western pine beetles are virtually inactive during the winter months but 

as soon as the weather becomes warmer, they will increase their activity. The 

first sign of western pine beetle attack is

the production of pitch tubes on ponderosa 

pines. The egg gallery that is constructed by 

western pine beetles is described as being 

“serpentine;” that is, it is a sinuous, winding gallery that can even cross itself at times 

(fig.  346). The beetles mate, and eggs are

deposited along the margins of the central 

gallery. Dendroctonus beetles pack these 

galleries with frass (boring dust and beetle 

excrement) once the eggs have been deposited. These eggs hatch out, and the tiny 

larvae feed briefly within the inner bark on

the phloem layer. However, after this brief 

period, the larvae turn out into the bark of 

the host tree and continue to tunnel within 

the outer bark. Pupation occurs within the 

outer bark of the host, and the next generation of adult beetles will emerge to renew the 

cycle. The combination of long, serpentine 

Figure 345. Western pine beetle adult 

(Dendroctonus brevicomis). Photo: Erich Vallery, 

USDA Forest Service-SRS-4552, Bugwood.org.

Figure 346. Western pine beetle galleries. 

Photo: Ladd Livingston, Idaho Department of 

Lands, Bugwood.org.

B A R K B E E T L E S
194 USDA Forest Service RMRS-GTR-241. 2010.

galleries and the absence of larval feeding beneath the bark of the host tree are 

key characteristics of western pine beetle galleries that allow positive identification even after the beetles have left the host.

Mixed Broods—One of the more confusing aspects of western pine beetles is 

the occurrence of mixed broods in a single tree under attack. This term refers 

to the presence of several different species of bark beetles attacking the same 

host simultaneously or within the same season. While in some cases, the various 

species will concentrate their activities in separate portions of the tree (e.g., Ips

spp. in the top, Dendroctonus adjunctus in the crown, and D. brevicomis in the 

bole), in many cases, the galleries of different species can be found immediately 

adjacent to each other. This phenomenon is poorly studied but has been noted 

for some time. Furniss and Carolin (ref. 50) list a number of species that will coattack a ponderosa pine. Within Colorado, the western pine beetle’s most common associates include the roundheaded pine beetle (D. adjunctus); the red turpentine beetle (D. valens); the pine engraver (Ips pini); and, occasionally, various 

borers (e.g., Cerambycidae and Buprestidae). The occurrence of mixed broods 

increases the difficulty of identification in the field, and it also implies that some

aspects of bark beetle biology are still not well understood.

Damage—Most of the time, the populations of these native insects are at low 

levels, and the beetles attack stressed, damaged, or weakened ponderosa pines. 

Trees damaged mechanically or by lightning or fire are often targets of attack,

as are diseased host trees. Western pine beetle populations often increase dramatically during periods of drought, and over-stocked stands are also subject to 

western pine beetle attack. These beetles kill host trees in a wide range of size 

classes, from 6-inch (15-cm), pole-sized ponderosa pines to very large “yellowbellies” with diameters over 3 ft (0.9 m). Populations of western pine beetles can 

increase when large numbers of susceptible hosts are present, and the large number of beetles can spread to other stands with generally low susceptibility. These 

outbreaks can last for several years and can affect forests over widespread areas.

Management—As with most bark beetles, the most economical and efficient

means of management is to maintain trees and stands in a healthy condition. 

Stocking reduction and creation of diverse stand conditions reduce overall 

susceptibility to western pine beetles. During times of drought, vigilance and 

prompt sanitation of infested hosts can reduce the overall impacts of western 

pine beetles. Programs that seek to remove infested trees require that managers 

have a good understanding of western pine beetle biology. They should be familiar with the occurrence of mixed broods and also be able to recognize the various 

stages of beetle development so that treatments can be prioritized.

Supplemental watering of high-value trees and stands has shown some efficacy in reducing western pine beetle attack. In addition, the use of chemical

sprays in landscape or recreational settings has proven to be highly effective at 

preserving individual high-value trees.

References: 37, 46, 50

B A R K B E E T L E S
USDA Forest Service RMRS-GTR-241. 2010. 195

Introduction to Defoliating Insects

Chewed, mined, and missing foliage

Insects that consume leaves or needles are classified as defoliators and are indicated by chewed, mined, or missing foliage. Damage from defoliating insects 

varies considerably with tree and insect species, feeding intensity, and the time 

of year that feeding occurs (table 17, figs. 347-349).Although the effects of most

defoliating insects are negligible, some may cause significant damage to individual trees. A few defoliators are important forest pests that can kill trees across 

large landscapes during outbreaks. Most of the important defoliating insects in 

conifers are the larval stages of moths, butterflies, or sawflies. In hardwoods,

beetle larvae and leaf-cutting bees are also damaging agents. Occasional defoliation is noted from other insects that tend to be more general feeders on rangeland, 

such as grasshoppers or adult scarab beetles.

General Features

General features of defoliating insects:

•Defoliation can kill trees directly or can predispose trees to other insects o

pathogens that can kill the trees.

•Generally, high levels of defoliation will occur for several consecutive year

before trees are killed.

•Defoliation that occurs later in the summer is typically less stressful to tree

than defoliation that occurs early in the growing season.

•Deciduous hardwood trees are more tolerant of defoliation because they ca

refoliate after early season defoliation, whereas coniferous tree cannot.

Figure 347. Douglas-fir 

tussock moths feed on 

foliage of Douglas-fir, true 

firs, and spruce. Photo: 

Ladd Livingston, Idaho 

Department of Lands, 

Bugwood.org.

Figure 348. Western spruce 

budworm is another important 

defoliator of spruce, Douglasfir, and true firs. Photo: Scott 

Tunnock, USDA Forest Service, 

Bugwood.org.

Figure 349. Western tent caterpillar is an important defoliator 

of aspen in the Rocky Mountain 

Region. Photo: Southwestern 

Region, USDA Forest Service.

D E F O L I A T O R S
196 USDA Forest Service RMRS-GTR-241. 2010.

D E F O L I A T O R S 

Table 17. Common defoliating insects in the Rocky Mountain Region and their active feeding seasons. Defoliator Host Feeding occurs Aspen leafminer (Phyllocnistis populiella) Aspen Late spring to late summer Bagworm (Thyridopteryx ephemeraeformis) Junipers, arborvitaes, and various hardwoods Late spring to late summer Boxelder leafminera (Caloptilia negundella) Boxelder Late spring to early fall; two generations per year Boxelder leafrollera (Archips negunanus) Boxelder; sometimes alder and honeysuckle Late spring to summer Cottonwood leaf beetle (Chrysomela scripta) Cottonwood Late spring to early fall Douglas-fir tussock moth (fig. 347) Douglas-fir, true firs, and spruce Late spring (bud burst) to midsummer

(Orgyia pseudotsugata)

Eastern tent caterpillara (Malacosoma americanum) Chokecherry, fruit trees, and other hardwood trees and shrubs Spring to midsummerElm leaf beetle (Xanthogaleruca luteola) Elm Late spring to early fallElm leafminer (Kaliofenusa ulmi) American elm Late spring-summerFall webworm (Hyphantria cunea) Hardwoods; many species Mid to late summerForest tent caterpillar (Malacosoma disstria) Aspen and other hardwoods Late spring (bud burst) to early summerLarge aspen tortrix (Choristoneura conflictana) Aspen Early spring to early summerLeafcutter beea Ash, redbud, lilac, rose, and others Early summerNeedleminers (Coleotechnites and others) Ponderosa pine, pinyon pine, white fir, and blue spruce Generally, late summer to late spring; some variation with speciesOak Leafroller (Archips semiferana) Gambel oak Early spring (bud burst) to early summerPandora Moth (Coloradia pandora) Lodgepole and ponderosa pines Mid to late summer to midsummer the next yearPine butterfly (Neophasia menapia) Ponderosa pine Late spring (bud burst) to midsummerPine sawflies (Neodiprion spp., Zadiprion spp.) Ponderosa and pinyon pines All seasons, depending on speciesSonoran tent caterpillara (Malacosoma tigris) Oaks Spring to midsummerTiger moth (Lophocampa spp.) Ponderosa pine, pinyon pine, Douglas-fir, white fir, and junipers Fall to early springUglynest caterpillara (Archips cerasivorana) Chokecherry and others Spring to late summerWestern pine budworm Ponderosa pine Spring to early or midsummer

(Choristoneura lambertiana ponderosana)

Western pine tussock moth (Dasychira grisefacta) Ponderosa pine Early spring to early summerWestern spruce budworm (fig. 348) Douglas-fir, true firs, spruce Spring (bud burst) to early summer

(Choristoneura occidentalis)

Western tent caterpillar (fig. 349) Aspen and other hardwoods Late spring (bud burst) to early summer

(Malacosoma californicum)

a Not discussed in this guide.
USDA Forest Service RMRS-GTR-241. 2010. 197

•Repeated years of defoliation become increasingly stressful to both deciduou

and coniferous trees.

•A significant amount of defoliation can slow and even stop tree growt

•Defoliation occurring in drought years or following a late freeze can be mor

stressful to trees.

•Some defoliators feed while protected in silken tents; in webbed, clipped foliage; or in folded foliage, and some mine while protected in the leaf tissue

Others feed in groups or may be covered with irritating or toxic hairs.

•Some species of defoliators feed in all seasons, including winte

•Many defoliating insects have a preference for the current or previous year’

foliage, and their feeding locations and life cycles are timed accordingly.

Aspen Leafminer

Serpentine mines impart silvery hue

Name and Description—Phyllocnistis populiella Chambers [Lepidoptera: 

Gracillariidae]

Adult aspen leafminers are tiny 

moths with a wingspan of about 1/4 inch 

(6 mm); narrow, lance-shaped, white

wings mottled with brown and black 

markings; and relatively long, threadlike antennae. Moths have an unusual 

resting posture of standing tall on their 

front legs with their bodies slanted 

downward with the tips of their hairfringed wings resting on the surface 

behind. Larvae are a pale, yellowish 

white color; very flat; and 1/10-1/4 inch 

(3-6 mm) long when mature. They

develop internally in serpentine tunnels they create within aspen leaves 

(fig. 350). Dark-colored pupae are found within the larval mine.Aspen leafminer

is a transcontinental species that apparently can be found everywhere aspen occurs across the western United States.

Host—Aspen; similar mines are found on cottonwood, poplar, and willow but 

are probably caused by other species

Life Cycle—Aspen leafminer has one generation per year. In early spring, more 

or less concurrent with aspen leaf flushing, moths emerge from hibernation and

feed for several weeks on nectar produced by glands near the base of young aspen leaves. After mating, single eggs are deposited on the edge of newly opened 

aspen leaves, and the female then folds the leaf edge over to form a protective 

shelter for her eggs until they hatch. When populations are low, one or two eggs 

Figure 350. Characteristic serpentine leaf mine with 

central line made by larva of Phyllocnistis populiella

on quaking aspen. Photo: William M. Ciesla, Forest 

Health Management International, Bugwood.org.

D E F O L I A T O R S
198 USDA Forest Service RMRS-GTR-241. 2010.

D E F O L I A T O R S 

are laid per leaf, but more will be deposited per leaf during outbreaks. Hatching 

larvae cut through the bottom of the egg directly into the leaf and begin feeding. They spend their immature life inside this leaf, creating a meandering or 

serpentine mine that has a center line composed of excrement. Larvae feed with 

sickle-shaped jaws on the tissue between the two leaf surfaces without breaking 

the leaf cuticle. The last larval stage does not feed. Pupation occurs inside the 

mine within a silken cell the larva constructs. New adults emerge in late summer before or during leaf senescence and are active for several weeks. In the 

fall, adult moths locate protected sites to overwinter. Other species of moths and 

flies also mine aspen leaves but do not create the characteristic mines and pupate

within them.

Damage—Mining of leaf tissue causes the leaves to dry out and turn brown, 

which may lead to premature leaf drop, especially in severe infestations. Large 

populations render a silvery hue to the appearance of aspen stands viewed from 

a distance (fig. 351). Periodic epidemics occur in Wyoming, and sustained outbreaks that cover up to half a million acres in 1 year have occurred several times 

in Alaska. During outbreaks, moths seeking overwintering shelter can become 

a minor nuisance by entering homes. Damage due to aspen leafminer feeding 

is described as being primarily cosmetic. However, heavily mined leaves have 

been shown to lose much of their photosynthetic capacity, which may reduce 

aspen tree growth. Sustained aspen leafminer outbreaks are suspected to cause 

branch dieback and top-kill, but this has not yet been proven. Similar leaf mining damage, in the shape of irregular blotches, can be caused by the larvae of the 

poplar blackmine beetle, Zeugophora scutellaris Suffrian.

Management—As with many leaf mining insects, natural enemies, including 

diseases, parasites, and predators, are the main causes for collapsing outbreak 

populations of aspen leafminer. Cannibalism can also occur in dense populations 

with many larval mines per leaf. It is also suspected that climate is influential in

determining aspen leafminer populations.

Aspen can tolerate significant defoliation. The fact that outbreaks collapse on

their own, combined with a lack 

of evidence indicating economic 

damage, means 

that there is little 

need for active 

management of 

aspen leafminer 

populations. In addition, there are no 

effective control 

measures for largescale outbreaks. 

Figure 351. Aspen showing the silvery hue indicative of a dense population of 

aspen leaf miner. Photo: Intermountain Region, Ogden Archive, USDA Forest 

Service, Bugwood.org.
USDA Forest Service RMRS-GTR-241. 2010. 199

Keeping trees in good health helps reduce the impacts of insect pests in general. Contact insecticides are not effective against leafmining insects during their 

most damaging stage because they are protected inside the leaf. Some insecticides that are carried systemically through the tree can be effective in controlling 

leaf mining insects.

References: 35, 50, 88, 109

Bagworm

Carries shelter made of clipped foliage and twigs

Name and Description—Thyridopteryx ephemeraeformis (Haworth) [Lepidoptera: Psychidae]

Male bagworm moths are sooty 

black with a densely hairy body, clear 

wings, and a wingspread of about 1 

inch (25 mm) (fig. 352). Female adults

are soft-bodied and grub-like. They 

are naked except for a circle of woolly 

posterior hairs and are yellowish-white 

in color with no functional legs, eyes, 

or antennae (fig. 353). Larvae, pupae,

and adult females live inside a bag constructed of silk, twigs, and leaves (figs.

354-357). Mature larvae are dark brown

and 3/4-1 inch (18-25 mm) long. The 

head and thoracic plates are yellowish 

and spotted with black (fig.  356). Bags

of mature larvae are 1 1/4-2 inches 

(3-5 cm) long and vary in appearance 

due to the use of host material in bag 

construction (figs. 356-357). Eggs are

small, white, and found inside female 

bags. Bagworm is a native defoliator 

that occurs throughout the eastern half 

of the United States, is commonly encountered in Kansas and Nebraska, and has been 

occasionally reported in South Dakota.

Hosts—Bagworm feeds on a wide variety 

of trees and shrubs but is primarily a pest 

on arborvitae and eastern redcedar. Other 

coniferous and broadleaf hosts include juniper, pines, spruces, apple, basswood, boxelder, elms, black locust, maple, oaks, persimmon, poplars, and willows.

Figure 352. Bagworm adult male. Photo: Pennsylvania 

Department of Conservation and Natural Resources, 

Forestry Archive, Bugwood.org.

Figure 353. Bagworm adult female removed from 

her bag. Photo: Lacy L. Hyche, Auburn University, 

Bugwood.org.

Figure 354. Young bagworm larva with bag on 

Colorado blue spruce. Photo: Rayanne Lehman, 

Pennsylvania Department of Agriculture, 

Bugwood.org.

D E F O L I A T O R S
200 USDA Forest Service RMRS-GTR-241. 2010.

D E F O L I A T O R S 

Life Cycle—Bagworm has one generation per year across almost its entire range. 

In September or October, adult males emerge and begin their mating flight, seeking the wingless females. Females remain within their bag, so mating takes place 

through the bag opening. The female’s bag also contains her empty pupal case, 

into which eggs are laid. Adults die, and eggs overwinter within their mother’s 

bag that is attached to a twig with silk. Eggs hatch in May or June, yielding

1/25 inch (1 mm) long glossy black or brownish larvae that almost immediately 

begin to spin their own bag. Young larvae, buoyed by long strands of silk, can 

be dispersed short distances by wind. The bag begins as a garland of pellets on a 

long silken strand around the larva’s body behind the legs, to which it adds twigs 

and leaf material, eventually becoming large enough to enclose the entire immature insect (fig. 354). Larvae move about carrying their tough, silken bag with

them. An opening is maintained at the top of the bag, through which the head and 

several segments protrude when the larva is feeding, moving, or enlarging the 

bag (fig. 356). There is a smaller opening on the bottom of the bag through which

excrement is passed. Larvae mature from August through September, firmly anFigure 355.

Mature bagworm 

larva in its silklined bag that was 

cut open. Photo: 

Lacy L. Hyche, 

Auburn University, 

Bugwood.org.

Figure 356.

Bagworm larva 

crawling out of 

its bag. Photo: 

Connecticut 

Agricultural 

Experiment 

Station, Bugwood.

org.

Figure 357. Several larval bags that show 

white, silk anchors around the branch. Photo: 

Lacy L. Hyche, Auburn University, Bugwood.

org.
USDA Forest Service RMRS-GTR-241. 2010. 201

choring their bag prior to pupation (fig. 357). Despite having wingless females,

bagworms are widely distributed due to wind-aided larval dispersal, unintended 

movement by humans, an ability to go long periods without food, a wide host 

range, and the production of many eggs by a single female.

Damage—Bagworm is seldom a forest pest of economic concern, but it is commonly a problem on trees and shrubs in urban settings, parks, and shelterbelts. 

Arborvitae and juniper can be killed by complete defoliation. Less severe defoliation results in reduced growth and weakens host plants, increasing susceptibility to opportunistic damaging agents. Mortality and gouting of twigs has been 

attributed to the girdling effect of the tight silken bands with which larvae attach 

the bags (fig. 357). Periodically, bagworm becomes exceedingly abundant, defoliating most any plant it can contact. Although young larvae are inconspicuous 

and have little impact, the seemingly sudden defoliation of tree tops and presence 

of unsightly, though highly visible, bags of large larvae firmly attached to twigs

and leaves causes considerable consternation. Bags may remain attached and 

intact long after the insects that constructed them have died.

Management—Bagworm populations vary widely in size from year to year. 

Disease, low winter temperatures, bird predation, and, especially, parasitic wasps 

affect population size and collapse the short-lived epidemics.

Because they are so conspicuous, overwintering bags and the eggs they contain can be picked from small trees and shrubs and then destroyed. Discarding, 

but not completely destroying removed bags may result in eggs surviving, 

hatching, and dispersing larvae that reinfest treated trees. Labeled, registered 

insecticides are available to use as an alternative means of control against small 

bagworm larvae in spring or early summer. However, by about early July when

larvae are more than 1/2 inch (13 mm) long, it is nearly impossible to kill them 

using insecticide. It is often at this time or later when bagworm infestations and 

associated defoliation become apparent.

References: 32, 174, 189

Cottonwood Leaf Beetle

Skeletonized leaves

Name and Description—Chrysomela scripta 

Fabricius [Coleoptera: Chrysomelidae]

Mature larvae of the cottonwood leaf beetle 

are about 1/2 inch (13 mm) long, black, and have 

two white spots on each side (fig. 358). Adults

are about 1/4 inch (6 mm) long and have yellow

wing covers with black stripes and black heads 

(fig. 359).

Host—Cottonwood, willow, and alder Figure 358. Cottonwood leaf beetle larvae. Photo: Whitney Cranshaw, Colorado 

State Forest Service, Bugwood.org.

D E F O L I A T O R S
202 USDA Forest Service RMRS-GTR-241. 2010.

D E F O L I A T O R S 

Life Cycle—There can be three to four generations per year, depending on location. All life stages can be found throughout the growing season. Bright yellow 

eggs are laid in groups on the undersides of leaves. Early instar larvae hatch

and feed gregariously, skeletonizing leaves and creating a windowpane effect 

(fig. 360). Older larvae and adults consume the entire leaf except for major veins

and petioles (fig. 361); adults also feed on the succulent new shoots.Adults overwinter in duff around the base of host trees or under loose bark on the tree.

Damage—Young trees are generally more affected than older trees. Heavily defoliated trees are weakened. Defoliation also reduces growth rate and can lead 

to deformed trees when young leaders are attacked. Young trees can be killed by 

repeated defoliation events.

Management—Management is not typically warranted. However, large populations can be a nuisance, especially in landscape situations. In heavy infestations, 

chemical sprays can be used to kill both adults and larvae.

References: 124, 174

Douglas-Fir Tussock Moth

Defoliates Douglas-fir, true firs, and spruce

Name and Description—Orgyia pseudotsugata (McDunnough) [Lepidoptera: 

Lymantriidae]

Figure 361.

Cottonwood leaf 

beetle damage. 

Photo: James 

Solomon, USDA 

Forest Service, 

Bugwood.org.

Figure 359. Cottonwood leaf beetle 

adult. Photo: Whitney Cranshaw, 

Colorado State University, Bugwood.org.

Figure 360. Windowpane 

feeding by cottonwood leaf 

beetle larvae. Photo: Lacy 

L. Hyche, Auburn University, 

Bugwood.org.
USDA Forest Service RMRS-GTR-241. 2010. 203

The Douglas-firtussock moth is a common defoliator

of Douglas-fir and true firs.Adult male moths are a nondescript, gray-brown moth with feathery antennae and 

a wingspread of 1-1 1/4 inches (25-32 mm) (fig. 362).

The female is flightless and notably different from the

male in that it has rudimentary wings and a large abdomen, usually about 3/4 inch (19 mm) long. Young larvae 

possess fine hairs; older larvae have two tufts behind the

head, one posterior tuft, and four dense tussocks located 

along the back (fig. 363). Larvae grow up to 1 1/4 inches 

(32 mm). Eggs are laid in a mass on top of the cocoon

from which the female moth emerged (fig. 364).

Hosts—In forested settings, the Douglas-fir tussock

moth prefers Douglas-fir. It is also occasionally found

on true firs or spruce. Forest infestations can be intense,

and other species of conifers surrounding Douglas-fir

trees are often also defoliated. In urban settings, blue 

spruce is attacked. Urban infestations are often confined

to individual trees, and the 

same trees may be attacked 

year after year, which can 

cause considerable damage 

or mortality.

Life Cycle—Douglas-fir

tussock moth has a 1-year 

life cycle and overwinters 

as eggs. Egg hatch coincides with bud burst. Larvae pass through four to six 

molts. Pupation occurs any time from late July to the end of August inside a thin

cocoon of silken webbing mixed with larval hairs. Adults appear from late July

into November, depending on the location. The female moth emits a sex pheromone that attracts males.

Damage—The first sign of attack appears in late spring as young larvae feed

on current year’s foliage, causing it to shrivel and turn reddish brown (fig. 365).

As larvae mature, they feed on older needles. Defoliation occurs first at tops of

trees and outer branches and then, as the season progresses, on lower crowns and 

inner branches of the host tree. During a severe defoliation event, trees will appear as skeletons once the damaged needles have fallen off, and cocoons and egg 

masses will be visible year-round in the lower tree canopy. Damage from severe 

defoliation can lead to tree death or predispose trees to subsequent bark beetle 

attack. Douglas-fir tussock moth can be one of the most damaging of western

defoliators.

Figure 362. Douglas-fir tussock moth adults: male (left) 

and female (right). Photo: 

Rocky Mountain Region, 

USDA Forest Service.

Figure 363. Douglas-fir tussock moth larva. Photo: Rocky 

Mountain Region, USDA Forest 

Service.

Figure 364. Tussock moth female, egg mass, and cocoon (from 

left to right). Photo: William M. Ciesla, Forest Health International, 

Bugwood.org.

D E F O L I A T O R S
204 USDA Forest Service RMRS-GTR-241. 2010.

D E F O L I A T O R S 

Management—Natural controls, including predators, parasitoids, and a nuclear 

polyhedrosis virus (NPV), keep the tussock moth populations low most of the 

time. The natural controls, especially 

the NPV, also act to bring populations 

back under control during an outbreak. 

Douglas-fir tussock moth populations

seem to follow a cyclical outbreak pattern, with outbreaks occurring every 

8-12 years and lasting for 2-4 years. If

applied control is desired, there are registered insecticides that might be used to 

reduce outbreak populations. The NPV 

has been made into a biocontrol (under 

the name TM-Biocontrol) and has been 

used in areas where rare Lepidoptera cooccur with the tussock moth. Availaiblity 

of this producted is limited. The microbial pesticide Bacillus thuringiensis var. 

kurstaki (B.t.k.) is not hazardous to most 

beneficial insects, birds,small mammals,

and aquatic systems. However, B.t.k. results against the tussock moth have not 

been consistent. Other contact chemical 

insecticides are also available for tussock moth management.

Tussockosis—Hairs on the tussock moth larvae can cause an allergic reaction 

in humans. The most common reaction is a skin irritation. Rashes, watery eyes, 

and sneezing are common symptoms. Avoid handling the larvae, and wash after 

exposure.

References: 50, 185

Elm Leaf Beetle

Chewed leaves look scorched

Name and Description—Xanthogaleruca luteola (Müller) (= Pyrrhalta luteola

[Müller]) [Coleoptera: Chrysomelidae]

The wing covers of overwintering adult elm leaf beetles are dark olivegreen (fig. 366). The wing covers change in color during spring to yellow or

a dull yellowish green with a longitudinal, dark stripe along the edges and a 

dark spot at the base of each wing cover (fig. 367). Adults are about 1/4 inch 

(6 mm) long with three black spots present on the thoracic shield behind their

heads. Eggs are bright yellow to yellowish orange, droplet-shaped, and laid in

Figure 365. Tussock moth damage on Douglas-fir 

tree. Photo: William M. Ciesla, Forest Health 

International, Bugwood.org.
USDA Forest Service RMRS-GTR-241. 2010. 205

clumps or irregular rows. Small larvae 

are dark and hairy. Mature larvae are 

1/2 inch (13 mm) long and dull yellow with two dark, longitudinal stripes 

along their side and black-colored 

heads, legs, and tubercles (fig. 368).

Pupae are bright orange-yellow with 

a few black bristles and are about 1/5

inch (5 mm) long. Elm leaf beetle is a

European species established in North

America for over 150 years and now 

occurs throughout most of the United 

States and eastern Canada.

Hosts—Elm leaf beetle feeds upon all

elms, particularly Siberian and English, as well as American, rock, slippery, and other native, Asian, and European species of elm.

Life Cycle—Elm leaf beetle has from

one to three generations per year, depending upon weather and location, 

with two per year being most common 

in the Great Plains. About the time 

elm buds begin to swell in late April 

and early May, overwintering adults 

emerge from protected sites and move 

to elm trees to feed and mate. After a 

few weeks, beetles will begin to lay 

eggs, typically in masses of 5-25 eggs, 

each attached to veins on the undersides of lower leaves, totaling 400-800

eggs per female. Larvae hatch in 10-14

days, feed for about three weeks, and 

then crawl down the trunk in search 

of pupation sites. Most pupate at the 

base of the tree, but some do so in bark 

crevices. Adults emerge in 10-15 days. 

Most will mate and reproduce, beginning a second generation, but some 

adults may seek a sheltered location 

and enter diapause to overwinter. The proportion of the elm leaf beetle population becoming dormant after one generation is weather-dependent, and larger 

second generations are associated with warmer weather. A partial or complete 

third generation may occur rarely. At the end of summer, all adult beetles move 

to dry, sheltered areas and go dormant until spring.

Figure 366. Overwintering elm leaf beetles in reproductive diapause. Photo: Whitney Cranshaw, Colorado 

State University, Bugwood.org.

Figure 367. Adult elm leaf beetles displaying coloration 

typical of the growing season. Photo: Clemson 

University, USDA Cooperative Extension Slide Series, 

Bugwood.org.

Figure 368. Mature elm leaf beetle larva showing 

skeletonizing feeding pattern. Photo: William M. 

Ciesla, Forest Health Management International, 

Bugwood.org.

D E F O L I A T O R S
206 USDA Forest Service RMRS-GTR-241. 2010.

D E F O L I A T O R S 

Damage—Elm leaf beetle is a serious

pest of shade and ornamental elms but is 

of little significance in the forest. Larvae

feed on the undersides of leaves between 

the major veins, seldom penetrating the 

upper surface, in a behavior called skeletonizing (fig. 368). Damaged leaf areas

dry out and turn brown, giving severely 

affected leaves and branches a scorched 

appearance (fig. 369). Leaf feeding by

adults results in a “shot hole” effect (fig.

370). Heavily injured leaves may be shed 

by the tree prematurely. Because young 

larvae require tender leaf tissue, the first

generation is usually the most damaging. Elm leaf beetle feeding can greatly

affect the appearance of elms, rendering them unsightly, and can reduce growth 

and weaken them, particularly when large beetle populations persist for several 

years. A secondary impact occurs when overwintering elm leaf beetles enter 

homes in swarms, particularly during warm winter episodes, which can be so 

severe that it can rival the box elder bug as the primary nuisance insect invader 

of homes in the Great Plains.

Management—Important natural controls include extreme winter temperatures 

and late spring frosts, as well as a native parasitic wasp and native insect predators, including earwigs, stink bugs, and a ladybird beetle. Attempts to introduce 

natural enemiesfrom Europe have met with some successin California, but there

are no reports of this being attempted in the Great Plains.

Destroying egg masses on leaves by hand can reduce localized, small populations, but care must be exercised to avoid damaging the ladybird beetle eggs that 

are somewhat similar in appearance.

There are several labeled, registered chemical and biological insecticides 

that readily control elm leaf beetle populations when sprayed on the foliage. 

Effective applications against the first generation generally suppress populations

sufficiently such that a second application is not necessary. In addition, soil systemic application of appropriate insecticide can also provide effective control.

Figure 369. Larval leaf skeletonizing dries and browns leaves, shown on Siberian elm. Photo: Whitney 

Cranshaw, Colorado State University, Bugwood.org.

Figure 370. Adult elm leaf beetle feeding 

produces a “shot hole” appearance, shown on 

American elm. Photo: Pennsylvania Department 

of Conservation and Natural Resources, Forestry 

Archive, Bugwood.org.
USDA Forest Service RMRS-GTR-241. 2010. 207

Another strategy is to apply a contact insecticide in about a foot-wide band 

around the trunk of infested trees just as larvae begin moving down to pupate. 

If this strategy is employed area-wide, defoliation by the second generation and 

nuisance populations of overwintering elm leaf beetles can be reduced. Migration 

from untreated trees in the area will accomplish rapid reinfestation.

Elm species, hybrids, and horticultural varieties vary in their resistance to elm

leaf beetle feeding. In ornamental settings, planting the most beetle-resistant and 

climate-appropriate elms will minimize elm leaf beetle problems.

References: 35, 50, 174

Fall Webworm

Wraps branches in silk in late summer

Name and Description—Hyphantria cunea (Drury) [Lepidoptera: Arctiidae]

Adult fall webworms are white with some orange 

markings on the body and legs. The wings often have 

some black spots and an expanse of about 1 1/4 inches 

(3.2 cm) (fig. 371). Eggs are light green or yellow,

globular, and laid in flat masses consisting of several

hundred eggs. Larvae are hairy with distinct, paired, 

dark spots on each segment of the back and are 1 

1/4-1 1/2 inches (3.2-3.8 cm) long at maturity. Larval 

color is variable, with two races recognized (figs. 372-

373). The black-headed, or northern race, has a black 

head with a pale yellowish or greenish body that has 

a dark stripe on the back and long white hairs rising 

from red or black tubercles (humps). Larvae of the 

Figure 371. Fall webworm 

adult. Photo: Gerald J. Lenhard, 

Bugwood.org.

Figure 372. Two forms of the red-headed race 

of the fall webworm larva. Photo: Lacy L. Hyche, 

Auburn University, Bugwood.org.

Figure 373. Examples of the black-headed race of fall 

webworm larvae. Note that both races can be present 

together and each is variable in coloration. Photo: Lacy L. 

Hyche, Auburn University, Bugwood.org.

D E F O L I A T O R S
208 USDA Forest Service RMRS-GTR-241. 2010.

D E F O L I A T O R S 

red-headed, or southern race, are yellowish tan with red or orange-colored heads 

and brownish hair that arises from reddish brown tubercles. The distribution of 

fall webworm in North America is transcontinental, extending from southern 

Canada to northern Mexico.

Hosts—Over 100 hardwood tree species, including alder, ash, choke-cherry, cottonwood, elm, maple, various fruit trees, walnut, and willow

Life Cycle—Fall webworm generally has one generation per year in the Rocky 

Mountain Region. Adults appear from late June to early July, fly at night, and lay

eggs on the underside of leaves. After hatching, the young larvae feed gregariously, at first on the epidermis but not the veins of both leaf surfaces, producing

a skeletonized pattern. As the larvae grow, they begin to feed on entire leaves, 

spinning silk wherever they go. Eventually, they enshroud leaves and then whole

branches in a loosely spun tent of silk, within which larval development and 

feeding occurs, so that leaf fragments, cast skins, and droppings become incorporated. The larvae, when disturbed, often twitch and wave their bodies synchronously in what is thought to be predator avoidance. Feeding continues until midSeptember, when full-grown larvae wander from their host plant in search of 

protected pupation sites. The insect overwinters in the pupal stage within a light 

silken cocoon in the soil, leaf litter, on the sides of buildings, or on tree trunks.

Damage—This insect causes minor defoliation in most forested situations. 

Large infestations can envelop and defoliate entire trees, which seems a relatively frequent occurrence on narrowleaf cottonwood along streams in Colorado. 

Substantial defoliation may weaken tree defenses to other opportunistic agents. 

As the most common tent-making defoliator in much of North America, fall 

webworm can cause loss of visual quality in ornamental plantings due to the 

unsightly tents (figs. 374-375). Wandering hordes of hairy, full-grown fall webworm larvae can be a substantial, if brief, nuisance.

Figure 375. Silken tents that contain fall webworm 

colonies on chokecherry. Photo: Whitney Cranshaw, 

Colorado State University, Bugwood.org.

Figure 374. Silken tents that contain fall webworm 

colonies can envelop whole branches or entire 

trees. Photo: Ronald F. Billings, Texas Forest 

Service, Bugwood.org.
USDA Forest Service RMRS-GTR-241. 2010. 209

Management—Generally, population control need not be attempted, as biological impact from fall webworm defoliation is inconsequential. Deciduous tree 

species can recover from substantial defoliation, particularly if impacted late in 

the season when most growth has already occurred. Typically, impacts to plant 

health cannot be observed until defoliation involves more than 20% of the foliage. Fall webworm populations are usually not maintained at high levels for 

consecutive years, although local outbreaks can be relatively common.

Fall webworm is food for many natural enemy species whose actions reduce 

population size and impact. Conservation of natural enemies includes restraint 

from using contact insecticides or judicious, well-timed applications when such 

beneficial organisms are not present.

Stressed or recently planted, smaller trees can be substantially weakened by 

intense webworm defoliation, potentially rendering them more vulnerable to opportunistic pathogens and/or insects. Aesthetic and nuisance considerations may 

prompt control attempts as well.

Fall webworm larvae and colonies are most susceptible to direct control when 

they are small. Hand picking larvae or destroying their tents can significantly

reduce a local population. Destroying tents or larvae by burning should not be 

done, as it can easily cause more damage to the tree than defoliation. Several 

insecticides are registered for fall webworm control and, to be most effective, 

must be applied before tents and the larvae they contain become large. Some 

registered insecticides have activity against Lepidoptera only, sparing the natural 

enemies. Limitations of these products include: the material must be ingested 

to kill webworms, may be washed away by rain, and will only kill larvae that 

are very small. Pesticide coverage on tall trees can be especially challenging to 

achieve. Area-wide control in a forest setting is not warranted.

References: 32, 35, 50

Forest Tent Caterpillar

Aspen defoliator makes no tent

Name and Description—Malacosoma disstria Hübner [Lepidoptera: Lasiocampidae]

Adult forest tent caterpillars are stout-bodied and light yellow to yellowbrown and have a wingspread of 1-1 1/2 inches (2.5-3.8 cm) (fig. 376). The

forewings have two darker oblique lines near the middle. Eggs are cemented

together and coated with a frothy, glue-like substance that hardens and turns a 

glossy dark brown (fig. 377). Newly hatched larvae are nearly uniformly black,

about 1/8 inch (3 mm) long, and conspicuously hairy. With each successive molt, 

the characteristic markings of mature larvae become more evident—pale blue 

lines along the sides of a brownish body with a mid-dorsal row of keyhole- or 

footprint-shaped, whitish spots on a black background (fig. 378). When fullgrown, caterpillars are about 2 inches (50 mm) long. Cocoons are composed of 

dense, yellowish silk with a powdery material dispersed between the strands.

D E F O L I A T O R S
210 USDA Forest Service RMRS-GTR-241. 2010.

D E F O L I A T O R S 

Hosts—Hosts include many hardwood tree species and preference varies by location. In the 

western United States, aspen is the preferred host, 

although alder, basswood, birch, cherry, cottonwood, elm, oak, poplar, various fruit trees, and 

willow are acceptable. If trees are defoliated, 

atypical hosts such as understory shrubs and the 

leaves of fruit and vegetable plantings may be fed 

upon by hungry, wandering larvae.

Life Cycle—All Malacosoma spp. have one generation per year and similar life cycles. Young 

larvae hatch in spring when leaves are beginning to unfold and are gregarious. Colonies stay 

together and move about in a line, following silk 

trails that are laid down by the leaders. Unlike 

other North American species of Malacosoma, the 

larvae do not construct tents. Instead, they spin silken mats on tree trunks or 

branches where they congregate when not feeding (fig. 379). The larvae feed for

5-6 weeks, becoming solitary in habit during the last stage. If large populations

result in complete tree defoliation, the last two larval stages will wander in search 

of food. When full-grown, larvae disperse and spin dense silk cocoons, within 

Figure 377. Egg masses of forest tent caterpillar, 

showing an old, hatched mass on the left and a new 

mass on the right. Photo: James B. Hanson, USDA 

Forest Service, Bugwood.org.

Figure 378. Mature larva of forest tent caterpillar 

displays the characteristic “keyhole” pattern down the 

back and distinctive blue markings. Photo: Whitney 

Cranshaw, Colorado State University, Bugwood.org.

Figure 379. When not feeding, forest tent 

caterpillars congregate on a silken mat to 

rest and to molt. Photo: James Solomon, 

USDA Forest Service, Bugwood.org.

Figure 376. Adult forest tent caterpillar. Photo: Whitney Cranshaw, 

Colorado State University, Bugwood.

org.
USDA Forest Service RMRS-GTR-241. 2010. 211

which they pupate. Cocoons can be found in folded leaves, bark crevices, or 

other sheltered sites. The moths, which emerge about 10 days later in late July,

are most active at night. Strong winds can carry the moths over long distances, 

and great numbers are attracted to lights. The eggs are laid mostly on uppercrown branches in masses of 100-350, which completely encircle small-diameter 

twigs. Inside the eggs, embryos develop into larvae that overwinter and hatch in 

the spring.

Damage—Forest tent caterpillar is the most widely distributed and destructive 

tent caterpillar in North America. In many areas, short-duration outbreaks result 

in intense defoliation, particularly to aspen. Forest tent caterpillar outbreaks have 

been known to occur across vast areas of the Lake States and prairie provinces 

of Canada, but less so in Colorado and Wyoming. Defoliation can result in reduced tree growth, top-kill, tree mortality, and impairment of aesthetic values. 

During years when forest tent caterpillars are especially abundant, they strip host 

plants of foliage and then wander across open ground in search of additional 

food sources, becoming a hazard for automotive traffic, trains, and pedestrians.

A major natural enemy of forest tent caterpillar, the “friendly fly” Sarcophaga 

aldrichi Parker [Diptera: Sarcophagidae], becomes extremely abundant and contributes to the termination of forest tent caterpillar outbreaks in aspen. Although 

they do not bite, clouds of this friendly fly alighting on people and regurgitating on them and their clothing can be exceedingly annoying. During forest tent 

caterpillar outbreaks, the loss of shade, the nuisance occasioned by wandering 

caterpillar hordes, and incessant harassment from the friendly fly can negatively

impact recreational use of an area.

Management—Forest tent caterpillar outbreaks are usually of short duration, 

lasting 3-5 years. Many factors regulate population size and contribute to outbreak collapse, including potent viral and fungal diseases, parasitic flies and

wasps, predatory insects and birds, starvation, and, occasionally, unseasonable 

cold weather immediately prior to, during, and right after egg hatch.

On small, ornamental trees and shrubs, the egg masses can be removed by 

hand and destroyed between July and the following spring. After hatching,

young colonies of larvae can be removed from branch tips or squashed while 

they are resting in dark clusters on the main stem, especially in the evening or 

on cool days. In June, when the caterpillars are migrating, a sticky or slippery

collar can be placed around the base of trees as a barrier, affording trees some 

protection.

Several insecticides are registered for forest tent caterpillar control, all of 

which are most effective if applied when the larvae are small. Some registered 

insecticides have activity against Lepidoptera only, which spares important insect 

natural enemies because they are immune. Limitations of such products include: 

some defoliation occurs after the insecticide is applied because the material must 

be ingested to take effect; the product is easily washed away by rain; and it will 

only kill larvae that are very small. Area-wide control in a forest setting using 

insecticide has been and will likely continue to be employed in locations with a 

D E F O L I A T O R S
212 USDA Forest Service RMRS-GTR-241. 2010.

D E F O L I A T O R S 

significant outbreak history in order to protect values and activities at risk such

as growth increment on high-value trees and recreation.

References: 11, 50

Large Aspen Tortrix

Larvae in rolled leaves

Name and Description—Choristoneura conflictana (Walker) [Lepidoptera: 

Tortricidae]

The adult large aspen tortrix is a moth with a wingspread of 1-1 1/4 inches (2.5-3.0 cm) (fig. 380). The

forewing is grayish with basal, middle, and outer 

brownish patches. Oval, pale green eggs are laid in 

rows that overlap like fish scales, forming large, flat

masses (fig. 381). Young larvae have yellowish or

pale green bodies and black heads. With each molt, 

larvae darken in color. Mature larvae are about 3/4 inch 

(15-21 mm) long and are grey-green to nearly black 

with dark spots on each segment and a dark shield and 

head (figs. 382-383). Pupae are light green when first

formed but soon change to a reddish brown to black 

color (fig. 384). Large aspen tortrix is a transcontinental, principally boreal species, occurring from Alaska to New 

Mexico, distributed along with its principal host, aspen.

Hosts—Aspen. During epidemics when aspen is completely 

defoliated, larvae will feed on other associated broad-leaved 

trees such as alder, birch, cottonwood, chokecherry, and 

willow and may consume the 

foliage of understory plants 

when starving.

Figure 380. Adult large aspen tortrix. Photo: Steven Katovich, USDA 

Forest Service, Bugwood.org.

Figure 381. Unhatched egg mass of large 

aspen tortrix. Photo: Steven Katovich, 

USDA Forest Service, Bugwood.org.

Figure 382. Mature, light green larva 

of large aspen tortrix. Photo: William 

M. Ciesla, Forest Health Management 

International, Bugwood.org.

Figure 383. Dark green 

larva of large aspen tortrix. Photo: K.B. Jamieson, 

Canadian Forest Service, 

Bugwood.org.
USDA Forest Service RMRS-GTR-241. 2010. 213

Life Cycle—There is one generation per year. Eggs are laid in mid-June through

July, mainly on the upper surfaces of leaves but almost anywhere if no aspen

foliage is available. Newly hatched larvae feed on leaf tissue but not veins, skeletonizing the leaf. Young larvae are often gregarious and can be found feeding 

within previously rolled leaves in old pupation sites. Dispersal to hibernation 

sites such as bark crevices and other protected places begins in early August 

and is completed by September. Larvae molt once and then overwinter within 

hibernaculae, which are white silk casings that each 

larva spins around itself. On warm days in early spring, 

the small larvae emerge from hibernaculae and move 

up the stems of aspen. The larvae mine into the buds, 

feeding on the young tissue. At this stage, secondary hibernaculae are often constructed at the base of the buds. 

In later stages, the larvae roll (fig. 384), fold (fig. 385),

or web leaves into shelters, within which they feed and 

eventually pupate. When disturbed, larvae writhe vigorously and often drop, suspended by a silk thread. During epidemics, larvae frequently wander in search of 

food, resulting in extensive webbing that covers grass 

and understory plants. Pupation occurs from mid-June

to mid-July, depending upon location and weather, and

lasts about 10 days. Adults are active for about a 2-week 

period from late June through July. Females are sluggish

and stay in the area of emergence, whereas males are 

more energetic and erratic in flight.

Damage—Large aspen tortrix becomes a problem only 

where aspen is a major component of the forest stand. 

Entire aspen stands and vast aspen forests have been

completely defoliated during outbreaks. Larval feeding 

in early spring sometimes causes complete defoliation 

before the buds have expanded.

Healthy, defoliated aspen usually grow new 

leaves by midsummer, but they are often smaller and fewer than normal, resulting in thinned 

crowns. Copious amounts of silk webbing in the 

understory can be annoying to people walking 

through heavily defoliated aspen stands. Growth 

loss is the major impact in the early stages of an 

outbreak. Repeated defoliation over a period of 

years can cause branch death that is most noticeable in trees growing on marginal sites. Weakened 

aspen are more susceptible to opportunistic 

agents such as plant diseases and insects. Despite 

growth loss, aspen usually withstand large aspen 

tortrix outbreaks with little tree mortality.

Figure 385. Pupa of large aspen 

tortrix protruding from leaf shelter. 

Photo: William M. Ciesla, Forest Health 

Management International, Bugwood.org.

Figure 384. Leaf roll held 

together by silk. Photo: 

Steven Katovich, USDA Forest 

Service, Bugwood.org.

D E F O L I A T O R S
214 USDA Forest Service RMRS-GTR-241. 2010.

D E F O L I A T O R S 

The impact of an outbreak depends upon many factors, primarily the frequency and severity of defoliation. Other considerations include wildlife use and 

recreational and aesthetic values of the aspen stand. Defoliation alone can reduce 

values, especially in high-use areas.

There are numerous similar looking caterpillar species on aspen that roll, tie, 

or fold leaves, many of which are also in the family Tortricidae. All are of minor 

consequence. Increases in large aspen tortrix populations are sometimes associated with increases in the populations of other aspen feeding insect species, such 

as tent caterpillars or leafminers.

Management—Outbreaks are characterized by the build-up of large populations 

that persist for 2 or 3 years and then suddenly collapse without human intervention. Though natural enemies, including insects, diseases, and birds, feed upon 

large aspen tortrix, the most significant contributors to outbreak collapse are

thought to be starvation and other debilitating effects of foliage depletion, as 

well as unseasonable cold weather early in spring that either kills young larvae or 

reduces their food supply by harming leaf tissue. Aspen can tolerate significant

defoliation. Because of the short duration of outbreaks of large aspen tortrix and 

associated insect defoliators, direct control is not generally recommended. In 

high-use areas like campgrounds, applying registered chemical or biological insecticides may be appropriate in order to reduce defoliation, masses of webbing 

in the understory, or aesthetic damage.

References: 13, 35, 50

Needleminers

Hollowed out needles

Name and Description—Pinyon needleminer—Coleotechnites edulicola

Hodges and Stevens [Lepidoptera: Gelechiidae]

Ponderosa pine needleminer—C. ponderosae Hodges and Stevens

White fir needleminer—Epinotia meritana (Heinrich) [Lepidoptera: 

Tortricidae]

Spruce needleminers—Endothenia albolineana (Kearfott) [Lepidoptera: 

Tortricidae] and Coleotechnites piceaella (Kearfott)

Needleminers are tiny moth larvae that feed inside conifer needles. They are 

locally common on pinyon and ponderosa pine and are less commonly found on 

white fir and spruce.Adult moths are rarely noticed, but the hollowed out needles

with tiny larvae or frass are distinctive (fig. 386). Larvae are particularly visible

in ponderosa pine needles when held into the sunlight (fig. 387). Exit holes are

also diagnostic on older hollowed out needles (fig. 388).

Hosts—Pinyon pine, ponderosa pine, white fir, and spruce are hosts to different species of needleminers in the Rocky Mountain Region. Lodgepole pine 

needleminers occur in California and Oregon but have not been reported in the 

Rocky Mountain Region.
USDA Forest Service RMRS-GTR-241. 2010. 215

Life Cycle—Pinyon needleminers lay eggs from early June through mid-July.

Larvae emerge soon after eggs are laid and bore into uninfested needles where 

they feed until fall. They overwinter inside the needles as dormant larvae. Feeding resumes in the spring, and larvae grow to about 3/8 inch (5 mm) long. Pupation occurs in late May. Ponderosa pine needleminers lay eggs in late summer 

inside previously mined needles. The newly hatched larva bores into the tip of a 

green needle and mines slowly through the winter, developing more rapidly as 

the weather warms and pupating in midsummer. White fir needleminers have a

similar life cycle but will mine about six needles before pupating in June or early

July. White fir needleminers hatch inAugust and September and spend the winter

within a needle. The two species of 

spruce needleminer that occur in 

the Rocky Mountain Region also 

overwinter as larvae. E. albolineana spends the winter protected 

in mats of webbing and dead needles attached to twigs (fig.  389).

Coleotechnites piceaella spends 

the winter inside of mined needles. 

Both species resume feeding in 

early spring and complete their development by late spring. Eggs are

laid in May and June.

Figure 387. Ponderosa pine needleminer. Photo: 

Southwestern Region, USDA Forest Service.

Figure 388. Pinyon needleminer-damaged 

needles. Photo: Robert Cain, USDA Forest 

Service.

Figure 389. Webbed needles from spruce needleminer. 

Photo: Whitney Cranshaw, Colorado State University, 

Bugwood.org.

Figure 386. Ponderosa pine needleminer. Photo: 

Scott Tunnock, USDA Forest Service, Bugwood.org.

D E F O L I A T O R S
216 USDA Forest Service RMRS-GTR-241. 2010.

D E F O L I A T O R S 

Damage—Damage 

first becomes evident

as foliage fades to yellow and brown. Early

needle drop, reduced 

growth, and tree mortality can all result 

from needleminer infestations. The severity of the infestation 

can vary significantly

from tree to tree, suggesting that individual 

trees have some resistance to these pests. Foliage thinning by needleminers can reduce tree vigor and 

increase susceptibility to bark beetle attack (fig. 390).

Management—Trees usually recover from needleminer damage without suffering serious injury. Properly timed insecticide applications have effectively controlled populations on landscape trees. Endothenia albolineana overwintering 

nests can be removed from infested spruce with a strong stream of water from a 

garden hose in early spring before bud flush.

References: 22, 35, 50

Oak Leafroller

Defoliates red and white oaks

Name and Description—Archips semiferana (Walker) [Lepidoptera: Tortricidae]

The oak leafroller is a common defoliator of oaks. In the central Rocky 

Mountains, the oak leafroller defoliates gambel oak. Moths are a light olive 

brown and have a diagonal rusty band on the forewing (fig. 391). The wingspan

is approximately 7/8 inch (22 mm) long. Mature larvae are approximately 4/5 inch 

(21 mm) long and are greenish yellow (figs. 392-393). Eggs are laid in a mass

containing 15-125 eggs and are covered with tan 

scales from the female’s abdomen.

Hosts—Gambel oak is the primary host in the 

Rocky Mountain Region, although ornamental 

plantings of various red and white oaks may be 

subject to infestation by the oak leafroller. Occasionally, the insect may be found on maples.

Life Cycle—The oak leafroller egg masses are deposited in the canopy of trees in July and August.

A majority of the egg masses are deposited on and 

Figure 391. Oak leafroller adult. Photo: 

John Himmelman, http://booksandnature.homestead.com/moth133.html.

Figure 390. White fir needleminer damage. Photo: Intermountain Region, 

Ogden Archive, USDA Forest Service, Bugwood.org.
USDA Forest Service RMRS-GTR-241. 2010. 217

overwinter in the middle 

half of the bole and branches of the canopy, and more 

occur on the southern side 

of the tree. Larvae emerge 

from eggs in spring at 

about the time that buds are bursting. Larvae feed 

within the protective folds of young leaf clusters and 

later roll or fold one or more entire leaves or leaf 

parts (fig. 394). The larvae pass through five instars.

Pupation takes place in the leaf roll in June to early

July, and the adults appear in July.

Damage—The oak leafroller defoliates oaks and is 

of particular significance in the mid-continent oak

woodlands, where red, scarlet, northern pin, white, 

and chestnut oaks have been defoliated. Outbreaks 

of this insect are infrequent in the Midwest and East

and may last from 3-5 years. In the central Rocky Mountains, outbreaks of the 

oak leafroller are sporadic. Defoliation may lead to weakened trees that succumb 

to other factors such as late frosts or wood borers. First-instar larvae produce 

webbing within folds of the young leaf cluster; later instars will web together 

leaves or roll leaves to provide for shelter and food.

Management—There is little information on the natural control agents of the 

oak leafroller, but natural enemies are thought to be important in preventing 

damaging populations. Mature larvae exhibit behavior typical of the members 

of its moth family, Tortricidae, when disturbed. They retreat rapidly, exit the 

shelter, and drop on a silken thread. To manage a local problem, horticultural oil 

is considered effective in controlling the overwintering egg stage of this insect. 

Applications should target the twigs where the egg masses are deposited.

Reference: 186

Figure 392. Oak leafroller 

larva. Image: Texas Agriculture 

Extension Service.

Figure 393. Oak leafroller 

larvae and damaged leaf. Photo: 

Northern Research Station, USDA 

Forest Service.

Figure 394. Rolled 

oak leaves (from 

Wilson 1972).

D E F O L I A T O R S
218 USDA Forest Service RMRS-GTR-241. 2010.

D E F O L I A T O R S 

Pandora Moth

Rare outbreaks leave pines tufted

Name and Description—Coloradia pandora (Blake) [Lepidoptera: Saturniidae]

The pandora moth is one of the largest forest insects in North America. 

Adults have a thick, heavy body 1-1 1/2 inches (2.5-3.8 cm) long and a wingspread of 3-5 inches (7-13 cm). Forewings are brownish grey, and hindwings 

are a light pinkish grey, each marked with a dark spot near the center above a 

transverse wavy line (figs. 395-396). Eggs are globular, about 1/8 inch (3 mm) 

in diameter, and are bluish green at first, later turning bluish grey. Eggs are laid

in clusters of 2-50 each. Newly hatched 

larvae have shiny black heads and dark 

brown to black, spiny bodies about 1/4

inch (6 mm) long. Intermediate stage

caterpillars have two narrow white 

lines marking their upper surface and 

an overall brown color. Mature larvae have an orange-brown head with a 

pale yellow-brown collar and are about 

2-3 inches (60-80 mm) long (figs. 397-

398). Their bodies start out dark brown 

and then change to greenish yellow, are 

marked with transverse yellow bands 

and a longitudinal white stripe and bear 

a few branched spines on each segment. 

These spines can produce a stinging 

sensation upon contact. The pupae are 

stout, dark reddish or purplish brown 

(fig. 399) and naked in the soil beneath

pines in a cell formed by the larvae.

The similar looking Black Hills pandora moth, C. doris Barnes, occurs in 

Colorado, Montana, South Dakota, and 

Wyoming, although little is recorded 

of its habits beyond a documented outbreak from 1938 to 1939 near Osage, 

Wyoming, and in the northern Black 

Hills. It apparently has a 1-year life 

cycle.

Hosts—Lodgepole and ponderosa pines

Life Cycle—The pandora moth requires 2 years to complete its life cycle, spending the first winter as larvae in the tree canopy and the second as pupae in the

litter or soil. Moths emerge from buried pupal cases in late June or July. During

Figure 397. Mature, healthy larva of pandora moth. 

Photo: Rocky Mountain Region Archive, USDA 

Forest Service, Bugwood.org.

Figure 395. Female adult pandora moth with threadlike, narrow antennae. Photo: Intermountain Region, 

Ogden Archive, USDA Forest Service, Bugwood.org.

Figure 396. Male adult pandora moth with 

feathery antennae. Photo: Terry Spivey, USDA 

Forest Service, Bugwood.org.
USDA Forest Service RMRS-GTR-241. 2010. 219

epidemics, thousands of these moths can be seen flying in the forest canopy.

Males seek the females, who generally do not fly until after mating. Females lay

an average of 80 eggs that are distributed in several clusters, usually on the bark 

and needles of pines. Eggs are occasionally deposited on ground litter or brush.

Eggs hatch in August following approximately 40-50 days of incubation. Young

larvae crawl up the trees and feed in clusters on needles of the terminal shoots 

on the outer branches. In moving from one feeding site to another, larvae travel 

together in single file. In autumn, larvae disperse and become solitary. Larvae

overwinter primarily in the second stage at the base of the needles, usually amidst 

some strands of silk, and may feed as weather permits. With the onset of warmer 

weather in spring, feeding resumes and larvae grow rapidly. Masses of greenish 

or brownish droppings and cast skins can be found under infested overstory trees 

due to the enormous quantity of needles consumed by caterpillars. Needles of all 

age classes are consumed, but the buds are not damaged. When fully grown in 

July or early August, larvae crawl down tree trunks and enter loose material to

pupate, preferring pumice or decomposed granite soils. Pupation typically lasts 

for about a year, but, in some areas at least, a substantial portion of the generation remains in the soil for 2 years and some individuals remain for up to 5 years.

Damage—Outbreaks, some covering vast acreage, have occurred only on areas 

with soils loose enough for larvae to bury themselves prior to pupation. This 

insect is rarely seen except during outbreaks, which occur at 20-30-year intervals and may last 6-8 years. The most recent outbreak in the Rocky Mountains

occurred from about 1959 to 1966 in Colorado and Wyoming. During outbreaks,

growth loss and mortality can be significant. Pandora moth has a 2-year life cycle, with feeding and moth flight occurring in alternate years, so that most of the

defoliation occurs every other year. Larvae feed primarily on older foliage, leaving trees with a tufted appearance. Because terminal buds are not damaged, even 

Figure 398. Mature, healthy larva of pandora moth (on 

fingers) and diseased larva with viral wilt (hanging). 

Photo: Donald Owen, California Department of Forestry 

and Fire Protection, Bugwood.org.

Figure 399. Pupae of pandora moth with 

smaller pupae of parasitic flies, visible at lower 

right. Photo: Southwestern Region, USDA 

Forest Service.

D E F O L I A T O R S
220 USDA Forest Service RMRS-GTR-241. 2010.

D E F O L I A T O R S 

severely defoliated trees usually recover completely. Heavy defoliation can predispose trees to 

attack by bark beetles, and repeated defoliation 

alone can kill pines, especially those stressed by 

other agents such as heavy dwarf mistletoe infection or drought. Moth flights to nearby sources of

light may be the first indication of an increasing

population of pandora moth.

Management—The pandora moth has many 

natural enemies, including insects (fig. 400),

diseases, rodents that dig up and eat pupae, and 

birds that consume all life stages. High soil temperatures during the pupation period can cause 

substantial mortality. Perhaps most significant in

causing the collapse of outbreaks is a viral wilt 

disease that devastates dense populations.

Insecticides are not generally needed because 

natural controls suppress outbreaks before significant mortality occurs. An appropriate, registered 

chemical insecticide would likely reduce larval 

populations, considering how exposed the feeding 

larvae are. Several prescribed burning treatments 

have been attempted to kill pupae and larvae with 

variable success. Pandora moth is one of many 

insects used as human food, although sporadic 

abundance limits its use as a staple.

References: 24, 35, 50

Pine Butterfly

Larvae feed on older needles

Name and Description— Neophasia menapia (Felder and Felder) [Lepidoptera: 

Pieridae]

This insect can be identified easily during any season. Look for single rows

of emerald green eggs from September to June on needles, colonies of immature

pale green larvae with black heads, or individual full-grown larvae about 1 inch 

(2.5 cm) long with two white, lateral stripes and green heads (fig. 401) from

June to August. Pupae, usually found during August, are also green with white

stripes and are attached to needles, branches, or stems. Adults are white butterflies with black wing markings and are seen flying around tree canopies from

August through September (fig. 402).

Hosts—Ponderosa pine

Figure 400. Mature larva of pandora 

moth, a victim of multi-brooded parasitic wasp, showing wasp cocoons. 

Photo: Donald Owen, California 

Department of Forestry and Fire 

Protection, Bugwood.org.
USDA Forest Service RMRS-GTR-241. 2010. 221

Life Cycle—Adults fly and lay their green eggs from August through October

(fig. 403). The eggs overwinter, and the larvae hatch the following June about the

time new foliage appears. The small larvae feed in clusters with their black heads 

oriented toward the tip of the needle. As the larvae mature, they feed singly. They 

pupate in late July for approximately 15-20 days (fig. 404).

Damage—Repeated defoliation can kill ponderosa pine, reduce radial growth up 

to 70%, or weaken trees enough to attract bark beetles. Under ordinary conditions, larvae feed only on older needles and are not a big problem, but during epidemics, they also consume new needles. In normal years, pine butterfly feeding

causes the twigs to be defoliated with the exception of the current year’s growth. 

Tree crowns then appear thin, and only tufts of new needles remain (fig. 405).

Figure 401. Pine butterfly 

larva. Photo: Ladd Livingston, 

Idaho Department of Lands, 

Bugwood.org.

Figure 402. Pine butterfly adult. 

Photo: Jerald E. Dewey, USDA Forest 

Service, Bugwood.org.

Figure 403. Pine butterfly 

eggs. Photo: Intermountain 

Region, Ogden Archive, USDA 

Forest Service, Bugwood.org.

D E F O L I A T O R S

Figure 404. Pine butterfly life cycle (from Johnson 1982).

Pine Butterfly Life Cycle

Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec.

Eggs

Adults

Pupae

Larvae
222 USDA Forest Service RMRS-GTR-241. 2010.

D E F O L I A T O R S 

Management—Management is generally 

not warranted. Natural 

control agents often 

keep populations in 

check and end epidemics. No correlation has been found 

between crown classes 

or diameters and resultant mortality from 

defoliation. Tree vigor 

prior to defoliation is 

important in a tree’s 

recovery from defoliation.

References: 28, 50, 93

Pine Sawflies

Feed on older needles, leaving distinctive stubs

Name and Description—Neodiprion spp. and Zadiprion spp. [Hymenoptera: 

Diprionidae]

Adult pine sawflies are thick-waisted wasps about 1/2 inch (10-12 mm) long 

(fig. 406). Females are yellowish brown, and the smaller males are mostly black,

bearing feathery antennae. Pine sawfly larval appearance varies by species and

by larval stage, but most are green or yellowish green in color with black, tan, or 

orange head capsules (figs. 407-408). Mature larvae are about 3/8 inch (10 mm) 

long. Both caterpillars (order Lepidoptera) and sawfly larvae have three pairs of

true legs, but sawfly larvae have six or more pairs of fleshy prolegs behind their

legs along the abdomen, while caterpillars have two to five pairs of prolegs. The

head and jaws of sawfly larvae point downward, perpendicular to the long axis

of the body, while caterpillar heads and 

jaws point forward, in line with the body 

length. Although not completely accurate, 

a quick test to distinguish sawfly larvae

from caterpillars can be done based on 

their behavior—sawflies frequently feed

in groups, and, when disturbed, the entire 

group tends to pulsate or wave their bodies, 

often in unison. This reaction is thought to 

be defensive. A papery, cylindrical cocoon 

coverssawfly pupae, which form in the soil.

Figure 405. Pine butterfly defoliation. Photo: Intermountain Region, Ogden 

Archive, USDA Forest Service, Bugwood.org.

Figure 406. Adult sawfly female (Neodiprion

spp.) laying an egg in a ponderosa pine needle. 

Photo: Michael R. Wagner, Northern Arizona 

University.
USDA Forest Service RMRS-GTR-241. 2010. 223

Eggs are laid in slits cut in the edge of living pine needles(fig. 409). There are six

pine-feeding Neodiprion spp. in the region and two pine-feeding Zadiprion spp. 

Differentiating among Neodiprion spp., especially among the five that feed on

ponderosa pine, can be difficult given the considerable resemblance and variation among closely related species and recent taxonomic changes.

Hosts—Pines, particularly ponderosa pine and pinyon, but rarely on other pines 

and Douglas-fir

Life Cycle—Neodiprion spp. pine sawflies have a 1-year life cycle that is similar

among species but differs in timing of life stages. They overwinter either as eggs 

inserted into pine needles or as full-grown larvae within cocoons in the ground 

litter or soil. The species that overwinter as eggs begin feeding upon hatching 

early in spring, while species that overwinter as larvae pupate in spring, emerge 

and mate as adults, and lay eggs in pine needles. Consequently, young Neodiprion spp. sawfly larvae are found in either spring-summer or summer-fall, feeding gregariously on older foliage. Young larvae consume only the outer needle 

tissue and leave the central ribs intact. The central ribs later turn yellow-brown 

and break off. Older larvae feed singly and consume most of the needle, often 

leaving a distinctive stub. Species of spring-summer feeders generally consume 

only old needles, completing their feeding before pine shoots have elongated, 

and enter the soil to pupate. Adults emerge late in summer and lay eggs in live 

needles to overwinter. Species of summer-fall feeders consume both old and new 

pine needles, complete feeding in late summer, and enter the litter or soil, forming cocoons in which they overwinter.

Figure 407. Late stage larvae of 

Neodiprion gillettei feeding on ponderosa pine. Photo: Southwestern Region, 

USDA Forest Service.

Figure 408. Late stage larvae of Zadiprion 

rohweri feeding on pinyon pine. Photo: 

Michael R. Wagner, Northern Arizona 

University.

Figure 409. Rows of 

Neodiprion autumnalis eggs 

inserted into ponderosa 

pine needles. Photo: Mark 

Harrell, Nebraska Forest 

Service.

D E F O L I A T O R S
224 USDA Forest Service RMRS-GTR-241. 2010.

D E F O L I A T O R S 

Zadiprion townsendi (Cockerell), the bull 

pine sawfly, is the only sawfly species whose

larvae feed on ponderosa pine in winter, weather permitting. The larvae are wormlike, dark 

green to black with gold flecking, and feed

gregariously (fig. 410). Larvae are about halfgrown by the onset of winter and are mature by 

May orJune. The mature larvae burrow into the

soil and spin a cocoon. Some pupate, emerge, 

mate, and lay eggs in pine needles, concentrating on the upper areas of trees that hatch that 

same summer. Others delay pupation and subsequent events until late summer or 

the following season. The entire life cycle takes 1-2 years to complete.

Zadiprion rohweri (Middleton) (fig. 408) reportedly has two generations per

year on pinyon.

Damage—Infested trees have sparse foliage and thin crowns. Consumption 

of older needles gives pines a tufted appearance that is created by current year 

needles growing on the tips of branches stripped of older foliage (fig. 411). Consumption of all old and new needles in one season has a greater impact on pine 

health. When dense larval populations descend together to enter the soil and spin 

cocoons, they can cause considerable consternation. Different species have different preferences for the size of host attacked and location on the host where they 

feed. Pine sawfliestypically attack open-grown trees or areas where pine is growing at a low density, often on poorer sites with dry, shallow soils. In some cases, 

prolonged drought is accompanied by pine sawfly outbreaks. The same trees are

frequently defoliated year after year while others nearby may remain largely unaffected. In general, defoliation causes slower growth. Repeated defoliation can 

result in top-kill. Although relatively rare, pine mortality may occur, particularly 

if bark beetles attack trees weakened by defoliation.

M a n a g e m e n t —

Typically, predators, parasites, 

viral disease, and 

foliage depletion 

manage to reduce 

pine sawfly outbreak populations 

within 2-3 years. 

Consequently, active management is 

often not necessary. 

When only a few 

colonies of larvae 

Figure 410. Late stage larvae of 

Zadiprion townsendii feeding on ponderosa pine. Photo: Michael R. Wagner, 

Northern Arizona University.

Figure 411. Feeding on old needles by Neodiprion autumnalis in the Nebraska 

Panhandle resulted in a tufted appearance on defoliated ponderosa pine. 

Photo: Todd Nordeen, Nebraska Game and Parks Commission.
USDA Forest Service RMRS-GTR-241. 2010. 225

are present on small pines, they can be picked off by hand or washed off with a 

high-pressure hose. Registered insecticides can be used effectively when larvae 

are present across an area in large numbers and some control is appropriate. Insecticides should be applied as soon as possible after egg hatch for best control 

on high-value pines. Note that biological insecticides that only target Lepidoptera are ineffective against sawflies. Because the last larval stages consume the

most foliage, control is often sought after most of the annual defoliation has occurred and larvae are too large to be killed efficiently by insecticide.

References: 35, 50

Tiger Moth

Tree-top tents appear early in conifers

Name and Description—Lophocampa ingens (H. Edwards) (= Halisidota ingens H. Edwards) [Lepidoptera: Arctiidae]

Adult moths have a wingspan of about 3 inches (8 cm); dark, reddish brown 

forewings that bear large, white oval splotches (fig. 412); and white hindwings.

Mature caterpillars are about 1 1/2 inches (3.8 cm) long and reddish brown to 

black in color with tufts of black and yellow hairs on their backs(fig. 413). These

hairs have been known to irritate the skin of some people on contact, causing a 

rash.

A subspecies of the similar looking silver spotted tiger moth L. argentata subalpina (French) feeds primarily on juniper and occasionally on pinyon.

Hosts—All native pines are hosts. Less commonly, Douglas-fir and white fir are

fed upon during outbreaks.

Life Cycle—The tiger moth has one generation per year. Adult moths emerge 

and lay eggsin July andAugust. During September and October, the small, darkcolored, hairy caterpillars hatch from eggs and begin feeding gregariously on 

needles in the upper branches of host pines, producing silk webbing. Larvae 

overwinter in the webs in groups and will feed during warmer periods of winter 

Figure 412. Tiger moth adult. 

Photo: Rocky Mountain Region 

Archive, USDA Forest Service.

Figure 413. Mature larva of the tiger moth, Lophocampa ingens. Photo: 

Whitney Cranshaw, Colorado State University, Bugwood.org.

D E F O L I A T O R S
226 USDA Forest Service RMRS-GTR-241. 2010.

D E F O L I A T O R S 

and early spring. A life cycle that includes larval feeding during winter and early 

spring is quite rare among temperate climate forest defoliator species. In April 

and May, larvae resume feeding continuously on older needles and expand their 

webs, binding needles with dense white silk (figs. 414-415). At this time, the larvae and their tent-like webs become very conspicuous. Tiger moth larvae reach 

maturity very early in the season and are among the largest caterpillars one can 

find on conifers in spring. Pupation occurs in June.

Damage—Larvae feed on older foliage, sometimes defoliating trees by early 

spring. They make large silk tents concentrated on the upper branches and tree 

tops, often enclosing the terminal growth. Tiger moth defoliation tends to be 

more common on younger pines, but generally occurs in natural, rather than 

landscape, settings. Although the webs and larvae can be very noticeable in late 

winter and spring, this insect causes minor defoliation. Permanent tree injury 

rarely results from feeding, which is usually limited to the upper foliage. Outbreaks are somewhat infrequent and usually collapse within 1 or 2 years.

Management—Populations of tiger moth usually remain at low levels due to 

the action of predators, parasites, diseases, and cold winters. Because outbreaks 

are brief, defoliation is rare in landscape settings, and plant injury is unlikely, 

management efforts against tiger moth are rarely warranted.

Due to the potential for skin irritation and the preference for upper foliage, 

hand picking probably will not be a successful method of population reduction. 

Pruning to remove tents and applying insecticide treatments are unnecessary because new shoots will elongate in late spring and early summer and will grow to 

obscure defoliated portions of the uninjured tree.

References: 35, 50

Figure 414. Tent with larvae of tiger 

moth in the top of ponderosa pine. 

Photo: William M. Ciesla, Forest Health 

Management International, Bugwood.org.

Figure 415. Tent with larvae of tiger moth in the top of ponderosa pine. Note the dense, silk webbing and consumption 

of older needles. Photo: Southwestern Region, USDA Forest 

Service.
USDA Forest Service RMRS-GTR-241. 2010. 227

Western Pine Budworm

Messy, webbed, clipped foliage

Name and Description—Choristoneura lambertiana (Busck) [Lepidoptera: 

Tortricidae]

The appearance and life cycle of this pine-feeding species complex is very 

similar to that of the Douglas-fir, spruce, and true fir-feeding western spruce

budworm, C. occidentalis, which is generally larger and rarely found feeding on 

pines. Forewings of adult western pine budworm are about 1/2 inch (11-15 mm) 

long and yellowish brown, red, cream, and silver (fig. 416). Eggs are oval, light

green, and laid in 25-50-egg clusters, overlapping shingle-like along pine needles. Young larvae are yellow-brown with a dark head capsule. Mature larvae are 

about 3/4 inch (17-19 mm) long and have a dark shield and head capsule and a 

smooth, brown-green body with paired white spots on each segment (fig. 417).

Characteristic of many Tortricidae, larvae wriggle vigorously when disturbed 

and will drop, suspended on a silken thread. 

Pupae are yellow-brown with darker brown 

abdominal segments (fig. 418). Distributed

across western North America, the western 

pine budworm species complex includes three 

described subspecies and an as yet unassigned 

additional form.

Hosts—Limber, lodgepole, and ponderosa 

pines; rarely whitebark pine; white fir if mixed

in stands of lodgepole pine. In other regions, 

Figure 418. Pupa 

of western pine 

budworm. Photo: 

Intermountain Region, 

Ogden Archive, 

USDA Forest Service, 

Bugwood.org.

Figure 416. Adult western pine budworm. Photo: 

Bernard J. Raimo, USDA Forest Service, Bugwood.

org.

Figure 417. Mature larva of western pine 

budworm. Photo: Scott Tunnock, USDA 

Forest Service, Bugwood.org.

D E F O L I A T O R S
228 USDA Forest Service RMRS-GTR-241. 2010.

D E F O L I A T O R S 

sugar pine is a host, hence the common name, 

sugar-pine tortrix.

Life Cycle—There is one generation per year. 

Eggs hatch in late summer, and the tiny larvae

do not feed but migrate to sheltered overwintering sites in branch and stem crevices. The 

larvae molt once and then hibernate through 

winter within silken shelters they spin around 

themselves. Larvae emerge in spring when 

shoot development is nearly complete, mining needle sheaths, old needles, and developing cones. As they grow, larvae feed on new, 

elongating needles. However, in some cases, 

larvae may continue to feed primarily on male 

and female cones. One to five larvae per shoot

will web new needles together and feed in this 

protective enclosure, which becomes littered 

with droppings and brown, partially chewed 

needles (fig. 419). Larvae pupate in these

loose silk and needle enclosures and emerge 

as adults in July and August. Eggs are deposited on the concave side of older needles, 

preferentially in the upper crown, hatching 2 

or 3 weeks later.

Damage—Occasional outbreaks lasting 1-3 years on limber, lodgepole, and 

ponderosa pines have been documented. Impacts include the loss of current year 

needles, destruction of shoots and developing cones, growth loss, and top-kill, 

but little or no tree mortality. Successive years of defoliation from 1973 to 1975 

in Colorado resulted in branch deformity, although severe damage was localized 

and of short duration. Pines weakened by western pine budworm defoliation 

may be more susceptible to mountain pine beetle and other bark beetles.

Management—Western pine budworm populations are subject to regulation by 

a large complex of insect and avian natural enemies as well as the effects of unseasonable temperature extremes and, during sustained outbreaks, starvation due 

to depletion of new growth.

Because populations are naturally regulated and impacts tend to be short-term 

and localized, active management is generally not attempted. No silvicultural 

methods have been developed against western pine budworm. When direct control is deemed necessary to reduce aesthetic or economic injury, application of 

appropriate, registered insecticide should be made in spring when larvae emerge 

from hibernation and begin feeding. This is usually just after buds begin to expand in May or June.

References: 35, 50, 88

Figure 419. Feeding damage on new 

growth. Photo: Intermountain Region, 

Ogden Archive, USDA Forest Service, 

Bugwood.org.
USDA Forest Service RMRS-GTR-241. 2010. 229

Western Pine Tussock Moth

Eruptive defoliator on poor sites

Name and Description—Dasychira grisefacta (Dyar) (= Paraorgyia grisefacta

[Dyar]) [Lepidoptera: Lymantriidae]

Adults are grayish brown moths with 

light and dark bands 

across the forewings 

and a wingspread of 

1 1/4-1 1/2 inches (3.2-

3.8  cm) (fig. 420).

Eggs are bluish white,

relatively large at about 

1/13 inch (2 mm) in diameter, and laid in lines 

or clusters (fig. 421). Mature caterpillars are rusty

brown with a dark head, four grey hair brushes on 

top, and numerous tufts of white and black hairs 

along the body. They are 2-2 3/4 inches (5-7 cm) 

long and have distinctive bundles of black hairs 

projecting in front of and behind the body (fig.

422). Silken cocoons have larval hairs incorporated into them (fig. 423) and envelop black pupae.

Dasychira grisefacta occurs from the Dakotas and 

southern Alberta west to the coast and south to 

Arizona. The closely related, conifer-feeding species D. pinicola (Dyar) and D. plagiata (Walker) 

are found farther to the east.

Figure 420. Western pine tussock moth adults. Photo: Whitney Cranshaw, 

Colorado State University, Bugwood.org.

Figure 422. Mature larva of the western pine 

tussock moth. Photo: William M. Ciesla, 

Forest Health Management International, 

Bugwood.org.

Figure 423. Characteristic Dasychira spp. cocoon with 

incorporated larval hairs. Photo: Herbert A. “Joe” Pase III, 

Texas Forest Service, Bugwood.org.

D E F O L I A T O R S

Figure 421. Eggs with newly 

hatched larvae of the western pine 

tussock moth. Photo: Bill Schaupp, 

USDA Forest Service.
230 USDA Forest Service RMRS-GTR-241. 2010.

D E F O L I A T O R S 

Hosts—Primarily ponderosa pine, although other known or reported hosts include pinyon, Douglas-fir, white and Engelmann spruces, and additional conifers

elsewhere

Life Cycle—The western pine tussock moth has a 1-year life cycle, with adults 

present in late July to mid-August. Eggs are laid in small clusters, usually on

pine needles, and hatch in 1-2 weeks. Young caterpillars feed externally on the 

top layers of the center of pine needles for a short period. After feeding, they 

seek sheltered locations, generally on the tree stem under bark scales, where 

they spend the winter. As soon as temperatures warm in the spring, the larvae 

resume feeding along needles and on staminate cones, growing large enough to 

consume entire needles by early summer. Most of the defoliation occurs then 

because older caterpillars consume the most foliage. The mature larvae spin a 

whitish cocoon with incorporated body hairs. Within the cocoon they undergo 

metamorphosis into a black pupa and emerge as a moth. The moths exit the 

pupal skin, mate, and females lay conspicuous, large eggs on pine foliage and 

elsewhere by late summer.

Damage—Because feeding begins before shoot expansion, old foliage is consumed first. However, eventually, pine foliage of all age classes will be eaten.

Extensive feeding gives a pine tree a distinct reddish brown hue due to retention of discolored needles that were incompletely consumed and/or killed 

(fig.  424). Growth loss often accompanies

defoliation. Because the pine tussock moth 

will eat all age classes of pine needles, it can 

completely strip a tree of foliage. This could 

kill a coniferous tree in 1 year. Partially defoliated trees might die if weakened by drought, 

previous defoliation, root disease, or other 

stresses. Defoliation-stressed trees may be at 

greater risk of attack from bark beetle. Two 

recent outbreaks in South Dakota, Nebraska, 

and Wyoming have resulted in some patches 

of ponderosa pine mortality, often associated 

with bark beetle attack on older, larger-diameter defoliated trees, although most of the 

defoliated ponderosa pine survived (fig. 425).

These two episodes occurred on ridges and 

escarpments of lands of low or marginal quality for pine growth, whereas within the Black 

Hills, better quality pine sites were not defoliated. One consequence of heavy defoliation is 

the litter fall of significant quantities of caterpillar excreta, and cast skins rich

in readily available nutrients may play a role in host tree recovery and compensatory growth once an outbreak subsides.

Figure 424. Extensive feeding by young 

western pine tussock moth larvae down 

the center of older ponderosa pine 

needles results in needle mortality and 

a reddish appearance. Photo: William 

M. Ciesla, Forest Health Management 

International, Bugwood.org.
USDA Forest Service RMRS-GTR-241. 2010. 231

Management—A viral disease 

appears to be important in natural control of dense populations, 

with infected larvae dying in 

a characteristic deflated pose

clinging to, yet limply dangling 

from, branches and twigs. Outbreak populations typically 

show a gradual decline over a 

several-year period, with documented outbreaks lasting no 

more than a few years. In many 

instances, no direct management action is needed.

Other than those promoting 

overall stand and tree health 

and vigor, specific silvicultural management tactics are 

not available. Depending upon 

resources and values at risk, 

pine tussock moths can be controlled with insecticide applications. However, because populations can increase 

rapidly and late-stage larvae do the most feeding, interest in direct control action often peaks each year after most defoliation has already occurred. At such 

times, when a mature larval population has nearly finished feeding, spraying

insecticides will not result in population suppression or resource protection. It is 

best to make the determination of purpose and need and to prepare for spraying 

before spring. Then monitor population levels as temperatures warm, spraying 

only when potential benefits outweigh costs and risks, and apply material as

early in the growing season as possible. Most of the time, this showy insect is 

rarely encountered.

Reference: 50

Western Spruce Budworm

Imparts reddish brown cast to forest

Name and Description—Choristoneura occidentalis Freeman [Lepidoptera: 

Tortricidae]

The Western spruce budworm is a widely distributed defoliator. The nondescript, brownish moths are 1/2 inch (13 mm) long and have a wingspan of 7/8-1 1/8

inches (22-28 mm). Both sexes are similar in appearance and are able to fly (unlike the Douglas-fir tussock moth) (fig. 426). Young larvae are yellow-green with

brown heads and distinctive white spots. Larvae change colors as they develop 

Figure 425. Consecutive years of severe defoliation on ponderosa pine can give trees a tufted appearance. These pines 

survived almost complete defoliation for several consecutive 

years. Photo: Bill Schaupp, USDA Forest Service.

D E F O L I A T O R S
232 USDA Forest Service RMRS-GTR-241. 2010.

D E F O L I A T O R S 

Figure 426. Western spruce budworm 

adult. Photo: Rocky Mountain Region, 

USDA Forest Service.

Figure 427. Western spruce budworm larva. Photo: Scott 

Tunnock, USDA Forest Service, Bugwood.org.

through six stages (fig. 427). Mature larvae are 1-1 1/4 inches (2.5-3.2 cm) long, 

have tan to light-colored heads and olive-brown or reddish brown bodies with 

large, ivory-colored areas. Eggs are oval, light green, about 1/25 inch (1 mm) 

long, and overlap like shingles on needles.

Hosts—Commonly found on Douglas-fir, Engelmann spruce, blue spruce, white

fir, and subalpine fir; occasionally found on pines when mixed with other hosts

Life Cycle—Western spruce budworm has a 1-year life cycle. Moths usually 

emerge from late July to early August. Eggs are laid on the underside of conifer

needles and hatch in about 10 days. Young larvae seek a sheltered place and 

overwinter in a silken casing called a hibernaculum. From early May to late 

June, larvae begin to feed within closed buds, 1-year-old needles, and new foliage. New foliage is preferred, followed by older needles. Larvae mature in 30-40

days and pupate in early July. Pupation usually lasts 10 days and is followed by

adult moth emergence.

Damage—Young larvae will feed by mining needles or newly swelling buds. 

Larvae feed mainly on new foliage but will feed on old foliage if all the new foliage has been destroyed. Cones and seeds 

can also be destroyed. Larvae and pupae 

can be seen in silken nests of webbed, 

chewed needles (fig. 428). Defoliation

occurs at tops of trees and outer branches. 

During a defoliation event, entire stands 

will have a brown appearance from the 

needle damage. Understory trees are most 

severely impacted (fig. 429). Repeated severe defoliation (4-5 years) can decrease

growth, cause tree mortality, or render 

weakened trees more susceptible to other 

damaging agents such as bark beetles.

Management—Western spruce budworm can be managed through silvicultural methods. Because larvae disperse 

on silken threads and often impact the 

Figure 428. Early instar webbing and feeding. 

Photo: Ladd Livingston, Idaho Department of 

Lands, Bugwood.org.
USDA Forest Service RMRS-GTR-241. 2010. 233

understory more intensively, removing 

the understory (thinning from below), 

lowering stand density, and maintaining tree species diversity can reduce 

the budworm populations. If more 

short-term, direct control is desired, 

then insecticides can be applied. The 

microbial insecticide Bacillus thuringiensis (B.t.) is available and is not hazardousto most beneficial insects, birds,

small mammals, and aquatic systems. 

Other contact chemical insecticides are 

also available for budworm management.

References: 45, 50

Western Tent Caterpillar

Silken tents in early spring

Name and Description—Malacosoma californicum (Packard) [Lepidoptera: 

Lasiocampidae]

Adults are heavy bodied 

with a wingspan of 1-2 inches 

(2.5-5 cm) (fig. 430). The color of the moths of both sexes 

varies from dark red-brown to 

yellow, tan, or gray. Forewings 

have a pair of lines that can be 

either darker or lighter than 

the wing color. Egg masses

are covered with a hardened, 

glossy material that varies in 

color from dark brown to pale 

gray (fig. 431). When first hatched, larvae are about 1/8 inch (3 mm) long and 

are dark brown to black in color with whitish hairs (fig. 432). Mature larvae are

about 2 inches (4.5-5.1 cm) long and are highly variable in color and markings.

Most larvae have a pale blue head capsule and body speckled with black markings(fig. 433). They have a mid-dorsalstripe formed by a blue-white to pale blue

dash on each body segment. The mid-dorsal stripe is edged with two bands that 

Figure 429. Western spruce budworm damage. Photo: 

Rocky Mountain Region, USDA Forest Service.

Figure 430. Adults of western tent caterpillar: female (left) and 

a male (right). Photo: Jerald E. Dewey, USDA Forest Service, 

Bugwood.org.

D E F O L I A T O R S
234 USDA Forest Service RMRS-GTR-241. 2010.

D E F O L I A T O R S 

may be black or yellowish orange, 

banded with black. The body is 

covered with orange or orangebrown hairs with white tips. When full-grown, caterpillars are about 2 inches 

(50 mm) long. Cocoons are of white silk dusted with a white to yellow powder. 

This insect is the most variable of the North American species of Malacosoma, 

in which six subspecies and several unclassified forms have been recognized.

Hosts—Hosts include a wide range of tree and shrub species. In the central and 

southern Rocky Mountains, quaking aspen is the preferred host. Other reported 

tree hosts include alder, cottonwood, crabapple, fruit trees, oak, poplar, and willow. Chokecherry is a host throughout its range, especially on the western slopes 

of Colorado. Other shrub hosts include bitterbrush, Ceanothus spp., mountain 

mahogany, nine-bark, serviceberry, sumac, wild currant, and wild rose.

Life Cycle—Western tent caterpillar has one generation per year. White silken tents 

in the branches of host plants provide evidence of western tent caterpillar presence 

in spring. These are generally visible shortly after bud burst. Larvae live and feed 

as a colony, enlarging the tent asthey grow (figs. 432, 434-435). If disturbed, larvae

wave their bodies and twitch. When not feeding, larvae remain in the tent, which they 

use as shelter, 

protection, and 

a molting site. 

Late-stage larvae become solitary feeders and 

no longer use the 

tent. The larvae 

mature in 30-42

days, depending 

on weather conditions. Pupation 

Figure 431. Female western tent caterpillar 

adult with an egg mass. Photo: Jerald E. 

Dewey, USDA Forest Service, Bugwood.org.

Figure 432. Hatchling larvae of western tent caterpillar. 

Photo: Whitney Cranshaw, Colorado State University, 

Bugwood.org.

Figure 433. Mature larvae of western tent caterpillar. Photo: William M. Ciesla, 

Forest Health Management International, Bugwood.org.
USDA Forest Service RMRS-GTR-241. 2010. 235

occurs within silken cocoons spun on branches in the remaining leaves of host 

trees, on non-hosts, or in leaf litter. The duration of the pupal stage is from 12-18 

days. Adult emergence usually takes place in late July to early August. Both males

and females are strong fliers and, during outbreaks, swarms of moths are common.

Eggs are laid in flat, clasping masses on the twigs and branches of host trees. Each

female lays a single egg mass. Live branches less than 3/4 inch (19 mm) in diameter 

are the preferred egg laying sites. Larvae hatch 3-4 weekslater but remain inside the

egg until the following spring.

Damage—Successive years of defoliation of aspen and other hosts can cause 

reduced diameter growth, branch dieback and top-kill, reduced production on 

fruit trees, and, in rare cases, mortality. In Colorado, entire aspen stands across 

large areas have been defoliated. During outbreaks, western tent caterpillar may 

be only one component in a complex of factors causing such widespread aspen 

defoliation, along with other insect defoliators and leaf diseases. Chokecherry 

fruit is an important food source for some indigenous cultures, and intense defoliation reduces fruit set. Chokecherry is also used as a food source by many 

wildlife species. Defoliation of range shrubs can reduce browse. Defoliated trees 

and tents are unsightly, and large hordes of wandering larvae are a nuisance in 

residential and recreation areas.

Management—Because western tent caterpillar is more of a nuisance than a 

damaging pest, infestations normally are allowed to run their course without 

intervention. A large complex of natural enemy species feeds upon western tent 

caterpillar and, together with unfavorable weather and foliage depletion, regulates population size. High populations usually collapse within a year or two due 

Figure 434. Tents indicate western tent caterpillar infestation, 

viewed from within an aspen stand. Photo: William M. Ciesla, 

Forest Health Management International, Bugwood.org.

Figure 435. Tent of western tent caterpillar on a mountain mahogany stem. 

Photo: William M. Ciesla, Forest Health 

Management International, Bugwood.org.

D E F O L I A T O R S
236 USDA Forest Service RMRS-GTR-241. 2010.

W O O D B O R E R S 

to natural controls, although outbreaks in the Southwest have lasted longer and 

caused significant aspen mortality.

When populations are low, the egg masses on small plants can be removed 

by hand and destroyed between July and the following spring. Hand picking can

inflict significant larval mortality. Destruction of occupied tents by hand or with

a brush leads to colony collapse. Pruning to remove egg masses, tents, or groups 

of larvae must be done sparingly to avoid injuring the plant. Using fire to accomplish this is not recommended as it damages the plant more than the defoliation.

Outbreaks in fruit trees, forests designated for timber or fiber production,

heavily used recreation sites, or around homes in urban-wildland interface areas 

may require direct control. Several chemical and biological insecticides are registered for western tent caterpillar control.

References: 27, 35, 50

Introduction to Wood Borers

Bore holes and tunnels into the wood

In general, wood-boring insects (figs. 436-

440) feed on a wide variety of stressed, dying,

or dead trees. Exceptions include some hardwood borers that attack apparently healthy, 

live trees and non-native wood borers for 

which natural enemies or host resistance are 

missing or absent. Adult wood borers lay eggs 

in or on a suitable tree, and the eggs hatch into 

larvae that mine under the bark. Some species 

will tunnel into the wood of the tree. Once 

larvae have completed development, new 

adults will chew an exit hole through the bark and emerge. Wood borers serve 

an important ecological role in facilitating tree decomposition by the holes and 

channels they produce in the woody tissue, phloem, and bark and by the decay 

fungi they bring with them.

Figure 436. Adult striped ambrosia beetle. 

Photo: Maja Jurc, University of Ljubljana, 

Slovenia, Bugwood.org.

Figure 437. Adult cottonwood borer. Photo: Charles T. 

Bryson, USDA Agricultural Research Service, Bugwood.org.

Figure 438. Adult green flatheaded pine 

borer. Photo: Sheryl Costello, USDA Forest 

Service.
USDA Forest Service RMRS-GTR-241. 2010. 237

General Features—General features of 

wood borers:

•Most only attack trees under stress or i

decline or those that are dying or dead.

•They develop under the bark and the lif

cycle is usually 1-3 years; adults occur 

outside the bark; larvae are found in 

stems, branches, or major roots.

•Larvae create meandering galleries tha

are packed with coarse to fine frass and

that often obscure galleries made by 

other insects.

•Cerambicidae form an oval-shape

exit hole, Buprestidae form a Dshaped exit hole, and Siricidae 

form a round exit hole.

•Some species are very large (woo

wasps look like very large wasps) 

and some have long antennae 

(longhorned beetles can have antennae longer than their body).

•Holes in the wood caused by woo

borers can be a weak point in the tree, leaving the tree prone to wind damage and 

can reduce the timber value.

Conifer Borers—In the Rocky Mountain Region, wood borers found in conifers 

only attack recently felled trees, dead or dying trees, trees severely stressed by 

drought, or trees attacked by bark beetles. For this reason, they are not considered a 

primary cause of tree mortality. Wood borers are often very active after large disturbances such as a fire. Where wood borer populations are high, larvae can be heard

chewing under the bark. Some species only mine in the outer layers of the tree, and 

others mine into the sapwood and heartwood of a tree. The types of hole that wood 

borers create distinguishes this group from other insects. Different insect families 

create differently shaped exit holes (see “General Features” above). Most wood borers in conifers are either beetles or wasps (table 18).

Hardwood Borers—Wood borers found in hardwoods in this region usually attack 

trees that are experiencing some form of stress, with the exception of clear-wing 

moths (Sesiidae), which attack apparently healthy trees. Borers found in hardwoods 

are problematic because they can be one of the factors leading to tree decline or 

death. Wood borers may attack hardwoods year after year, and attacks may eventually cause branch death or create a weak point in the tree that is prone to wind 

breakage. Some hardwood borers like the cottonwood borer develop in roots and 

can cause severe damage to young trees. Wood borers found in hardwoods can be 

beetles, moths, or a single wasp species (table 18).

References: 29, 35, 50

Figure 439. Adult clear-wing moth. Photo: James 

Solomon, USDA Forest Service, Bugwood.org.

Figure 440. Adult wood wasp, pigeon tremex. Photo: 

Kenneth R. Law, USDA APHIS PPQ, Bugwood.org.

W O O D B O R E R S
238 USDA Forest Service RMRS-GTR-241. 2010.

W O O D B O R E R S 

Table 18. Common wood-boring insects in the Rocky Mountain Region. Insect Family Host Signs Agrilus quercicola Buprestidae Oak species, specifically Gambel oak Wide, meandering galleries under the bark; D-shaped holes in the wood Alder borera (Saperda obliqua) Cerambycidae Alder and birch Oval-shaped holes in the wood Ambrosia beetles (fig. 436) Scolytidae, Platypodidae Most conifers Fine, white sawdust at base of dead/dying trees Bronze poplar borer (Agrilus liragus) Buprestidae Aspen Meandering or zig-zag galleries under bark; D-shaped exit holes Carpenterworm (Prionoxystus robiniae) Cossidae Certain hardwoods, depending Sap spots and fine frass mixed with sap on the region Cottonwood borer (fig. 437) Cerambycidae Cottonwood, poplars, and Found in roots; small seedlings can snap at root collar(Plectrodera scalator) occasionally willowsElm borer (Saperda tridentate) Cerambycidae American elm and other elms Meandering galleries; oval-shaped exit holeFlatheaded borers (fig. 438) Buprestidae Most conifers and hardwoods Meandering galleries; D-shaped exit holeJuniper borers Cerambycidae, Juniper Meandering galleries; oval-shaped exit hole

Buprestidae

Lilac (Ash) borer (fig. 439) Sesiidae Ash and lilac Rough, gouging wounds under bark of large branches and trunk; (Podosesia syringae) old pupal skin attached to the treePoplar borer (Saperda calcarata) Cerambycidae Aspen; occasionally cottonwood Oozing wounds on bark, often associated with some frass; 

and poplar chronic infestations

Roundheaded borers Cerambycidae Most conifers and hardwoods Meandering galleries; oval-shaped exit holePitch moths (discussed in Bud, Shoot, Pyralidae Pinyon, ponderosa, and other Large gouges that ooze light pinkish pitch on trunk and Branch, and Terminal Insects section) pines, occasionally large branchesWood wasps (fig. 440) Siricidae Most conifers and some hardwoods Meandering galleries; round exit holea Not discussed in this guide.
USDA Forest Service RMRS-GTR-241. 2010. 239

Agrilus quercicola

Attacks boles and branches of stressed oaks

Name and Description—Agrilus quercicola Fisher [Coleoptera: Buprestidae]

Agrilus quercicola, a flatheaded wood borer, attacks stressed oaks. Adults

are small, approximately 1/3 inch (8 mm) long, and have small antennae and 

the characteristic oval body shape. The head, thorax, and abdomen coloration 

is consistently metallic and 

varies in shades of green and 

copper, with the thorax usually more copper-colored and 

the abdomen usually greener. 

The elytra (wing covers) are 

consistently black (fig. 441).

Larvae are white, legless 

grubs with the appearance of 

a flattened head.

Hosts—Oak species; specifically associated with native stands of Gambel oak,

Quercus gambelii

Life Cycle—There is one generation annually. Adults begin to emerge in late May 

from D-shaped exit holes (fig. 442). Peak flight is mid-June, and flight continues

into August. Eggs are laid in the outer layers of the bark. Larvae develop under

the bark in the cambium layer and then tunnel 

into the sapwood and heartwood (fig. 443).

Agrilus quercicola overwinters as larvae under 

the bark.

Figure 441. Adults of Agrilus quercicola. Photo: Andrea Sever, 

Colorado State University.

Figure 442. D-shaped exit holes of Agrilus 

quercicola and round-shaped exit hole of 

a parasitoid (upper right). Photo: Andrea 

Sever, Colorado State University.

Figure 443. Agrilus quercicola larval 

galleries. Photo: Andrea Sever, Colorado 

State University.

W O O D B O R E R S
240 USDA Forest Service RMRS-GTR-241. 2010.

W O O D B O R E R S 

Damage—In 2003 and 2004, there were numerous reports of a borer attacking

and damaging nursery and ornamental oaks along the Colorado Front Range and 

Durango-Cortez areas. It was determined that these attacks were caused by an 

outbreak of A. quercicola, which was also attacking and damaging surrounding 

native Gambel oak (fig. 444). A build-up of borer populations in Gambel oak

was due to years of drought conditions combined with late frost, causing stress 

and dieback of Gambel oaks. The most obvious sign of an A. quercicola attack 

was the wide, meandering galleries under the bark. Holes that penetrate into the 

wood were also present. Emerging adults left D-shaped, cleanly cut exit holes.

Management—This native insect rarely achieves pest status. Populations of A. 

quercicola naturally decline with normal rainfall that lessens stress on oak trees.

Reference: 153

Ambrosia Beetles

White frass on boles

Name and Description—Trypodendron sp., Gnathotrichus sp., Xyleborus

sp. [Curculionidae: Scolytinae]

Platypus sp. [Curculionidae: 

Platypodidae]

Ambrosia beetles attack stressed, 

dying, or dead trees. There are several 

species that belong to two different 

bark beetle families (figs. 445-446).

Adult ambrosia beetles are generally small, reddish brown to nearly 

Figure 444. Damage to gambel oak in Douglas County, Colorado, due to Agrilus quercicola. Photo: Andrea 

Sever, Colorado State University.

Figure 445. Adult striped ambrosia beetle, 

Trypodendron lineatum. Photo: Maja Jurc, University 

of Ljubljana, Slovenia, Bugwood.org.
USDA Forest Service RMRS-GTR-241. 2010. 241

black, cylindrical 

beetles from about 

1/8-3/16 inch (3-5 mm) 

long. Larvae are small, 

white, legless grubs 

similar to bark beetle 

larvae.

Hosts—Most western 

conifers

Life Cycle—There 

may be one or multiple 

generations per year. Ambrosia beetle adults attack spring through fall, depending on the species. Ambrosia beetles develop through four stages typical to bark 

beetles: egg, larva, pupa, and adult. For some species, all stages overwinter in the 

wood, and other species overwinter as adults in the duff and litter on the forest 

floor. Most species have a fairly wide host range.

Damage—Ambrosia beetles attack weakened, dying, and recently cut or killed 

trees. They can attack freshly cut 

lumber and lumber in decks before it is dried, and they can cause 

pinhole defects and dark staining in the outer wood. Galleries are formed in the sapwood or 

heartwood and damage the wood. 

Because ambrosia beetles tunnel 

into the wood, they are considered wood borers rather than bark 

beetles in this guide. Adults introduce ambrosia fungi that stain the 

wood, and lower its value. Ambrosia beetles feed on the fungus 

rather than the wood. The most 

obvious sign of an ambrosia 

beetle attack is the fine, white

boring dust that accumulates at 

the base of the tree and in the 

bark crevices (fig. 447). Adults

bore straight into the tree, creating perfectly round, smalldiameter holes. If the bark is 

removed, the entrance points of 

adult ambrosia beetles and galleries are distinctive and are often surrounded by a dark brown 

Figure 446. Adult Wilson’s white-headed ambrosia beetle, Platypus 

wilsoni. Photo: Pest and Diseases Image Library, Australia, Bugwood.org.

Figure 447. Fine, white boring dust produced by ambrosia 

beetles. Photo: Ronald F. Billings, Texas Forest Service, 

Bugwood.org.

Figure 448. Entrance holes of adult ambrosia beetles. Photo: 

W.H. Bennett, USDA Forest Service, Bugwood.org.

W O O D B O R E R S
242 USDA Forest Service RMRS-GTR-241. 2010.

W O O D B O R E R S 

or black fungal stain (fig. 448). Damage caused by ambrosia beetles can vary

greatly among locales. In some areas, aggressive control programs are required 

to reduce economic damage to wood products.

Management—Management is focused on preventing attacks because ambrosia beetles attack weakened or recently dead trees. Removing and processing 

wood quickly is the best way to prevent damage. Management can also be done 

through proper handling of wood products. Proper handling methods include: 

milling or debarking susceptible logs prior to the attack period, storing logs in 

an area safe from attack, and creating mill conditions unfavorable to beetle development.

Reference: 50

Bronze Poplar Borer

Attacks stressed aspen

Name and Description—Agrilus liragus Barter and Brown [Coleoptera: Buprestidae]

The bronze poplar borer is a member of a family commonly known as flatheaded or metallic boring beetles. The adult beetle is blackish with a faint metallic 

green lustre and shiny, bronzed underside, 1/4-1/2 inch (7-13 mm) long, narrow, 

and slightly flattened. Eggs are cream-colored, oval, and about 1/20 by 1/33 inch 

(1.2 by 0.8 mm). The larva is much longer than the adult, narrow and flat, about 1

1/2 by 1/10 inch (30-40 by 2-3.5 mm), and its

the prothorax is slightly wider than the rest 

of the body (figs. 449-450). The rear segment has a pair of dark, anal spines.

Host—The bronze poplar borer typically 

attacks weakened aspen. Trees that are partially girdled, such as those heavily gnawed 

by elk or carved by campers, are commonly 

attacked. Overmature or injured trees, those 

damaged by poplar borer (Saperda calcarata) or cankers, and young trees released 

from the dominance of other trees are also 

susceptible to attack.

Life Cycle—The life cycle typically takes 2 

years, though borers in 

severely weakened trees 

in southerly locations 

usually complete a generation in 1 year. Adults 

emerge from June to

August. They feed on 

Figure 449. Larva of 

the bronze poplar borer, 

slightly displaced from its 

gallery on the underside 

of live bark. Photo: Mike 

Ostry, USDA Forest 

Service, Bugwood.org.

Figure 450. Larva of 

the bronze poplar borer. 

Photo: Jim Worrall, 

USDA Forest Service.
USDA Forest Service RMRS-GTR-241. 2010. 243

aspen foliage for about one week before laying eggs in bark crevices in groups 

of five to eight. Eggs hatch in about two weeks. Young larvae bore through bark

to the cambium. Feeding is mostly along the cambium, though larvae occasionally move into the phloem and cortex. Generally, larvae bore into the wood to 

molt and then come back to the cambium. A completed gallery typically shows 

four such departures, suggesting five instars. Pupation occurs in outer sapwood

or thick bark in spring.

Damage—Larval galleries are 1/50-1/8 inch (0.6-3.0 mm) wide and can be over

39 inches (1 m long), but they are never straight. In severely weakened trees, 

galleries usually meander without any distinct pattern. In vigorous hosts, a zigzag or sinuate gallery is the rule (figs. 451-454). Galleries weave back and forth

Figure 452. Fresh gallery in live bark with frass. Photo: 

Jim Worrall, USDA Forest Service. Figure 451. External appearance of Agrilus

sp. attack on a live aspen. Photo: Jim 

Worrall, USDA Forest Service.

Figure 453. When the cambium produces callus in a gallery 

in live bark, a raised pattern of wood is left behind after the 

bark deteriorates. Photo: Jim Worrall, USDA Forest Service.

Figure 454. Typical zig-zag gallery. A 

gallery generally extends down from the 

origin and is packed with frass. Galleries 

tend to be more random in severely weakened trees. Photo: James Solomon, USDA 

Forest Service, Bugwood.org.

W O O D B O R E R S
244 USDA Forest Service RMRS-GTR-241. 2010.

W O O D B O R E R S 

across the grain with successive loops closer together in the most vigorous hosts. 

Adult emergence holes in the outer bark are distinctly D-shaped. Multiple attacks 

may contribute to mortality or result in girdling and direct mortality.

Management—Maintenance of high tree vigor reduces the likelihood of attack, 

as does prevention of mechanical injuries and diseases. Natural control includes 

a variety of egg and larval parasites. Woodpeckers are particularly important. 

According to one study, up to 40% of larvae, pupae, and young adults are taken

by woodpeckers.

References: 99, 161

Carpenter Ants

Wood dust and honeycombed wood

Name and Description—Camponotus spp. [Hymenoptera: Formicidae]

Carpenter ants tunnel into the wood 

of stumps, logs, dead standing trees, the 

dead interior of living trees, and wooden 

portions of buildings. They do not eat 

wood; rather, they excavate cavities in 

decayed wood for nests to raise their 

young. Adult carpenter ants are this 

Region’s largest ants at 1/4-1/2 inch (6-13

mm) long and they can be black or black 

and red ants (fig. 455). Adults have elbowed antennae and a constricted waist. 

Larvae are small, white, legless grubs 

(fig. 456).

Hosts—Dead and decaying wood of all tree species

Life Cycle—The queen carpenter ant works alone in 

founding the colony and lays eggs that develop into 

the first worker ants in about 30-40 days. Worker

ants (infertile females) carry food (insects, honeydew, etc.) into the colony for the developing larvae. 

The queen continues to lay eggs, and colonies can 

become quite large. Completely developed and fertile queen females are produced spring to midsummer and can been seen flying in swarms. Winged

males are also produced at this time and they mate with the queens. Queens then 

disperse and establish a new colony. Queens shed their wings shortly after finding a suitable site for beginning a colony.

Damage—Carpenter ant damage to forest trees is usually very minor. In some 

cases, carpenter ant excavations can be so extensive that they can cause a loss of 

Figure 456. Carpenter ant larvae, 

Camponotus sp. Photo: Whitney 

Cranshaw, Colorado State University, 

Bugwood.org.

Figure 455. Carpenter ant adult, Camponotus sp. 

Photo: Edward H. Holsten, USDA Forest Service, 

Bugwood.org.
USDA Forest Service RMRS-GTR-241. 2010. 245

structural integrity in the tree and lead to wind 

breakage. Carpenter ants sometimes damage 

young conifers by gnawing around the root 

collars. The most obvious sign of a carpenter ant colony is the large amount 

of sawdust-like borings piled on the ground beneath entrance holes (fig. 457).

The galleries are also distinctive with their vertical, honeycomb appearance and 

smooth walls that are free of boring dust (fig. 458). These might be confused

with termite activity, but termites do not typically occur at elevations higher than 

the pinyon-juniper woodlands in the forests of the Rocky Mountain Region, and 

fecal pellets from termites are distinctively different from carpenter ant boring 

dust.

Management—Carpenter ants play important, beneficial roles in that they contribute to decomposition of woody debris by excavating decayed wood, prey on 

small insects, and serve as food sources for wildlife species. Management in the 

forest is seldom needed, but removing and processing wood quickly is the best 

way to prevent damage. Because carpenter ant excavations lead to a loss in structural integrity, trees should be treated as falling hazards. In rare cases, carpenter 

ants will infest wooden structures, and control in these situations can be difficult.

References: 35, 50

Carpenterworm

Sapstaining on bark of hardwoods and frass at base of tree

Name and Description—Prionoxystus robiniae (Peck) [Lepidoptera: Cossidae]

The carpenterworm is widely distributed in the United States and southern 

Canada where it breeds in a variety of hardwoods. In Colorado, the insect is 

most common in shelterbelt settings in the eastern portion of the state. The adult 

is a large, stout-bodied moth with mottled forewings (fig. 459). The female is of

lighter coloration than the male and has a wingspan of about 3 inches (7.6 cm).

Full-grown larvae are greenish white with a shiny, dark brown head, approximately 2-3 inches (5.0-7.6 cm) long, and they have very few, simple hairs on

their surface (fig. 460).

Figure 457. Sawdust-like borings from carpenter ants. 

Photo: Dave Powell, USDA Forest Service, Bugwood.org.

Figure 458. Carpenter ant damage. 

Photo: Edward R. Werner, USDA Forest 

Service, Bugwood.org.

W O O D B O R E R S
246 USDA Forest Service RMRS-GTR-241. 2010.

W O O D B O R E R S 

Hosts—Hosts include oak, elm, maple, 

willow, cottonwood, black locust, boxelder, sycamore, and ash, but the insect 

exhibits regional host preferences. In the 

Rocky Mountain Region, poplars are the 

major hosts in the mountain areas and 

green ash and elms are the major hosts in 

the Great Plains.

Life Cycle—Carpenterworms begin to 

appear in May in the Kansas and Nebraska and in June or early July in colder regions. The female deposits eggsin groups

in bark crevices near wounds or under vines, lichens, or moss. Young larvae bore 

directly into the inner bark or enter it through openings and then bore directly 

into the wood. Larval development may require 1-3 or 4 years, depending on

latitude. The larva maintains an open tunnel to the exterior, through which it 

expels boring dust (fig. 461). Mature larvae pupate near the tunnel opening. The

pupa wriggles to the mouth of the tunnel and continues until its head and thorax 

are protruding from the tree. Even after the adult moth has departed the tree, the

pupal case usually remains in place, sticking out of the opening.

Damage—The earliestsign of attack by carpenterworms are sap spots and fine frass

mixed with the sap. Carpenterworms tunnel in the sapwood and heartwood of trunks 

and branches of cottonwoods. Carpenterworms seldom kill trees outright, although 

heavily riddled 

small trees may be 

broken off by the 

wind (fig. 462).

Open-grown trees, 

or trees growing on poor sites 

with dry, shallow 

soils such as dry 

ridgetops or ridge 

slopes, are especially subject to attack and damage. 

Wounds, associate 

with the tunnel 

opening, usually 

heal in 1-2 years, 

leaving oval to irregular bark scars 

that remain as evidence of attack for 

10-20 years.

Figure 459. Carpenterworm adult. Photo: James 

Solomon, USDA Forest Service, Bugwood.org.

Figure 461. Carpenterworm tunnel and exit hole. 

Photo: James Solomon, USDA Forest Service, 

Bugwood.org.

Figure 460. Carpenterworm 

larva. Photo: William H. 

Hoffard, USDA Forest 

Service, Bugwood.org.
USDA Forest Service RMRS-GTR-241. 2010. 247

Management—This insect mainly 

attracted attention due to its impact 

on shade, ornamental, and windbreak 

trees, but loss of oak timber value 

may also be significant. Natural enemies suppress the carpenterworm but 

do not keep damage at acceptable levels. Woodpeckers are able to excavate 

larvae from galleries, and many birds 

feed on the adults. Cultural practices 

that promote tree vigor, prevent bark 

injuries, and remove brood trees help 

to minimize damage. Trunk-applied 

insecticides timed with the use of sex 

attractants to correspond with egg 

hatch are effective in preventing infestation.

Reference: 161

Cottonwood Borer

Striking black and white longhorned beetle

Name and Description—Plectrodera scalator (Fabricius) [Coleoptera: Cerambycidae]

The cottonwood borer is a 

common wood borer of cottonwood and poplars and infests the 

bases and roots of living trees. 

The insect is widely distributed 

throughout the eastern United 

States and the Midwest; however, 

it is most common in the Great 

Plains from Texas to South Dakota 

and throughout the Mississippi 

River Valley. Within Colorado, 

the cottonwood borer is restricted to the southeastern portion of the state. The 

adult is an elongate, robust, strikingly patterned beetle that is approximately 

1-1 3/4 inches (2.5-4.4 cm) long and 1/2 inch (13 mm) wide (fig. 463). Mature

larvae are approximately 2 inches (50 mm) long (fig. 464), and the pupa is approximately 1 1/2 inches (3.8 cm) long (fig. 465).

Hosts—Host trees include cottonwood, poplars, and, occasionally, willows. 

Eastern cottonwood is the preferred host tree throughout its range.

Life Cycle—Adults emerge in late June through mid-August in Kansas. Adults

live about one month and feed on leaf petioles, new twig growth, and tender 

Figure 463. Cottonwood borer adult. Photo: Charles T. 

Bryson, USDA Agricultural Research Service.

Figure 462. Carpenterworm damage to wood of host 

tree. Photo: University of Idaho.

W O O D B O R E R S
248 USDA Forest Service RMRS-GTR-241. 2010.

W O O D B O R E R S 

bark. Adult feeding may 

cause shoots to break or 

shrivel and die. Females 

dig shallow pits in the soil 

at the root collar and then 

shred the bark with their 

jaws to make an oviposition site. The female deposits a single egg at each 

site and then partially fills

the soil pits. The young larvae mine downward in 

the inner bark and soon begin to etch the wood of 

the root. Galleries extend downward into the taproot 

of 1- and 2-year-old seedlings. In larger trees, larvae usually do not penetrate the wood deeper than 

1 inch (2.5 cm) and often hollow out areas of 2-3 

inches (5-7.5 cm) in diameter, particularly in large 

roots. The larva may take 1 or 2 years to complete 

development. The larvae pupate from April through 

early July.

Damage—The cottonwood borer attacks trees of all 

sizes. Cottonwood borers may infest nursery stock, 

causing young trees to die and shrivel, or, if they 

survive, the young trees often break at the root collar 

when handled. Injury to older trees is hard to detect 

unless the soil is removed to expose the root collar 

and the upper roots. The larvae bore into the heartwood of infested trees. In plantations, infested trees are often riddled with larvae, 

but they seldom die from the injury. Severely infested trees may break at the base 

during periods of high wind.

Management—Natural and planted cottonwood stands on poor sites such as 

sand flats and heavy clays are more heavily infested than stands on good sites.

Also, shelterbelt plantings on the Great Plains have been severely damaged by 

the cottonwood borer. The cottonwood borer has few natural parasites and predators because most of its life cycle is spent in the tree below ground-level. Extended flooding in lowlands often kills many larvae.

Reference: 161

Elm Borer

Extensive mining loosens bark

Name and Description—Saperda tridentata Olivier [Coleoptera: Cerambycidae]

Figure 464. Cottonwood borer larva. Photo: James Solomon, USDA 

Forest Service, Bugwood.org.

Figure 465. Cottonwood 

borer pupa. Photo: James Solomon, 

USDA Forest Service, Bugwood.org.
USDA Forest Service RMRS-GTR-241. 2010. 249

The elm borer is a common wood borer 

of North American elms. The adult beetle is 

1/3-2/3 inch (8-17 mm) long with three orange-red 

oblique crossbars on the wing covers and narrow stripes on the margins of the wing covers 

and pronotum (body segment behind the head). 

Three sets of two black spots occur on the pronotum, at the base of the wing covers, and near the 

apex of the wing covers (fig. 466). Mature larvae

are white, legless, and approximately 1/2-1 inch 

(13-25 mm) long.

Hosts—American elm is the favored species, although slippery elm and cedar elm are hosts. The elm borer probably attacks 

other native elm species, but this beetle does not infest English elm, an introduced species.

Life Cycle—The elm borer has one generation 

per year. Adults appear in spring, with the males 

emerging slightly before females. The beetles 

mate and then feed extensively on foliage, petioles, and young twigs. At night, females carve 

out niches in the bark of host trees and deposit 

their eggs. After hatching, the larvae feed initially in the outer corky layer of bark and later 

move to the phloem layer, creating extensive 

galleries (fig. 467). In early August into October, larvae bore into the sapwood and create 

chambers in the woody tissue of the tree where 

they overwinter. In March and April, the larvae pupate, which requires 15-33 days. Adults 

emerge through a round exit hole (fig. 468) following pupation.

Damage—Fresh sappy wood of trees weakened 

from drought, disease, or other causes are favored by ovipositing beetles. The first sign of attack is the appearance of thin, light-colored foliage 

followed by scattered dying of branches. Inspection 

of the trunk and branches reveals small egg niches 

in the bark. Within a few days, small pieces or ribbons of reddish frass are extruded from tiny openings in bark crevices. After attack has progressed, 

patches of the bark may be easily pulled from the 

tree. Removing the bark reveals a mass of mines or 

burrows in the inner bark. After a brood completes 

development, numerous round holes, about 1/8 inch 

Figure 466. Elm borer 

adult. Photo: Pennsylvania 

Department of Conservation 

and Natural Resources, 

Forestry Archive, Bugwood.org.

Figure 467. Elm borer larvae and galleries. Photo: James Solomon, USDA Forest 

Service, Bugwood.org.

Figure 468. Elm borer emergence 

hole. Photo: James Solomon, USDA 

Forest Service, Bugwood.org.

W O O D B O R E R S
250 USDA Forest Service RMRS-GTR-241. 2010.

W O O D B O R E R S 

(3 mm) in diameter, are present on the surface of the bark. Larvae penetrate the 

wood only to pupate, doing so at depths of 1/8-1/4 inch (3-6 mm).

Management—Because trees weakened by drought, mechanical damage, air 

pollution, disease, and insect defoliators are especially susceptible to attack, 

practices should be followed to keep trees vigorous. Severely infested trees 

should be removed and destroyed to reduce the borer population. Six species of 

wasp parasitoids help to reduce infestations but often do not prevent economic 

damage.

References: 161

Flatheaded Wood Borers 

(Metallic Wood Borers)

Oval exit holes on dead and dying trees

Name and Description—[Coleoptera: Buprestidae]

Flatheaded wood borer beetles attack 

stressed, dying, or dead trees. There are 

many species that belong to the beetle family 

Buprestidae. Adult flatheaded wood borers

are small to relatively large beetles (1/4-2 1/2

inches [6-64 mm]) with small antennae and

a characteristic oval body shape (figs. 469-

470). Adult Buprestidae are called metallic

wood borers because they are iridescent or 

metallic looking underneath and sometimes 

on top (fig. 470). Larvae are white, legless

grubs similar to bark beetle larvae, but the 

body shape is elongate, and the head 

area is different than bark beetle larvae. The larval head is small, and the 

next body segment (thorax) is much 

broader than the following segments 

and usually has a hardened plate on 

the top and bottom, giving the appearance of a flat head (fig. 471).

The larva can be distinguished from 

roundheaded wood borer larva by 

noting the flat head characteristic,

which has been likened to a horseshoe nail.

Hosts—Most western conifers, also found in hardwoods

Figure 469. Adult large flatheaded pine 

heartwood borer, Chalcophora virginiensis. 

Photo: Sheryl Costello, USDA Forest Service.

Figure 470. Adult green flatheaded pine borer, 

Phaenops gentilis. Photo: Sheryl Costello, USDA Forest 

Service.
USDA Forest Service RMRS-GTR-241. 2010. 251

Life Cycle—The life cycles of different species 

varies from 1 to many years. Flatheaded wood 

borer adults attack spring through fall, depending 

on the attacking species. Eggs are laid in the outer

layers of the bark. Larvae develop under the bark 

in the phloem. Several species also tunnel into the 

sapwood and heartwood. Most overwinter as larvae 

under the bark.

Damage—Flatheaded wood borer beetles attack 

weakened, dying, and recently cut or killed trees. 

They can also attack freshly cut timber before it is 

dried. Larvae that tunnel into sapwood and heartwood can frequently damage logs and wood products. 

Adults aid in wood decomposition by introducing yeasts, 

bacteria, and wood-rotting fungi that lead to tree rot and 

checking in the wood. In some instances, these processes 

occur within a couple of years. The most obvious sign of 

a flatheaded wood borer attack is the wide, meandering

galleries under the bark with tightly packed, fine boring

dust. Holes that penetrate into the wood are most likely 

due to wood borer larvae. Emerging adults leave oval,

cleanly cut exit holes (fig. 472).

Management—Because flatheaded beetles do not attack

healthy conifers, management is focused on preventing attacks on recently dead or felled trees. Removing 

and processing wood quickly is the best way to prevent 

damage. Management can also be done through proper 

handling of wood products. Proper handling methods include milling or debarking susceptible logs prior to the 

attack period and storing logs in an area safe from attack.

Melanophila spp.—Several species of wood borers are attracted to fire. In fact, species of the genus

Melanophila possess specific pit-sensing organs

that detect infrared radiation produced by forest 

fires. As a result, these beetles are often seen by

firefighters laying eggs on recently burned trees.

Metallic wood borers can be responsible for biting 

firefighters. Melanophila spp. have been known to 

build up their numbers in fire-damaged hosts and

emerge to attack adjacent, otherwise healthy, trees. 

Such “outbreaks” are generally short-lived.

Emerald Ash Borer—The emerald ash borer, Agrilus planipennis Fairmaire, is an exotic flatheaded

borer that kills live ash trees but is not currently 

Figure 471. Flatheaded wood borer 

larva. Photo: Sheryl Costello, USDA 

Forest Service.

Figure 472. Exit holes of 

adult flatheaded wood borer. 

Photo: Hannes Lemme, 

Germany, Bugwood.org.

Figure 473. Adult emerald ash 

borer, Agrilus planipennis. Photo: 

Marianne Prue, Ohio Department 

of Natural Resources, Division of 

Forestry, Bugwood.org.

W O O D B O R E R S
252 USDA Forest Service RMRS-GTR-241. 2010.

W O O D B O R E R S 

found in the Rocky Mountain Region (fig. 473). It was accidentally introduced

into the Lake States and is responsible for killing millions of ash trees. The beetle 

can be found in live and recently dead ash trees. There is great potential for it to 

spread to other states by moving beetle-infested ash (e.g., moving infested firewood and nursery stock). Several native species resemble the emerald ash borer, 

and identification should be confirmed by a specialist.

References: 29, 35, 50

Juniper Borers

Juniper mortality or branch dieback

Name and Description—Black-horned juniper borer—Callidium texanum

(Fisher)

Juniper borer—Atimia huachucae (Champlain and Knull)

Juniper twig pruner—Styloxus bicolor (Champain and Knull)

[Coleoptera: Cerambycidae]

Several roundheaded and 

flatheaded borers can kill

drought-stressed junipers or 

cause noticeable dieback (fig.

474). Three common roundheaded borers are discussed 

here. Larvae are white, cylindrical, legless grubs that vary in 

size. Often, only the wood mining and exit hole are apparent, 

and the specific species of borer

that killed the tree cannot be determined. Species that have been 

verified to occur in the Rocky

Mountain Region include the 

juniper borer and 

the black-horned 

juniper borer (fig.

475) in trunks and

branches and the 

juniper twig pruner 

in twigs and small 

branches (fig.

476). Damage can

be extensive before symptoms are 

apparent. Often, a 

Figure 474. Juniper killed by wood borers. Photo: 

Southwestern Region, USDA Forest Service.

Figure 475. Black-horned juniper borer. Photo: Southwestern 

Region, USDA Forest Service.

Figure 476. Juniper twig pruner damage. 

Photo: Robert Cain, USDA Forest Service.
USDA Forest Service RMRS-GTR-241. 2010. 253

large portion of the tree or the entire tree dies before the insects’ exit holes are 

noticed.

Hosts—Junipers

Life Cycle—Life cycles vary with species.

Black-horned juniper borer (Callidium texanum) larvae bore beneath the 

bark, making very wide, wavy galleries that distinctively score the outer sapwood. Older larvae excavate oval tunnels deep in the wood and spend the winter 

within affected branches or boles. Adults emerge throughout the warm months of 

the year. There is one generation per year. Adult beetles are rather short-horned 

for cerambycids, dark blue or black, and less than 1/2 inch (1 cm) long. These 

roundheaded borers leave an oval or rectangular exit hole.

Juniper borers (Atimia huachucae) are small roundheaded borers about 

1/4 inch (6 mm) long and generally have a 1-year life cycle. Adults emerge from

mid-May through June and feed on the foliage. Females lay eggs under bark

scales, especially near wounds. The larvae feed just below the bark and pack 

fibrous frass behind them as they tunnel. Larvae overwinter as pupae under the

bark.

The juniper twig pruner (Styloxus bicolor) is a slender roundheaded borer. 

Adults are 1/4-1/2 inch (7-1 mm) long and have a reddish orange head and brownish to black body with narrow, tapered wing covers that do not completely cover 

the abdomen. Larvae mine small branches and cause twig dieback. Damage is 

most apparent during dry periods. Eggs are laid on branches, often near an intersection of twigs, 1-2 ft (30-60 cm) from the branch tip. Larvae kill twigs by

boring through the center. The life cycle may take as long as 2 years to complete.

Damage—Larval feeding produces girdling wounds that kill branches or entire 

trees. Damage may occur gradually over several years before obvious symptoms 

are visible.

Management—No management strategies have been developed for these insects in natural areas.

References: 22, 35

Lilac (Ash) Borer

Pupal skins extrude from trunk

Name and Description—Podosesia syringae (Harris)

 [Lepidoptera: Sesiidae]

Adult lilac (ash) borers are moths that vary in color from brown to yellow to orange. They have clear 

wings with a span of about 1-1 1/8 inches (26-28 mm)

and appear wasp-like (fig. 477). Larvae are about

1 inch (2.5 cm) long and are white with brown heads 

(fig. 478). Figure 477. Adult ash borer. 

Photo: Daniel Herms, Ohio State 

University, Bugwood.org.

W O O D B O R E R S
254 USDA Forest Service RMRS-GTR-241. 2010.

W O O D B O R E R S 

Hosts—Ash and lilac

Life Cycle—There is one generation per year. Mature borer larvae overwinter in tunnels under 

the bark. Adult moths emerge 

from March through June to lay

eggs on the bark of host trees. 

The larvae bore into trunks and 

branches of the sapwood of trees 

during the summer. Galleries 

can be up to 6 inches (15 cm)

long.

Damage—The mining of the 

larvae causes branch dieback 

(fig. 479). It can also lead to

broken branches. The leaves on 

affected branches turn brown as 

the branch dies. Extensive mining can also lead to tree death. 

Entrances to larval mines often

appear as sunken or cankered 

areas on the bark of the trunk or 

branch. Dark, moist sawdust can 

be found around the gallery entrance (fig. 480). Pupal skin remaining in the bark is often also 

observed (fig. 481).

Management—Avoid damaging trees—maintaining trees in 

good health reduces their susceptibility 

to attack. There are chemical sprays that 

are highly effective at preventing attacks, 

but they must be used at the onset of moth 

flight. Trees and branches that have been

attacked can be removed and destroyed to 

kill maturing larvae.

References: 39, 174

Figure 479. Ash borer damage. Photo: James Solomon, 

USDA Forest Service, 

Bugwood.org.

Figure 480. Ash borer gallery entrance. Photo: James 

Solomon, USDA Forest 

Service, Bugwood.org.

Figure 481. Pupal skin pulled out from trunk 

during adult emergence. Photo: Whitney 

Cranshaw, Colorado State University, Bugwood.

org.

Figure 478. Ash borer larva. Photo: James Solomon, USDA 

Forest Service, Bugwood.org.
USDA Forest Service RMRS-GTR-241. 2010. 255

Poplar Borer

Oozing varnish on stressed aspen stems

Name and Description—Saperda calcarata Say [Coleoptera: Cerambycidae]

The poplar borer is a cerambycid, a member of a family commonly 

known as roundheaded wood borers or 

longhorned beetles. The adult beetle 

is elongate, 4/5-1 1/8 inches (2-3 cm) 

long, and grayish blue in color with 

fine, brown dots (fig. 482). Antennae

are about as long as the beetle’s body. 

Larvae are yellowish white, cylindrical, 

and about 1 1/8-1 1/2 inches (3-3.8 cm) 

long (fig. 482). The head is broader than

the rest of the larva’s body.

Hosts—Within the Rocky Mountain 

Region, the poplar borer typically attacks weakened aspen. Increases in 

poplar borer activity have been specifically noted in drought-stricken, defoliated, sun-scalded, and partially cut 

aspen stands. Borers favor large-diameter trees but can infest trees as small 

as saplings. Brood trees are common, where female borers lay eggs on the same 

trees from which they emerged.

Life Cycle—The life cycle typically takes 2-3 years. 

Adults emerge in July and August and feed on aspen

leaves and new shoots. Female beetles chew slits in the 

bark of aspens (often the same trees the females emerged 

from) and deposit one or two eggs. Eggs hatch in about

two weeks. Young larvae begin feeding on bark tissue 

and eventually mine the host’s sapwood. Larvae expel 

frass through enlarged entrance holes along the host’s 

trunk. Pupation occurs in pupal cells constructed near 

the lower end of larval mines. The poplar borer typically 

overwinters as a pupa, emerging as an adult beetle the 

following summer.

Damage—Wet spots with oozing sap mixed with frass 

along the trunk are signs of poplar borer (fig. 483). Frass

becomes coarse and excelsior-like as larvae develop. As 

larval feeding advances, frass may be seen at tunnel entrances along the trunk, in bark crevices, and in piles at 

Figure 482. Life stages (larva, pupa, and adult) and 

gallery pattern of the poplar borer. Excelsior-like frass 

is particularly indicative of advanced stages of poplar 

borer activity. Photo: James Solomon, USDA Forest 

Service, Bugwood.org.

Figure 483. Frass and 

oozing sap along the trunk 

of aspen indicates the 

presence of poplar borers. 

Photo: Jim Worrall, USDA 

Forest Service.

W O O D B O R E R S
256 USDA Forest Service RMRS-GTR-241. 2010.

the base of trees. Galleries (tunnels) typically meander in the sapwood (fig. 482)

and total length may approach 1 inch (2.5 cm). Adults emerge from oviposition sites. Small trees may be killed by larval girdling alone, but large trees are 

seldom killed by this alone. Poplar borer egg deposition, excavation, and emergence activities provide infection courts for numerous canker and decay fungi. 

Multiple borer attacks and resulting tunnels reduce the tree’s stability, and heavily infested trees are prone to wind breakage.

Management—Maintenance of tree vigor reduces the likelihood of attack, as 

does prevention of mechanical injuries and diseases. Control includes a variety 

of egg and larval parasites. Fungi and bacteria readily colonize egg niches, often 

resulting in extensive mortality of larvae. Woodpeckers are particularly effective 

predators. Partial cutting of aspen to remove borer-infested individuals is not 

recommended.

References: 50, 99, 159

Roundheaded Wood Borers 

(Longhorned Beetles)

White, splinter-like boring frass under bark

Name and Description—[Coleoptera: Cerambydicae]

Roundheaded wood borer beetles attack stressed, dying, or dead trees. There 

are many species all belonging to the beetle family Cerambycidae. Adult roundheaded wood borers are small to relatively large beetles (1/4-2 1/2 inches [6-64

mm]) and often have long antennae. The long 

antennae give them the name longhorned beetles 

(fig. 484). Larvae are white, legless grubs similar

to bark beetle larvae, but the body shape is elongate, and the head area is primarily mouthparts 

without the more defined head capsule seen on

bark beetle larvae (fig. 485).

One common group of longhorned beetles is 

the pine sawyers, Monochamusspp. Eggs are laid

in egg niches chewed by the females, and larvae 

bore into sapwood and heartwood. Adults are 

large (1 inch, 25 mm long) with large antennae 

(fig. 484) and can be mistaken for the similarappearing exotic Asian longhorned hardwood 

beetle (not established in this Region). Distinctive 

spots and antennal patterns can be used to identify the species. Pine sawyers are also responsible 

for spreading the pinewood nematode that causes 

W O O D B O R E R S 

Figure 484. Adult pine sawyer, 

Monochamus clamator. Photo: Sheryl 

Costello, USDA Forest Service.

Figure 485. Roundheaded wood borer 

larva. Photo: Sheryl Costello, USDA 

Forest Service.
USDA Forest Service RMRS-GTR-241. 2010. 257

pine wilt, a fatal disease of Scots, Austrian, and other non-native pines, that is 

currently affecting these species in the Midwest.

Hosts—Most western conifers; also found in hardwoods

Life Cycle—The life cycle frequently spans 1-3 years, but some species can take 

many more years to complete development. Roundheaded wood borer adults 

attack spring through fall, depending on species. Eggs are laid in the outer layers of the bark or by first chewing a slit (referred to as an egg niche) in the inner

bark and depositing eggs into the moist inner layers of the bark. Larvae develop 

under the bark in the cambium. Several species also tunnel into the sapwood and 

heartwood. Most overwinter as larvae under the bark.

Damage—Roundheaded wood borer beetles attack weakened, dying, recently 

cut, and fire damaged or killed trees and can attack freshly cut timber before it is

dried. Larvae that tunnel into the sapwood and heartwood can frequently damage logs and wood products. Adults aid in wood decomposition by introducing 

yeasts, bacteria, and wood-rotting fungi that lead to tree rot and checking in the 

wood. In some instances, these processes can occur within a couple of years. The 

most obvious sign of a roundheaded wood borer beetle attack is the egg niches 

that some beetles chew on the outer bark or the white, splinter-like boring frass 

that can be found under the bark (fig. 486). Larvae produce wide tunnels that

meander under the bark. Holes that penetrate into the wood are most likely due 

to wood borer larvae. Emerging adults leave broadly oval to circular, cleanly cut

exit holes (fig. 487).

Management—Because roundheaded beetles do not attack healthy trees, management should focus on preventing attacks on recently dead or felled trees. Removing and processing wood quickly is the best way to prevent damage. Management can also be done through proper handling of wood products. Proper 

handling methods can include milling or debarking susceptible logs prior to the 

attack period and storing logs in an area safe from attack.

References: 29, 35, 50

Figure 486. White, splinter-like boring frass produced by roundheaded wood borers when feeding. 

Photo: Sheryl Costello, USDA Forest Service.

Figure 487. Exit hole of adult roundheaded wood borer. 

Photo: Sheryl Costello, USDA Forest Service.

W O O D B O R E R S
258 USDA Forest Service RMRS-GTR-241. 2010.

W O O D B O R E R S 

Twolined Chestnut Borer

Top-down attacks on oaks can take years to kill

Name and Description—Agrilus bilineatus (Weber) [Coleoptera: Buprestidae]

Adult twolined chestnut borers are slender, 

black beetles with a bluish to greenish hue that 

are 1/5-1/2 inch (5-13 mm) long with two faint, 

yellowish stripes along their back (fig. 488).

Larvae are legless, white, slender, and about 1 

inch (2.5 cm) long when fully grown with two 

spines at the tip of the abdomen (fig. 489) and

have an enlarged, flattened front end, characteristic of the Buprestidae. Twolined chestnut 

borer larvae live under the bark in cylindrical, winding tunnels tightly packed with fine

grain sawdust and excrement (fig. 490). Pupae

form in chambers at the end of these tunnels. 

Twolined chestnut borers occur in southern 

Canada and throughout the eastern United 

States, including in Kansas, Nebraska, and 

South Dakota.

Hosts—Oaks, especially bur oak in South Dakota; the word chestnut in the common name 

refers to the beetle’s past status as a principal 

pest of American chestnut, Castanea dentata.

Life Cycle—The twolined chestnut borer has one 

generation per year. Adults are active from April 

to August, depending on the location and temperature. After emerging, adults fly to the crowns

of oak trees and feed on foliage before moving to 

the branches and trunks to mate. Females lay their 

eggs in small clusters in bark cracks and crevices. 

Larvae hatch within 1-2 weeks. The larvae burrow 

through the bark to the cambial region where they 

construct meandering galleries (fig. 490), lightly

scoring the sapwood (fig. 491). Galleries may be

straight rather than serpentine in host trees that are 

highly stressed. When fully mature, usually in August to October, larvae burrow into the outer bark 

and construct individual chambers in which they 

overwinter. If the bark is thin, the larvae construct 

chambers in the outer sapwood. Larvae pass the winter in a doubled-over position. Pupation occurs the following spring. Adults emerge soon after through 

Figure 488. Adult twolined chestnut borer. 

Photo: Robert A. Haack, USDA Forest 

Service, Bugwood.org.

Figure 489. Twolined chestnut borer larva. 

Photo: Robert A. Haack, USDA Forest 

Service, Bugwood.org.

Figure 490. Twolined chestnut borer 

larvae with galleries on the inside 

of the bark. Photo: James Solomon, 

USDA Forest Service, Bugwood.org.
USDA Forest Service RMRS-GTR-241. 2010. 259

distinctive D-shaped exit holes about 

1/5 inch (5 mm) wide that they chew 

in the bark. In rare cases, it may take 

2 years for larvae to complete their 

growth and development, especially 

when originating from eggs laid late in 

the growing season or at the northern 

extent of the insect’s range.

The bronze birch borer, Agrilus anxius

Gory, has a life cycle similar to that of the 

twolined chestnut borer. It can be a devastating pest of stressed or off-site mature 

birch trees, and it attacks young birch that 

have been recently transplanted.

Damage—Adult twolined chestnut borers primarily attack oaks that are weakened by drought or trees that are suppressed or declining. Urban oaks that suffer 

stress from trunk and root injury, soil compaction, and changes in soil depth 

are equally vulnerable to attack by this pest. Oaks that have been defoliated 

by insects or debilitated by root disease may also be attacked by the twolined 

chestnut borer. Although full-grown larvae are not much thicker than 1/13 inch 

(1-2 mm), they are able to construct galleries of sufficient depth to girdle and kill

branches and trees. The first symptom of borer attack is usually wilted foliage

that appears on scattered branches 

during late summer. The foliage 

on infested branches wilts prematurely and turns brown but remains 

attached to the branches for several 

weeks or months before dropping 

(fig. 492). Such branches will die

and produce no foliage the following year (fig. 492). Trees can

be killed in the first year of attack;

however, death usually occurs after 2-3 successive years of borer 

infestation. Typically, the crown is 

attacked during the first year, with

the remaining live portions of the 

branches and trunk being infested 

during the second and third years. 

Practically nothing can be done 

to save infested portions of a tree 

once symptoms become visible 

because, at that time, the damage 

to the host tree is nearly complete.

Figure 491. Outer sapwood on the tree trunk engraved by twolined chestnut borer larval galleries. 

Photo: Minnesota Department of Natural Resources, 

Bugwood.org.

Figure 492. White oak infested by twolined chestnut 

borer; photo shows branch mortality and faded leaves. 

Photo: Steven Katovich, USDA Forest Service, Bugwood.org.

W O O D B O R E R S
260 USDA Forest Service RMRS-GTR-241. 2010.

W O O D B O R E R S 

Management—Because this insect attacks stressed oaks, any action that reduces 

stress will lower the probability of infestation. This includes thinning overstocked 

stands to increase vigor, limiting oak defoliation with insecticide application, and 

watering during drought. Management programs should first attempt to prevent

attack from occurring, but if it happens, several control options are available.

Natural controls include limited impact from a parasitic wasp attacking borer larvae and mortality from woodpecker predation that can cause significant

population reduction within infested trees. Some direct cultural controls take 

advantage of the sensitivity of twolined chestnut borer larvae to rapid drying of 

the host tissues by appropriately timed cutting or pruning of infested material. 

Infested material can be chipped, burned, solarized, or treated in a way that kills 

resident borers, including treatment with pesticide. Girdled trees can be used as 

traps, attracting twolined chestnut borers that lay eggs on hosts that will dry out 

before larvae can mature. Preventive pesticide treatments can be used to protect 

high-value shade and ornamental trees from becoming infested but require multiple annual applications. Incorporating knowledge of twolined chestnut borer 

biology and the local timing of life cycle events into these control and prevention 

activities is essential for their success.

References: 58, 174

Wood Wasps (Horntails)

Stinger-like ovipositor drills into wood

Name and Description—[Hymenoptera: Siricidae]

There are several species of wood wasps, all belonging to the wasp family 

Siricidae. Adults are medium to large wasps, 1/2-1 1/2 inches (13-38 mm) long, 

and can be distinguished from common wasps by their thick waists (figs. 493-

494). Identification is sometimes difficult due to the large range of the adult size.

Adults are colored blue, black, or reddish brown and have red, ivory, or yellow 

markings. Flying adults make a noisy, buzzing sound. Both sexes possess a short, 

hornlike process at the end of their bodies. They do not possess a stinger; instead, 

females have a long stinger-like ovipositor for laying eggs under the bark of trees 

(fig. 493). Larvae are white, legless grubs similar to bark beetle larvae, except

Figure 493. Adult wood wasp, Urocerus gigas. Photo: 

David Leatherman, Colorado State Forest Service, 

Bugwood.org.

Figure 494. Adult wood wasp, Sirex juvencus. 

Photo: Sheryl Costello, USDA Forest Service.
USDA Forest Service RMRS-GTR-241. 2010. 261

the body shape is elongate (fig. 495). The larva has a small

spine at the posterior end of the body.

Hosts—All except one western species attack conifers 

and most will attack trees in several different genera. The 

one exception, the pigeon tremex, attacks hardwoods.

Life Cycle—The life cycle spans 1 to several years, and 

many species take at least 2 years to complete development. Wood wasp adults attack summer through fall, depending on species. Females lay eggs deep in the wood 

by inserting their long, flexible ovipositor. Larvae develop

under the bark in the cambium and tunnel into the sapwood and heartwood. Most overwinter as larvae under the 

bark.

Damage—Wood wasps most frequently attack weakened, 

dying, and recently cut or killed trees and can attack freshly cut timber before it is dried. They are particularly attracted to fire-damaged

trees. For this reason, they are well-known to wildland firefighters. A female

wood wasp will land on the substrate of potential host material and survey the 

site, using its antennae to determine the suitability of the material. Once an acceptable location is found, she will withdraw the ovipositor from its sheath and 

begin to “drill” down into the wood. At an acceptable depth, the egg is laid, and 

the female withdraws the ovipositor to begin the process anew. Larval tunnels 

in the sapwood and heartwood can frequently damage logs and wood products. 

Adults aid in wood decomposition by introducing yeasts, bacteria, and woodrotting fungi that lead to decay and checking in the wood. In some instances, 

these processes can occur within a couple of years. The most obvious signs of a 

wood wasp attack are the meandering galleries under the bark packed with fine

boring dust and finding larvae that have a spine on the end. Holes that penetrate

into the wood are also signs of wood borer larvae. Emerging adults leave round,

clean-cut emergence holes where they emerge.

Management—Because wood wasps do not attack healthy trees, management 

is focused on preventing attacks on recently dead or felled trees. Removing and 

processing wood quickly is the best way to prevent damage. Management can 

also be done through proper handling of wood products. Proper handling methods include milling or debarking susceptible logs prior to the attack period and 

storing logs in an area safe from attack.

An Introduced Pest—An exception to the “non-pest” status of siricids is the 

recent introduction of the European wood wasp, Sirex noctilio Fabricius into the 

eastern United States. This exotic species attacks living hosts, particularly pine 

trees. These insects have been introduced to a number of important pine-growing 

regions worldwide, including in Australia, New Zealand, and Brazil, where they 

have had an economic impact upon commercial softwood production. Although 

currently confined to the eastern United States, risk assessment studies have indicated that suitable conditions exist throughout the western United States, and 

Figure 495. Wood wasp 

larva. Photo: Stanislaw 

Kinelski, Polish Forest 

Research Institute, Bugwood.

org.

W O O D B O R E R S
262 USDA Forest Service RMRS-GTR-241. 2010.

S A P - S U C K I N G I N S E C T S , G A L L F O R M E R S , A N D M I T E S 

several important tree species have been proven acceptable hosts (including 

lodgepole and ponderosa pine). Forest managers need to be aware of this situation. If siricids are found to be associated with living host trees, samples should 

be taken for expert identification.

References: 29, 35, 50, 64

Introduction to Sap-Sucking Insects, 

Gall Formers, and Mites

Honeydew, spittle masses, growth deformities, 

or dried out foliage

Sap-sucking insects feed on the sugary sap produced in foliage and transported in the soft phloem tissue beneath the bark. Some insects’ feeding affects plant 

growth hormones, causing distinctive foliage or shoot deformities called galls. 

Gall formers feed protected within the gall. They may suck sap, as do gall-forming adelgids, or pierce and scrape the surface, as do some gall-forming midges. 

Mites (Acarina) are tiny arthropods that are more closely related to spiders and 

ticks than to insects. The class Acari contains 45,000 described species, but this

is possibly only a portion of the total number of species yet to be described. 

Spider mites puncture individual cell walls on the outside of foliage and suck out 

the contents. This leads to a characteristic mottling of the foliage.

There are other types of mites that will be seen and noticed in a forested setting, but most are quite inconspicuous. One group of mites that may catch the 

observer’s eye is the mites that are found associated with bark beetles. Opening 

up a bark beetle gallery or looking at an individual bark beetle may reveal the 

presence of mites. These bark beetle associates fulfill a wide array of functions,

including being predators of bark beetle eggs and larvae, grazers of associated 

fungi, and even predators of other mites. Because mites are wingless, they must 

hitchhike to new bark beetle galleries on their bark beetle hosts. This pattern of 

movement is referred to as phoresy. These mites are an integral part of the bark 

beetle system.

General Features—

•Hosts and symptoms of sap feeding insects, gall formers, and mites are described in table 19 (see figs. 496-499

•Sap-sucking insects like aphids an

scales feed on foliage, twigs, branches, and, occasionally, on the trunks 

of trees. These insects often produce 

large amounts of sweet, sticky honeydew that can sometimes be seen as a 

fine mist coming from the trees. Honeydew can coat branches and objects 

Figure 496. The boxelder bug is a seed-feeding 

insect that may overwinter in and around buildings. Photo: William M. Ciesla, Forest Health 

Management International, Bugwood.org.
USDA Forest Service RMRS-GTR-241. 2010. 263 Table 19. Common sap-sucking and gall forming insects and mites in the Rocky Mountain Region. Insect/mite Host Symptom Black pine needle scalea (Nuclaspis californica) Ponderosa and Austrian pine Tiny, black discs on needles Boxelder buga (fig. 496) (Leptocoris trivittatus) Seeds of boxelder and other maples Notable home invader in fall Cooley spruce gall adelgid (Adelges cooleyi) Spruce and Douglas-fir Galls on spruce; woolly material on Douglas-fir needles and twigs Eriophyid mites (many species) Aspen, oak, maples, and others Areas of deformed foliage, often red or pink-colored European elm scale (Gossyparia spuria) Elms, primarily American elm Honeydew, black sooty mold, and scales on branches Giant conifer aphids(Cinara spp.) Many conifers Honeydew and aphid colonies Hackberry budgall psyllida Hackberry Enlarged, spherical-shaped killed buds

(Pachypsylla celtidisgemma)

Hackberry nipplegall makera Hackberry Nipple-shaped swellings on leaves

(Pachypsylla celtidismamma)

Oystershell scale (Lepidosaphes ulmi) Many hardwoods Grey modeling on barkPetiole gall aphid (Pemphigus spp.) Aspen, cottonwood, and other poplars Marble-sized galls on leaf petiolePine needle scale (Chionaspis pinifoliae) Pines, spruce, and occasionally other conifers Tiny, white, teardrop-shaped scales on needlesPinyon needle scale (Matsucoccus acalyptus) Pinyon pine Tiny, black, bean-shaped bumps on year-old needlesPinyon spindle gall midgea Pinyon pine Football-shaped swelling joining needles with the fascicle

(Pinyonia edulicola) (fig. 497)

Pitch or Resin midgea (Cecidomyia spp.) Ponderosa pine, primarily Resin pits with many tiny, bright red midge larvaeSpider mites (Oligonychus spp., Spruce, junipers, and others Dried foliage; dusty appearance

Tetranychus spp. and others)

Spittlebugsa (Aphrophora spp.) (fig. 498) Primarily junipers, oaks, and herbaceous understory Frothy masses on twigs and foliage

plants; also in southwestern dwarf mistletoe

Western conifer seed buga Seeds of pines and Douglas-fir Damaged seed; can invade homes in the fall(Leptoglossus occidentalis) (fig. 499)a Not discussed in this guide.

S A P - S U C K I N G I N S E C T S , G A L L F O R M E R S , A N D M I T E S
264 USDA Forest Service RMRS-GTR-241. 2010.

S A P - S U C K I N G I N S E C T S , G A L L F O R M E R S , A N D M I T E S 

below the trees, which can subsequently be covered by black sooty mold.

•The majority of sap-sucking insect

belong to the orders Hemiptera (true 

bugs) and Homoptera (aphids, leafhoppers, scales). Common gall formers 

belong to the insect orders Homoptera 

(adelgids), Hymenoptera (cynipid gall 

wasps), and Diptera (gall midges) and 

to the mite order Acari, which includes 

gall-forming mites (eriophyid mites) 

and spider mites.

•Spider mite injury is usually related t

drought conditions.

•Spider mites produce very fine webbing that causes a distinctive dirty appearance due to the accumulation of dust, cast mite skins, and egg

•There are many natural enemies that help control sap-sucking insects an

mites. Insecticide use can sometimes prolong infestations by killing natural 

enemies of these insects.

•There are a number of non-native sap-sucking insects that have been introduced to other parts of the country that have become serious forest pests. Th

central Rocky Mountain Region does not have serious problems with nonnative sap-sucking insects in its conifer forests, but several non-native species 

are found in deciduous landscape trees.

•Sap-sucking insects such as leafhoppers are known to vector certain plan

diseases (e.g., X-disease on chokecherry).

Figure 497. Pinyon spindle gall. Photo: Brian 

Howell, USDA Forest Service.

Figure 498. Frothy mass covers 

spittlebug on juniper. Photo: Whitney 

Cranshaw, Colorado State University, 

Bugwood.org.

Figure 499. The western conifer seed bug may seek overwintering sites around buildings in the fall. Photo: Whitney 

Cranshaw, Colorado State University, Bugwood.org.
USDA Forest Service RMRS-GTR-241. 2010. 265

Cooley Spruce Gall Adelgid

Conspicuous galls on spruce

Name and Description—Adelges cooleyi (Gillette) [Homoptera: Adelgidae]

Adelges cooleyi are aphid-like insects whose activity causes conspicuous 

galls on spruce trees and premature needle cast of Douglas-firs.

Hosts—Blue and Engelmann spruces and Douglas-fir

Life Cycle—These adelgids have 

a complex, 2-year life cycle when 

both spruce and Douglas-fir are

present.

Spruce—Immature females 

(with a “woolly” wax covering) 

overwinter beneath young branches. They mature in the spring and 

place large numbers of eggs near 

branch tips in proximity to developing buds. Eggs hatch near the

time of bud break, and nymphs 

feed at the base of newly developing needles. Galls form as the tree’s 

growth response to saliva that is 

introduced during this feeding activity (fig. 500). Adelgids grow and

develop within the galls. During the summer, fully developed galls begin to dry out, 

and winged adelgids emerge. Most of these 

winged insects then leave the spruce tree in 

search of a suitable Douglas-fir host.

Douglas-fir—Female adelgids deposit eggs on Douglas-fir needles. Multiple

generations of wingless adelgids are then 

produced. They appear as cottony tufts scattered among the foliage (fig. 501). In late

summer, some woolly adelgids migrate to 

spruce trees and produce eggs. These give 

rise to the population that will overwinter 

among spruce branches. Wingless adelgids 

stay on Douglas-fir trees and produce offspring that will overwinter on those trees.

Damage—

Spruce—Where both spruce and Douglasfir are present, galls are formed on branch tips

Figure 500. Galls formed on branch tips of spruce. Photo: 

Whitney Cranshaw, Colorado State University, Bugwood.

org.

Figure 501. Wingless adelgids appear as 

cottony tufts on Douglas-fir. Photo: Whitney 

Cranshaw, Colorado State University, 

Bugwood.org.

S A P - S U C K I N G I N S E C T S , G A L L F O R M E R S , A N D M I T E S
266 USDA Forest Service RMRS-GTR-241. 2010.

S A P - S U C K I N G I N S E C T S , G A L L F O R M E R S , A N D M I T E S 

of spruce. Impact to spruce in general forest settings is usually minimal. However, gall formation on spruce does cause the death of the branch tip, and, with 

extremely heavy infestations, some crown deformity may result. Galls range 

from 1/2-3 inches (13-75 mm) long, and nymphs can be found within them when 

they are green. By late summer, galls dry and turn brown. They may persist for 

several years. Galls are often mistaken for cones. Many spruce trees express resistance to gall formation. For trees that are not resistant, gall formation is often 

concentrated on well-shaded sides (north and east) of the crown.

Douglas-fir—Nymphs pierce needles and feed on fluids. Needles then become discolored, often distorted, and drop from the tree prematurely. Also, as 

insects feed on needles, they secrete honeydew. Sooty mold often grows on the 

honeydew. Nymphs also feed on developing cones. Significant damage to cones

can negatively impact seed production. Galls are not formed on Douglas-fir.

Management—Control of these insects and their resulting galls (spruce) or needle drop (Douglas-fir) is not necessary in forest settings. However, their impact

may be significant to trees in nurseries, plantations, or ornamental settings. Old

galls can be removed from ornamental spruce trees to improve aesthetics, but 

this does not influence existing or future adelgid populations. For ornamental

trees, control efforts must occur before gall formation, typically as early as April. 

Several insecticides are registered for use to control adelgids and vary somewhat, depending upon host tree species (spruce or Douglas-fir).

References: 33, 35, 50

European Elm Scale

Elm branch dieback and shiny, sticky leaves

Name and Description—Gossyparia (= Eriococcus) spuria (Modeer) [Homotera: Eriococcidae]

European elm scale is often first noticed on

urban elms when honeydew produced by feeding scales becomes a nuisance on parked cars or 

outdoor furniture. Branches on infested trees are 

typically blackened by sooty mold growing on the 

honeydew. Closer examination of the branches 

reveals female scales, which appear as gray or 

reddish brown discs surrounded by a white, waxy 

fringe less than 1/8-1/2 inch (<10 mm) long (fig.

502). Tiny, orange crawlers can be seen on leaves 

along main veins in summer (fig. 503).

Hosts—Elms, primarily American elm

Life Cycle—Eggs hatch in June and July from beneath mature female scale coverings. Summer is 

spent as tiny, yellow nymphs on the undersides 

Figure 502. European elm scale. 

Photo: Whitney Cranshaw, Colorado 

State University, Bugwood.org.
USDA Forest Service RMRS-GTR-241. 2010. 267

of leaves. The insects crawl back to 

branches in late summer and attach 

themselves to twigs and branches. 

Female scales cover their bodies 

with a gray, waxy covering, and 

they overwinter in the second instar. 

Winged and unwinged males, when 

present, overwinter in small white 

cocoons. In spring, females grow 

and become a dark red-brown and 

produce copious amounts of honeydew. Reproduction can be sexual or 

asexual.

Damage—Honeydew from European elm scale feeding can be a nuisance. Yellowing leaves may drop early, and heavy feeding can cause branch 

dieback or death.

Management—Natural enemies include parasitic wasps on female branch scales 

and predatory plant bugs, mites, and spiders on leaves. On landscape elms, properly timed application of horticultural oils or insecticides is effective. Soil applications of the systemic insecticide imidacloprid have been very effective.

References: 35, 98

Gall (Eriophyid) Mites

Unique and sometimes colorful leaf distortions

Name and Description—Eriophyidae family

Eriophyid mites are soft-bodied, tiny to

microscopic mites that feed on many species of plants. They are unique among the 

mites and other arachnids because they 

have only two pairs of legs instead of four, 

and they are shaped more carrot-like than 

spider-like. Eriophyid mites pierce the

plant cells and suck up the plant’s juices. 

Many are found on leaves, but some feed 

on flowers, fruits, or buds. Most species

are restricted to feeding on one plant species and many are undescribed.

Hosts—Hardwoods and conifers

Life Cycle—There are several overlapping 

generations during the plant growing season. Growing-season forms include both 

Figure 503. Newly hatched crawlers emerging from 

female scales. Photo: Whitney Cranshaw, Colorado State 

University, Bugwood.org.

Figure 504. Aceria macrorrhyncha on maple. 

Photo: Milan Zubrik, Forest Research Institute, 

Slovakia, Bugwood.org.

S A P - S U C K I N G I N S E C T S , G A L L F O R M E R S , A N D M I T E S
268 USDA Forest Service RMRS-GTR-241. 2010.

S A P - S U C K I N G I N S E C T S , G A L L F O R M E R S , A N D M I T E S 

sexes. Erirophyid mites develop through four stages: egg, first nymph, second

nymph, and adult. Dormant-season forms are fertile females that hide under bud 

scales or other protected sites. Species are often hard to identify because growing 

and dormant season forms can be different shapes and sizes.

Damage—Infestations of eriophyid mites produce a variety of symptoms on 

plants. The saliva in some mites produce bladder, bead, pouch, or finger galls

of bright colors; others may cause leaf bronzing or brittleness. Some induce irregular leaf blisters and some stunt or distort buds or flowering parts (figs. 504-

506). In fact, it is often possible to identify a causal organism by the type of

gall induced on the host. Eriophyid mites can induce changes in plant hairs that

produce velvety or cottony patches called erineum (fig. 507-508).

Management—Plants can tolerate large populations of mites, and management 

is often not needed. When management is desired, affected leaves or plant parts 

can be pruned or destroyed. A horticultural oil spray can be applied in the fall to 

smother overwintering females of some species. However, the horticultural oil 

may also remove beneficial predaceous mites.

References: 35, 41, 50

Figure 508. Colomerus vitis on grape. Photo: Jody Fetzer, New 

York Botanical Garden, Bugwood.org.

Figure 506. Eriophyes parapopuli on aspen. Photo: 

Whitney Cranshaw, Colorado State University, 

Bugwood.org.

Figure 507. Eriophyes calcercis on Rocky 

Mountain Maple. Photo: William M. Ciesla, 

Forest Health Management International, 

Bugwood.org.

Figure 505. Eriophyes celtis on hackberry. 

Photo: Whitney Cranshaw, Colorado State 

University, Bugwood.org.
USDA Forest Service RMRS-GTR-241. 2010. 269

Giant Conifer Aphids

Feed on sap of twigs, branches, and roots

Name and Description—Cinara spp. [Homoptera: Aphidae]

Giant conifer aphids are softbodied, generally gregarious 

insects that are often found in 

large groups. They have piercing-sucking mouth parts that are 

used to feed on the sap in twigs, 

branches, trunks, and roots. Most 

species are restricted to feeding on one genus of tree; many 

attack only one tree species. 

Giant conifer aphids are large 

aphids, up to 1/4 inch (6 mm)

long, and are long legged and dark colored 

(figs. 509-510). Aphids have a pair of tubelike structures on their abdomen and can be 

winged or wingless.

Hosts—Many different conifer species. 

Each Cinara species is specific to a tree genus or tree species. About three dozen species occur in the Rocky Mountain Region.

Life Cycle—There are several generations 

a year. Eggs hatch in the spring into small

aphids. They molt through several stages, 

becoming larger aphids each time. Females 

give birth to live young, except with the last 

summer generation when eggs are laid on 

needles or bark. Aphids secrete honeydew as 

they feed and other insects, especially ants, bees, 

and wasps, feed on the honeydew (fig. 511). Honeydew is also a good growth medium for sooty 

mold.

Damage—Heavy infestations of giant conifer 

aphids cause yellowing of the foliage and reduce 

tree growth, especially in young trees (fig. 512).

Populations are usually highest in late spring and 

may crash by early summer. The best evidence of 

an aphid population is the presence of honeydew (clear, sugary, sticky liquid), 

ants attracted to honeydew, or ladybeetles that feed on aphids. The juniper aphid, 

C. sabinae (Gillette and Palmer), is common on juniper and can be destructive. 

Figure 510. Spruce shoot aphid, Cinara 

pilicornis. Photo: Andrea Battisti, Università di 

Padova, Bugwood.org.

Figure 511. Ants feeding on honeydew 

secretions. Photo: E. Bradford Walker; 

Vermont Department of Forests, Parks 

and Recreation; Bugwood.org.

Figure 509. Powdery juniper aphids, Cinara pulverulens. 

Photo: Whitney Cranshaw, Colorado State University, 

Bugwood.org.

S A P - S U C K I N G I N S E C T S , G A L L F O R M E R S , A N D M I T E S
270 USDA Forest Service RMRS-GTR-241. 2010.

S A P - S U C K I N G I N S E C T S , G A L L F O R M E R S , A N D M I T E S 

Giant conifer aphids can also 

be found in Christmas tree 

plantations. Damage can be 

difficult to distinguish from

damage caused by other sucking insects and needle diseases. Aphids can be confused 

with other sucking arthropods 

such as mites, mealybugs, or 

scale insects.

Management—Natural controls, including natural enemies such as ladybeetles, 

lacewings, syrphid flies, or

parasitic wasps, usually bring 

aphid population under control. Often, damage is not 

noticed until the aphid population declines from natural 

controls. If direct control becomes necessary, aphids can 

be controlled with insecticides. However, trees should be inspected for natural 

enemies before an insecticide is applied. Furthermore, because giant conifer 

aphid populations can build up by early spring, application timing is often critical.

References: 35, 50

Oystershell Scale

Tiny, armored scale on many hardwoods

Name and Description—Lepidosaphes ulmi (Linnaeus) [Homoptera: Diaspididae]

Oystershell scale is believed to have arrived in North 

America in the 1700s with 

European settlers. It now has

a worldwide distribution and 

is one of the more common armored scales on hardwoods in 

the Rocky Mountain Region. 

Full-grown female scale coverings are comma-shaped and 

resemble miniature oyster shells 

Figure 512. Aphid damage on juniper (photo taken from side of 

tree). Photo: Sheryl Costello, USDA Forest Service.

Figure 513. Oystershell scales and eggs revealed under a 

lifted scale covering. Photo: Whitney Cranshaw, Colorado State 

University, Bugwood.org.
USDA Forest Service RMRS-GTR-241. 2010. 271

attached to bark on twigs or 

branches (fig. 513). Mature

scales are gray or brown, 

range from 1/16-1/8 inch 

(1.5-3 mm) long, and have 

parallel, arcing ridges. 

Heavy infestations can impart a gray, mottled look to 

the bark (fig. 514). Older

scales can stay attached to 

the tree for several years. 

Pale yellow crawlers are 

tiny and nondescript. Tiny 

adult males are winged and 

rarely seen.

Hosts—Oystershell scale 

has been recorded on over 

125 species of plants, mostly hardwoods, and is most 

commonly found on aspen 

and other poplars, ash, maples, willows, and lilac.

Life Cycle—The insects overwinter in eggs beneath the female scale covering. 

Upon hatching in May or June, the young insect crawls about looking for a suitable site to begin feeding. Some may be carried by the wind to other trees at this 

time. This stage is called the crawler stage, and the young are only mobile for 

a few days. Once settled down to feed, they develop the waxy shell that covers 

their body. The scales have relatively long mouthparts for their size, and they 

feed on the contents of individual plant cells. There is one generation per year 

in the gray form of oystershell scale, with winged males appearing in summer 

and mating with sessile females. Two generations are reported from the brown 

race of oystershell scale, but this form of the insect is less common in the Rocky 

Mountain Region.

Damage—Heavy infestations of this insect can encrust entire branches and 

cause branch dieback and can kill plants.

Management—Natural enemies are generally adequate to reduce populations 

of this insect. Heavy infestations may warrant management on high-value trees.

Oystershell scales can be physically removed from branches and stems with 

a plastic dish scrubber, but be careful not to damage thin bark on the trees. 

Horticultural oil or insecticide application can also give good control if timed 

properly. Insecticides are only effective against the crawler stage, which normally lasts a few days but may be extended over weeks. Insecticide control is 

best targeted at the beginning of the emergence period and requires monitoring 

for the presence of crawlers.

References: 34, 156

Figure 514. Oystershell scale infestation on aspen. Photo: Whitney 

Cranshaw, Colorado State University, Bugwood.org.

S A P - S U C K I N G I N S E C T S , G A L L F O R M E R S , A N D M I T E S
272 USDA Forest Service RMRS-GTR-241. 2010.

S A P - S U C K I N G I N S E C T S , G A L L F O R M E R S , A N D M I T E S 

Petiolegall Aphids

Swollen or disfigured leaves of poplars

Name and Description—Pemphigus spp. [Homoptera: Aphididae]

Adult petiolegall aphids are about 1/13 inch 

(2 mm) long, usually pale green with a dark 

thorax, and covered with a waxy substance. 

They have short, thread-like antennae and lack 

the terminal abdominal tubules called cornicles 

that are characteristic of other aphids. At certain 

times of the year, adults have four clear wings. 

Immatures, called nymphs, look like smaller 

versions of the adults. Pemphigus spp. aphids 

are cryptic; they are found within galls on trees 

(fig. 515) or on roots of their alternate hosts.

Eggs are relatively large, given the size of laying females but are less than 1/25 inch (1 mm).

Hosts—Aspen, cottonwood, and other Populus

spp. trees in winter, alternating with annual herbaceous host plants in summer

Life Cycle—Petiolegall aphids have complex, 

1-year life cycles that alternate between two 

sets of hosts. Although probably similar in 

life cycle, considerable confusion exists regarding species’ identities within the genus 

Pemphigus, so there may be more species 

involved than are named here. The poplar 

petiolegall aphid (P. populitranversus Riley) 

(fig.  515) life cycle is used as an example.

Eggs are laid in fall in bark cracks on Populus

spp. trees. In spring, more or less coincident 

with the flushing of the poplar foliage, eggs

hatch into nymphs that will all become asexually reproducing females. Nymphs 

feed on developing leaf petioles through tubular, sucking mouthparts. Feeding 

induces the host plant to produce a swollen growth, called a gall that envelops 

the developing aphid (fig. 516). As this overwintering form matures, it produces

young that remain within the gall until full-grown. These new adult females have 

wings. The gallssplit open, and the winged adults fly during late June and July to

their summer hosts. Using cracks in the soil, they locate and colonize the roots of 

plants in the cabbage family. Colonies of nymphs secrete a waxy substance that 

is believed to protect them from excess moisture. Several asexual female generations may be produced on these summer hosts. At the end of summer, winged 

adults are produced that fly back to their winter hosts where they give birth to

Figure 515. Characteristic marble-sized 

galls on the petiole of cottonwood leaves 

induced by the poplar petiolegall aphid, 

Pemphigus populitransversus. Photo: 

Herbert A. “Joe” Pase III, Texas Forest 

Service, Bugwood.org.

Figure 516. When a gall is cut open, numerous petiolegall aphids can be found inside. 

Photo: Herbert A. “Joe” Pase III, Texas Forest 

Service, Bugwood.org.
USDA Forest Service RMRS-GTR-241. 2010. 273

small, mouthless males and females that mate, 

and then the males die. After mating, each female, 

which is less than 1/25 inch (1 mm) long, lays one 

egg that is almost as large as she is and then dies.

The poplar petiolegall aphid forms a spherical green gall with a transverse slit on the petiole 

of plains cottonwood (fig. 515). It is also a rootinfesting pest on cruciferous crops in summer, 

where it is known as the cabbage root aphid.

The lettuce root aphid, P. bursarius (Linnaeus), 

is a European insect that was accidentally introduced into North America. It produces a flask-like

gall on the petioles of Lombardy poplar and hybrid poplars (fig. 517). It is an occasional pest of

lettuce crops and is known from lambsquarter and 

carrot.

The sugarbeet root aphid, P. populivenae Fitch, forms an elongate gall on 

the mid-vein on the upper side of Populus spp. leaves, such as narrowleaf cottonwood, and infests the roots of sugarbeets and other garden plants. Gall shape 

and location are variable.

The poplar vagabond aphid, Mordwilkoja vagabunda (Walsh) [Homoptera: 

Aphididae], may be confused with Pemphigus spp., although its characteristic 

leathery, distorted galls differ greatly in shape (figs. 518-519). Poplar vagabond

aphids have a life cycle similar to other gall-making aphid species in that it overwinters as an egg and alternates between winter and summer host plants. Newly 

hatched aphids feed on developing leaves of aspen and certain cottonwoods, 

which produce an enlarged, twisted clump of growth at the leaf base (fig. 518).

Several generations of aphids occur within the folds of such galls, which may 

contain as many as 1600 individuals each. The green gallsfade in color and harden, tend to remain on trees, and may not be visible until after normal leaf fall (fig.

519). Galling is 

concentrated in 

the upper third 

of the tree. This 

Region’s summer host plants 

are not known, 

although loosestrife is an 

important summer host in 

other areas.

Figure 517. Gall produced by the lettuce 

root aphid, Pemphigus bursarius. Photo: 

Gyorgy Csoka, Hungary Forest Research 

Institute, Bugwood.org.

Figure 518. A poplar vagabond aphid that 

was feeding on developing aspen leaf 

produced a gall of distorted growth that will 

dry and remain after leaf fall. Photo: Dave 

Powell, USDA Forest Service, Bugwood.org.

Figure 519. Dried gall from poplar vagabond aphid feeding. Photo: Whitney 

Cranshaw, Colorado State University, 

Bugwood.org.

S A P - S U C K I N G I N S E C T S , G A L L F O R M E R S , A N D M I T E S
274 USDA Forest Service RMRS-GTR-241. 2010.

S A P - S U C K I N G I N S E C T S , G A L L F O R M E R S , A N D M I T E S 

Damage—None of the Pemphigus species are economic pests for foresters or 

arboriculturists. Galled leaves tend to fall from the tree prematurely. Severe infestations can render trees unsightly but cause little to no other injury. There are 

many other organisms, including aphid, fly, and mite species, that cause galls to

form on Populus spp. leaves and branches, each with characteristic gall shape 

and location. Other than aesthetic considerations, these organisms are not known 

to damage their tree hosts. Serious economic damage can occur on the alternate 

annual herbaceous summer hosts.

Management—Excessive gall formation on host trees can be indicative of an

unhealthy condition, relief from which might reduce galling. In some cases, gallforming insects prefer hosts that are given abundant moisture and/or fertilization, 

so galls can be a problem in well-tended landscape settings. Given the lack of 

injury, active management of petiolegall aphid populations is not indicated. If 

control is desired, application of horticultural oil in spring as leaves begin to 

expand should kill the overwintered eggs.

References: 35, 50, 88

Pine Needle Scale

Tiny, white, tear drop-shaped scales 

on conifer needles

Name and Description—Chionaspis pinifoliae (Fitch) [Hemiptera: Diaspidae]

Adult female pine needle scale insects are about 1/8 inch 

(3 mm) long, dark orange, and wingless under an armor scale 

covering they secrete that is about 1/3 inch (8 mm) long. They 

are the most conspicuous form, with the scale being almost 

pure white, elongate oval, and yellow at the apex (fig. 520).

Developing males are similar to developing females, though 

smaller at 1/25 inch (1 mm) long and more slender. The minute 

adult males have wings when mature, but are rarely present 

in western populations. The immature crawler stage is generally oval, light purple to reddish brown, and tiny (fig. 520).

Rusty brown-colored eggs are deposited under the scale covering. The characteristic white armor coverings can be seen 

on needles any time of the year, although the insects beneath 

the coverings may not all be alive. Pine needle scale is distributed throughout North America 

but is most common in the northern 

half of the United States and southern Canada.

Hosts—Pines, spruces, white fir,

Douglas-fir, and cedar; may be

Figure 520. Close-up of small 

immature and larger mature 

female pine needle scales 

with newly-hatched brownish 

crawlers. Photo: E. Bradford 

Walker, Vermont Department of 

Forests, Parks and Recreation, 

Bugwood.org.
USDA Forest Service RMRS-GTR-241. 2010. 275

common on shelterbelt and ornamental plantings of native and introduced pines, 

especially mugo pine

Life Cycle—The pine needle scale has two generations per year in much of Colorado. Twenty to 30 eggs are laid in the fall by the female and overwinter under 

her scale covering. Some females may survive winter and lay eggs in spring. 

As they lay eggs, the females gradually shrink in size while the scale fills with

eggs. Regardless of overwintering stage, egg hatch occurs over a relatively brief 

period sometime from late April to mid-June, depending upon local weather and

location. The nymphs, or crawlers, then wander over the needles for a few days, 

select a needle, insert their sucking mouth parts, and begin feeding on sap. Wind 

is responsible for most of the dispersal of crawlers to uninfested trees. Females 

are sessile after settling and will remain on one needle for the rest of their lives. 

Feeding nymphs turn yellow and lose their appendages by molting. Combining 

their cast skins with waxy secretions, the pine needle scales create the characteristic protective white covering. When present, males also remain in place, 

developing as a scale and feeding until they emerge from the covering as minute, 

winged adults, flying off in search of females. Pine needle scales mature by early

July. Clusters of eggs are then laid with or without mating in a white, waxy sack

under the scale covering. Scales of this second generation mature by fall and lay 

overwintering eggs under the scale covering. In areas where there is one generation per year, scales mature by the end of summer and lay overwintering eggs.

Damage—Pine needle scale feeding causes a yellowing of the foliage in the area 

surrounding each sessile scale, and these areas coalesce when trees are heavily 

infested. Consequently, the foliage on heavily infested trees becomes discolored. 

The waxy, white, secreted coverings of the scale insects and black mold that 

grows on scale exudates often combine to give infested trees a grayish appearance (figs. 521-522). Heavy infestations can result in premature needle shed. On

pines, infestations can result in a marked reduction in needle length and a diminished growth rate. Outbreaks are frequently confined to limited areas on a given

tree. Prolonged infestations, rare in natural forests in this Region, can kill branches and even young trees and can weaken larger trees, predisposing them to attack 

by opportunistic insects and diseases. Planted pines can be severely damaged. 

Scale infestations are often associated with factors that negatively affect host tree 

vigor. In addition, 

conditions that 

adversely affect 

scale predators 

and parasites such 

as road dust or 

pesticide application can result in 

significant infestations.

Figure 521. Heavy infestations of pine needle scale on blue spruce. Photo: 

Whitney Cranshaw, Colorado State University, Bugwood.org.

S A P - S U C K I N G I N S E C T S , G A L L F O R M E R S , A N D M I T E S
276 USDA Forest Service RMRS-GTR-241. 2010.

M a n a g e m e n t —B e -

ing sessile, pine needle 

scales are the target of 

many predatory and 

parasitic insects and other natural enemies that 

frequently exert a high 

degree of control. Of particular note is a lady beetle species whose annual 

life cycle is synchronized 

with that of the scale. Severe cold temperatures 

are thought to limit outbreaks by reducing survival of the overwintering stages.

In planted settings, proper tree care is a primary management strategy against 

scale insects, including pruning heavily infested branches. Refrain from planting pines or other hosts along dusty roads or in areas of heavy air pollution. 

Minimizing tree stress is key to keeping scale populations low, which will also 

lower susceptibility to other insect and disease pests.

Direct control options include the use of horticultural oils and insecticidal soaps, 

which are not toxic to and, therefore, will help conserve insect natural enemies active against the scales. Additional registered insecticides are also available, but they 

have wider non-target insect toxicity. Armored scales are more difficult to kill with

insecticide due to their protective covering. Applications should be carefully timed 

against the crawler stage, which immediately follows egg hatch.

References: 32, 35, 50, 88

Pinyon Needle Scale

Black bumps on pinyon needles

Name and Description—Matsucoccus 

acalyptus (Herbert) [Homoptera: Margarodidae]

Pinyon needle scales appear as small, 

black, bean-shaped bumps on the surface of 1-year-old pinyon needles (fig.

523). These tiny, sap-sucking insects 

kill the needles and seriously weaken 

pinyon pines in woodlands and in urban 

landscapes.

Host—Pinyon pine

S A P - S U C K I N G I N S E C T S , G A L L F O R M E R S , A N D M I T E S 

Figure 522. Heavy infestations of pine needle scale on pine. Photo: Scott 

Tunnock, USDA Forest Service, Bugwood.org.

Figure 523. Scales infesting one-year-old needles. 

Photo: Robert Cain, USDA Forest Service.
USDA Forest Service RMRS-GTR-241. 2010. 277

Life Cycle—Adult, wingless females emerge 

from scale coverings in late winter or early 

spring and mate with winged males (fig. 524).

Emergence time varies with temperature across

the range of the insect. In Colorado, emergence 

occurs in late March or early April. Most males 

emerge from scale coverings the previous fall 

and spend the winter in silk webs in litter beneath 

the tree. A few males don’t enter this stage until 

early spring. Mated females crawl along the bark 

to egg-laying sites around the root collar, on the 

undersides of large branches, in branch crotches, 

or in cracks of rough 

bark. Yellow eggs 

are laid in clusters 

held together by 

white, cottony webbing (figs. 525-526).

Occasionally, egg 

masses are found 

several feet from the 

base of the tree on a 

rock or log. About 

four weeks after 

eggs are laid, tiny, 

red eye spots can 

be seen in the eggs 

with the aid of a 

hand lens. Nymphs, 

called crawlers, 

emerge about 7-10 

days after eye 

spots appear. They 

climb to the ends of 

branches and settle on the previous year’s new growth. After inserting tube-like 

mouth parts into the needle, they become immobile, cover the body with wax, and 

turn black. By late August, they will molt in place to the more visible, bean-shaped 

second stage, which is about 1/16 inch (1.5 mm) long.

Damage—Reduced new growth and stunted needles are common on trees suffering repeated attacks (fig. 527). Heavy infestations frequently kill small trees

and predispose weakened larger trees to attack by other insects, especially bark 

beetles, which can kill trees.

Management—No control strategies have been developed for woodlands. However, potential damage from these pests on landscape pinyons can be drastically 

Figure 525. Cottony webbing and eggs 

deposited under branches and in branch 

crotches. Photo: Robert Cain, USDA Forest 

Service.

Figure 526. Eggs, webbing, and 

expired female scale. Photo: 

Southwestern Region Archives, 

USDA Forest Service.

Figure 524. Winged male mating with 

emerging female pinyon needle scale. 

Photo: USDA Forest Service.

S A P - S U C K I N G I N S E C T S , G A L L F O R M E R S , A N D M I T E S
278 USDA Forest Service RMRS-GTR-241. 2010.

S A P - S U C K I N G I N S E C T S , G A L L F O R M E R S , A N D M I T E S 

reduced by destroying eggs before they hatch. Dislodge egg masses from the 

tree with a strong stream of water from a garden hose. After washing down the 

tree, rake up all the material around the base of the tree and destroy or remove 

it. Chemical insecticides are registered to control the pinyon needle scale, but 

timing of the spray application is critical for success. Apply insecticides to the 

bark and branch crotches when females are moving to egg-laying sites or as 

soon as crawlers begin to emerge from eggs. Examine eggs with a hand lens,

and be ready to spray shortly after the crawler’s red eve spots are visible. Once 

scales have established themselves on the needles, they become more difficult

to control.

Reference: 22

Spider Mites

Dry or dusty looking foliage

Name and Description—Family Tetranychidae

The spider mites family includes about 1600 species, some of which are

well-known due to their habits that are often injurious to plant life. These mites 

range in size from 1/25-1/20 inch (0.9-1.25 mm) long and are often visible to 

the naked eye. They are spider-like in appearance with two body parts and four 

pairs of legs. The common name, spider mites, is derived from mites’ habits 

of spinning webbing on the plants they have colonized. Most frequently, it is 

Figure 527. Only the new growth remains green on heavily infested pinyons. Photo: Robert 

Cain, USDA Forest Service.
USDA Forest Service RMRS-GTR-241. 2010. 279

the discovery of the webbing that will lead an 

observer to determine the infestation of spider 

mites (fig. 528).

Hosts—Hardwoods and conifers

Life Cycle—Spider mites typically develop 

through five stages: egg, nymph, two immature

stages, and adult. Many overwinter in the egg 

or adult female stage, and there are often several generations a year, with a generation being 

completed in as little as a week.

Damage—Unlike spiders, spider mites are herbivorous and feed upon host plants by piercing 

individual plant cells with their mouthpart and 

sucking out the fluids. The damage takes on a

stippled yellow or brown appearance as large 

numbers of cells are affected on an infested 

plant part. Some species will feed upon a wide 

range of host plants, while others have a very 

limited host range. The presence of webbing, 

which may contain a large number of visible mites, and damage to plant tissue 

are indicative of spider mite activity.

Spider mites rarely achieve pest status in a forested setting. However, in 

landscape situations, greenhouses, and, at times, in forest plantations or other 

planted areas, spider mites can damage high-value plants. Even in these circumstances, an outbreak of spider mites is indicative of another condition that 

is allowing the spider mites to thrive. Outbreaks of spider mites are frequently associated with hot or dry conditions, and, even in forested settings, their 

numbers increase during periods of drought. Another situation in which spider mites can reach outbreak levels is when host plants are subjected to large 

amounts of dust like what might be found along a dirt road. Finally, spider mite 

outbreaks can be caused by the use of insecticides or fungicides that have been 

employed to deal with another pest. The use of these chemicals can kill off the 

organisms that normally keep spider mites under control, resulting in a secondary outbreak of the spider mites.

Management—Control of spider mites needs to start with investigation of the 

cause of an infestation. Often, removal of a causal factor results in the rapid 

and complete collapse of a spider mite infestation. Sometimes these causal factors can be somewhat obscure; in one case, spruce spider mites were infesting 

large numbers of host trees in an urban setting. Only later was it discovered 

that frequent spraying for mosquitoes contributed to the outbreak of spider 

mites. Adjustment of the spray schedule caused the spider mite population to 

decline.

Reference: 35, 41, 50

Figure 528. Spruce spider mites, 

Oligonychus ununguis (Tetranychidae). 

Note the fine webbing. Photo: USDA 

Forest Service, Intermountain Region, 

Bugwood.org.

S A P - S U C K I N G I N S E C T S , G A L L F O R M E R S , A N D M I T E S
280 USDA Forest Service RMRS-GTR-241. 2010.

B U D A N D S H O O T I N S E C T S 

Introduction to Bud, Shoot, Branch, and 

Terminal Insects

Damaged tips and branches

The insects presented in this section are those that feed only within or on 

buds, shoots, or branches (table 20). These include insects primarily in the orders Lepidoptera (moths and butterflies) and Coleoptera (beetles). Bark beetles

and twig beetles that are found in twigs and branches are presented in the 

Introduction to Bark Beetles entry. Aphids, scales, and midges that feed on 

shoots and branches are presented in the Introduction to Sap-Sucking Insects, 

Gall Formers, and Mites entry.

General Features—

•Twig-boring insects often mine out the pith or center of developing shoot

•Resin may or may not be presen

•Pitch mass-forming insects feed protected within the soft pitch mas

•The most serious damage, stunting, forked tops, or multiple leaders occurs t

sapling conifer trees when the main stem or leader is affected.

•These insects can be important in young, intensively managed stands or replanted forested area

Table 20. Common insects that damage buds, shoots, branches, and terminals.

Insect Host Symptom

Pine tip moth Pines Hollowed out shoots often with undeveloped 

or partially developed needles

Juniper twig pruner Junipers Branch dieback; hollowed out twigs

(discussed in the 

Wood Borers 

section)

Pitch moths Pinyon pine and ponderosa Large, irregular pitch masses

pine

Pitch nodule moth Pinyon pine Flagging and nodules of pitch at small 

branch/twig intersections

Western spruce Spruce, white fir, and Early instar larvae mine in buds

budworm (discussed Douglas-fir

in the Defoliators 

section)

Terminal weevils Spruce and lodgepole pine Dying leader; often with a shepherd’s crook

Twig beetles Pines and Douglas-fir Tiny holes; tip or branch flagging

(discussed in the

Bark Beetles section) 
USDA Forest Service RMRS-GTR-241. 2010. 281

Pine Tip Moth

Hollowed out shoots

Name and Description—Rhyacionia spp. [Lepidoptera: Olethreutidae]

Western pine tip moth—R. bushnelli Busck

Nantucket pine tip moth—R. frustrana Comstock

Southwestern pine tip moth—R. neomexicana Dyar

Larvae of pine tip moths are yellowish with 

black heads and are less than 1/2 inch (13 mm) 

long. Adults, though rarely seen, have forewings 

mottled with yellowish gray and reddish brown 

(fig. 529). Hind wings are gray. Wingspans extend

to about 3/4 inch (19 mm).

Hosts—Ponderosa pine

Life Cycle—A single generation per year occurs 

in most parts of the Intermountain West. Winter 

is passed as pupae in cocoons in the litter or soil 

beneath infested trees. Adult moths emerge by 

late May or early June, mate, then females deposit eggs on needles, buds, and

shoots of young trees. Newly hatched larvae either feed between or mine the 

needles. Later instars feed inside needle sheaths or buds and then enter new 

shoots and mine within developing shoots. Larvae complete growth by midsummer or fall, emerge from shoots, and drop to the ground to pupate, where 

they spend the winter (fig. 530).

Figure 529. Adult pine tip moth. 

Photo: Scott Tunnock, USDA Forest 

Service, Bugwood.org.

Figure 531. Pine tip moth larva. Photo: 

Whitney Cranshaw, Colorado State University, 

Bugwood.org.

Figure 532. Pine tip moth 

damage. Photo: John 

Guyon, USDA Forest 

Service, Bugwood.org.

B U D A N D S H O O T I N S E C T S

Figure 530. Western pine tip moth life cycle (from Johnson 1982).

Western Pine Tip Moth Life Cycle

Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec.

Eggs

Adults

Pupae

Larvae
282 USDA Forest Service RMRS-GTR-241. 2010.

B U D A N D S H O O T I N S E C T S 

Damage—Larvae mine shoots and buds of young pines—this is especially 

damaging in plantations, even-aged natural stands, and ornamental plantings 

(fig. 531). Infested trees are often deformed and growth is reduced. Damage is

unsightly but seldom fatal. Pine trees less than 25 ft (7.6m) tall are most commonly affected (fig. 532).

Management—Damage is seen in upper and mid-crowns of young ponderosa 

pines. Larval feeding activity distorts and kills both terminal and lateral shoots, 

which often stunts growth and deforms trees. No control is necessary in forest 

settings. Ornamental trees may be protected by applying insecticides to the 

foliage and soil in the spring.

References: 50, 93, 167

Pitch Moths

Oozing pitch masses on trunks and branches on pines

Name and Description—Dioryctria spp. [Lepidoptera: Pyralidae]

Pine pitch moth—D. ponderosae Dyar

Zimmerman pine moth—D. zimmermani (Grote)

Pitch moths attacks appear as large, oozing masses of pitch that form at the 

wound site in response to larval feeding beneath 

the bark. Adult moths are rarely observed and are 

difficult to differentiate from other members of the

genus. They are grey with white zig-zag markings 

and are up to 3/5 inch (15 mm) long. Full-grown 

larvae are 3/5-1 inch (15-25 mm) long, dirty white, 

pinkish, orange, light green, or light brown and are 

marked with rows of dark spots (figs. 533-534).

Several species that bore beneath of bark of 

pines and some other conifers are encountered in 

the Rocky Mountain Region. Most notable is the 

pinyon pitch mass borer, which causes large, oozing, soft pitch masses (fig. 535) on pinyon pine, and

less noticeably, on ponderosa pine. This insect is also 

called the ponderosa twig moth and has caused notable damage on young pines in Nebraska. Other related 

species may produce less pitch or cause a clear sticky 

sap flow from branches or on the trunk of the tree.

Zimmerman pine moth is an eastern species that has 

become established along Colorado’s Front Range. It 

causes popcorn-like masses of pitch on the trunk 

and branches (fig.  536).

Austrian pines are most 

Figure 533. Pitch moth larva on 

ponderosa pine under bark. Photo: 

Southwestern Region, USDA Forest 

Service.

Figure 534. Pitch moth larva 

in pitch mass on ponderosa 

pine. Photo: Whitney Cranshaw, 

Colorado State University, 

Bugwood.org.
USDA Forest Service RMRS-GTR-241. 2010. 283

frequently attacked, often at the junction of the trunk 

and a branch.

Hosts—Pinyon pine, ponderosa pine, lodgepole pine, 

Austrian pine, Scots pine, and, occasionally, Douglas-fir and true firs

Life Cycle—Pitch moth life cycles vary with species and can require 1 or 

2 years for one generation. Pinyon pitch mass borer requires 2 years to complete its life cycle, and eggs are laid over an extended period from June through

August. Eggs are laid in bark crevices or near bark wounds, including in previous attack sites and pruning cuts. Newly hatched larvae tunnel under the bark, 

forming irregular galleries or elongated gouges in the sapwood. Larvae pupate 

in silk-lined chambers in their tunnels or in the pitch mass in spring or early 

summer. The Zimmerman pine moth has one generation per year and does not 

bore under the bark until spring. They feed under bark, causing large amounts 

of pitch to be produced. Eggs are laid in August, and larvae hatch in late summer and seek overwintering sites under bark scales.

Damage—Larger branches, limbs, and trunks of young trees are attacked. Repeated attacks can seriously weaken trees and kill branches. The most severe 

damage is to trees less than 20 ft (6 m), especially in urban areas. The insects are

rarely a problem on larger trees or in the forest environment.

Management—In forests and woodlands, no control strategies are warranted. 

Properly timed insecticide applications have successfully reduced damage on 

landscape trees. Individual larvae can be removed from the pitch mass or from 

under the bark with a knife or similar tool. Avoid pruning or mechanical injury 

to the bark during the summer months when adult moths are seeking egglaying sites.

References: 35, 50, 62, 63

Pitch Nodule Moths

Tip dieback with pitch nodules on pines

Name and Description—Pinyon Pitch Nodule Moth—Retinia (= Petrova) arizonensis (Heinrich) [Lepidoptera: Tortricidae]

Ponderosa pitch nodule moth—R. metallica (Busck)

Figure 536. Zimmerman pine 

moth damage. Photo: Whitney 

Cranshaw, Colorado State 

University, Bugwood.org.

B U D A N D S H O O T I N S E C T S

Figure 535. Pitch mass caused by Dioryctria ponderosae on pinyon 

pine. Photo: Brian Howell, USDA Forest Service.
284 USDA Forest Service RMRS-GTR-241. 2010.

B U D A N D S H O O T I N S E C T S 

Pitch nodule moths are seldom noticed, 

but the damage they cause is distinctive. 

Moths are rusty brown with mottled forewings and have a 3/4-inch (19 mm) wingspan 

(fig. 537). Attacks are characterized by fading branch tips and nodules of pitch formed 

at the insects’ feeding sites. The pitch 

nodules are hollow balls of pitch that are 

1/2-1 inch (10-25 mm) long, round, smooth, 

and that are often light purple or red. On 

pinyon pines, they are most often found at 

the crotch of two or more twigs. The fading 

twigs eventually lose their needles and fall off.

Hosts—Pinyon pine, ponderosa pine

Life Cycle—Pitch nodule moths have one generation per year. The small, rusty 

brown moths emerge through holes in the pitch nodule in July or early August.

Eggs are laid on needle sheaths of the current year’s growth. Newly hatched

larvae feed on young needles before boring into the bark at nodes or whorls of 

twigs or branches. Pitch nodules form at feeding sites, and larvae feed and spend 

the winter protected within the distinctive nodule. Full-grown larvae are about 

1/2 inch (13 mm) long and reddish yellow with a black head and a dark area behind the head. Pupation occurs inside the pitch nodule in June. Pupae move just

below the surface of the pitch before they emerge as adults.

Damage—Leaders are occasionally damaged, and trees with multiple terminals 

may result. Insect populations and damage can be abundant in localized areas. 

On ponderosa pine, attacks occur on the terminal growth (fig. 538).

Management—In the forest, topkilling of pinyon is rarely important, and damage to ponderosa pine 

is minor. On valuable ornamental 

trees, control the insect by pruning and destroying the infested tips 

as they fade in May or early June

before the adult moths emerge. On 

branches and stems, destroy larvae 

by crushing them within the pitch 

nodule. Pinyons being dug from infested areas for landscape planting 

should be inspected, and pitch nodules should be crushed or removed.

References: 22, 35, 50

Figure 538. Pitch nodule moth in damaged ponderosa 

pine shoot. Photo: Whitney Cranshaw, Colorado State 

University, Bugwood.org.

Figure 537. Wings of pinyon pitch nodule 

moth. Photo: Gilligan, www.tortricidae.com.
USDA Forest Service RMRS-GTR-241. 2010. 285

White Pine Weevil and Lodgepole Pine 

Terminal Weevil

Drooping, dying leaders on spruce and pines

Name and Description—Pissodes spp. [Coleoptera: Curculionidae]

White pine weevil—P. strobi (Peck)

Lodgepole terminal weevil—P. terminalis Hopping

White pine weevil (P. strobi) and lodgepole terminal weevil (P. terminalis) infest 

terminals on spruce and lodgepole pine. 

Adults are rough-surfaced, brown to black 

beetles that usually have spots and patches 

of white, yellow, or brown scales and are 

about 1/4 inch (6 mm) long (fig. 539). The

head is prolonged into a slender beak or 

snout used for feeding. Larvae are small, 

white, legless grubs and resemble bark beetle larvae (fig. 540).

Hosts—White pine weevil: Engelmann and blue spruce and, occasionally,

lodgepole pine

Lodgepole terminal weevil: lodgepole pine

Life Cycle—There is one generation per year. The adults emerge in April and 

May and make feeding punctures on terminal growth of the preceding year. 

Eggs are laid in May or June, and the developing larvae feed first in the cambium region and later mine and pupate in the pith. Larvae girdle and kill the 

terminals. White pine weevil larvae kill the terminals once the current year’s 

growth has begun to elongate, resulting in the death of 2 years of growth, compared to lodgepole terminal weevils that only kill the current year’s growth. 

Starting in August, adults emerge and feed on needles, buds, and twigs. Adults 

overwinter in forest litter; larvae and pupae overwinter in infested terminals.

Damage—Larvae attack and kill or seriously injure current and previous year’s 

terminal shoots of young trees. This causes tops to die and leads to reduced 

height growth, forked tops, and crooked stems. Open-grown, even-aged stands 

of young spruce and lodgepole pine are highly susceptible. The first sign of

attack occurs in the spring when copious resin flow from small punctures near

Figure 540. Lodgepole terminal weevil 

larva, Pissodes terminalis. Photo: 

Intermountain Region, Ogden Archive, 

USDA Forest Service, Bugwood.org.

Figure 539. White pine weevil adult, 

Pissodes strobi. Photo: E. Bradford Walker, 

Vermont Department of Forests, Parks and 

Recreation, Bugwood.org.

B U D A N D S H O O T I N S E C T S
286 USDA Forest Service RMRS-GTR-241. 2010.

Figure 541. White pine weevil damage. Photo: Scott 

Tunnock, USDA Forest Service, Bugwood.org.

Figure 542. Pupal cells or “chip cocoons.” Photo: Ladd 

Livingston, Idaho Department of Lands, Bugwood.org.

the top of the leader can be seen. 

The leader begins to droop in 

midsummer and is brittle and 

brown by fall (fig. 541). Large

holes made by emerging beetles 

may be visible on the dead leader in late summer and fall. One 

of the diagnostic characteristics 

of terminal weevils is the construction of chip cocoons. Chip 

cocoons are the oval, pupal cells 

that have been surrounded with 

shredded wood fiber and are

constructed partly in the bark 

but mostly in the sapwood (fig.

542). These chip cocoons remain in the wood long after the 

beetles emerge.

Management—Severe problems 

with this insect are rare in forested areas in the Rocky Mountain Region, and direct control is 

not often warranted. However, 

damage is sometimes significant

in landscape situations, and it 

is possible to apply insecticides 

during the spring when adults are 

active and laying eggs. Only the upper areas of the spruce or lodgepole 

pine tree need to be treated. Focus 

should be placed on trees growing 

in open-grown, even-aged stands.

References: 35, 50

B U D A N D S H O O T I N S E C T S
USDA Forest Service RMRS-GTR-241. 2010. 287

Glossary

A

abdomen—Posterior or hindmost portion of an insect’s three main body 

divisions.

abiotic—Non-living factors such as temperature, moisture, and wind.

adult—Full-grown, sexually mature insect; usually has wings, in contrast 

to larvae, which lack wings.

advanced decay—Late stages of the decay process in which a fungus has 

produced its characteristic type of wood decay.

aecium, plural aecia—One of the many types of fruiting bodies formed 

during the life cycle of rust fungi; a cup-like structure producing 

aeciospores.

alternate host—One of two dissimilar plants infected by a different spore 

type of a rust or infested by a different life stage of an insect.

anamorph—Imperfect state of a fungus; produces asexual spores.

annual—An event that occurs once a year, or something that lasts 1 year 

or season; also, completing the life cycle in 1 year.

antenna, plural antennae—A pair of appendages used as sensory organs 

that are located on the head, usually above the eyes in insects.

apothecium, plural apothecia—Cup-shaped fruiting body of ascomycete 

fungi.

arthropod—An invertebrate creature characterized by an exoskeleton, 

segmented body, and paired, jointed limbs; includes insects, 

arachnids, and crustaceans.

Ascomycetes—A class of fungi that produces spores by sexual reproduction in a sac-like structure called an ascus. Many leaf and canker 

diseases are caused by Ascomycetes.

ascospore—Spore produced by sexual reproduction in Ascomycetes.

asexual spores—Spores produced vegetatively in fungi without going 

through genetic recombination.

B

Basidiomycetes—A class of fungi that produces spores by sexual reproduction in or on the outside of a structure called a basidium. 

Rusts and most decay fungi are Basidiomycetes.

biotic—Relating to living organisms.

blight—A disease or injury that results in rapid discoloration, withering, 

cessation of growth, and death of parts without rotting.

blue stain—Coloration of wood infected by fungi with blue, brown, or 

black hyphae; a group of fungi that cause blue stain.

boring dust—Tiny particles of bark or wood produced by insects as they 

tunnel in woody plant structures.

brood—All of the bark beetle offspring produced by a single set of parents 

that hatch and mature at about the same time.
288 USDA Forest Service RMRS-GTR-241. 2010.

broom—An abnormal proliferation of branches or twigs on a single 

branch.

brown rot—A light to dark brown decay of wood that is friable and cracks 

into more or less cubical segments in the advanced stage; caused 

by fungi that attack mainly the cellulose and associated carbohydrates. Residue left is predominantly lignin.

bug—An insect in the order Hemiptera, the “true bugs.”

burl—Abnormal proliferation of plant tissue, typically a globose growth 

on a tree stem or branch.

C

callow adult—Young adult bark beetle that is pale brown in color and has 

not emerged from under the bark.

callus—Woody healing tissue that attempts to grow over wounds or otherwise damaged tissue.

cambium—Layer of actively dividing cells between the xylem (sapwood) 

and the phloem (inner bark) of trees that forms additional conducting tissue, therefore increasing the girth of a stem, branch, 

or trunk.

canker—An oval or circular killed area on a stem or branch; usually with 

a shrunken surface.

chlorotic/chlorosis—Yellow appearance of normally green foliage caused 

by loss or lack of chlorophyll.

cocoon—A covering spun or constructed by a larva as a protection to the 

pupa.

conidia—Asexual spores produced on microscopic structures called 

conidiophores.

conk—The large, often bracket-like fruiting bodies of wood-consuming 

fungi (Basidiomycetes).

complement—The set of foliage produced by evergreen trees during one 

growing season.

cornicle—Paired, secretory, tubular structures protruding from the top of 

an aphid abdomen.

cortex—Outer layer of bark.

crawler—The active first instar of a scale insect.

cryptic—Hidden or concealed; not readily apparent.

cubical decay—Decayed wood breaking up into distinct cubes. 

D

DBH—Diameter at breast height. Tree diameter measured at 4.5 ft from 

the ground.

decay—The decomposition of wood or other substrate by microorganisms; also called rot.

decline—Gradual reduction in health and vigor as a tree is in the process 

of dying slowly.

declivity—Slope of elytra at posterior end of the beetle.
USDA Forest Service RMRS-GTR-241. 2010. 289

defoliation—Premature removal of foliage.

defoliator—An insect or pathogen that feeds on foliage, physically removes needles or portions of needles, or causes tissue necrosis 

and premature needle drop.

dessication—Rapid drying of plant parts.

diapause—Dormant phase.

dieback—Dead apical parts, usually twigs or limbs; also the process of 

dying from the outside in.

disease—A prolonged disturbance of the normal form or function of a tree 

or its parts

distal—Near or toward the free end of an appendage; far from the point of 

attachment or origin.

dorsal—Refers to the upper surface of an organism, appendage, or part.

E

egg gallery—Tunnel constructed under the bark of host trees by adult 

beetles for the purpose of laying eggs. Egg galleries maintain a 

fairly constant width with increasing length but are sometimes 

associated with wider constructions, such as entrance points and 

nuptial chambers.

egg niche—A small recess constructed in woody plant tissue by a female 

insect for the purpose of egg deposition. 

elytra—Leathery front wings of beetles that cover the membranous hind 

wings when the insect is not in flight; not used for flying. 

elytral declivity—The posterior, down-sloping portion of the elytra on 

bark beetles.

endemic—Characteristic of populations at normal background levels.

epidemic—Large-scale, temporary increase in population; characteristic 

of a population at an unusually high level.

erineum—An abnormal growth of hairs induced on the epidermis of a 

leaf by certain mites. 

etched—Lightly imprinted on a surface.

exotic—An introduced, or non-native, insect or pathogen.

exudate—Material that has oozed to the outside surface through small 

openings.

F

fascicle—An individual needle bundle or cluster on coniferous species.

flagging—Conspicuous recently dead or dying shoots or branches on a 

live tree that have discolored foliage still attached.

forewings—The first pair of wings from the head.

frass—Solid excrement of insects, particularly of larvae; particles are usually rounded pellets.

fruiting body—Any of a number of kinds of fungal reproductive structures that produce spores.
290 USDA Forest Service RMRS-GTR-241. 2010.

fungus, plural fungi—A member of the group of saprophytic and parasitic organisms that lack chlorophyll, have cell walls made of 

chitin, and reproduce by spores; includes molds, rusts, mildews, 

smuts, and mushrooms. Fungi absorb nutrients from the organic 

matter in which they live. Not classified as plants; instead, fungi 

are placed in the kingdom Fungi.

G

gall—Abnormal proliferation of plant tissue stimulated by insects, pathogens, or abiotic influences.

gallery—A tunnel constructed by an insect in which it lives, feeds, or deposits eggs.

generation—Development of insects from egg to adult; a brood.

genus—A taxonomic grouping of species with some set of similar 

characteristics.

girdle—To destroy or remove the cambium around the complete circumference, typically resulting in death of all tissues beyond the point 

of girdling.

gouting—Abnormally thickened or swollen branch tips or branches, 

caused by insects, pathogens, or abiotic influences.

gregarious—Living or feeding in groups.

grub—Thick-bodied, usually sluggish, larva of a beetle.

H

heartrot—A decay restricted to the heartwood.

heartwood—Central cylinder in tree trunks consisting of dead xylem tissue; functions as mechanical support.

hibernaculum, plural hibernacula—A tent-like structure or sheath constructed from a leaf or other material, in which a larva hides or 

hibernates.

hindwings—The second pair of wings from the head.

hip canker—A canker on one side of a pine stem with flared edges, caused 

by western gall rust.

hindwings—On insects having two pairs of wings, the pair of wings farthest from the head.

honeydew—Clear, sugary, liquid excretion of aphids or scales. Becomes 

sticky and shiny when dried.

host—Plant infected by a pathogen or infested by insects.

hypha, plural hyphae—Microscopic filaments of fungal cells.

hysterothecium, plural hysterothecia—Elongated fungal fruiting bodies 

that open with a slit, found in Ascomycete fungi. Common to 

many needle cast fungi.

I

incipient decay—The earliest stages of wood decay in which the wood 

may show discoloration but is otherwise not visibly altered.
USDA Forest Service RMRS-GTR-241. 2010. 291

infection—The process or result of a pathogen establishing itself on or 

within a host.

infection court—The point of invasion of a disease organism into its host.

instar—Life stage of a larva between successive molts.

L

laminated decay—Selective, more extensive decay in the spring wood 

than in summer wood, causing the wood to separate into sheets 

or laminae along annual rings.

larva, plural larvae—An immature form of an insect that undergoes 

complete metamorphosis, such as a caterpillar, grub, or maggot.

larval gallery—Tunnel constructed by developing beetle larvae as they 

feed under the bark. Larval galleries tend to grow wider with 

length and are often constructed at a right angle from the parent 

gallery.

latent infection—Infection by a disease organism that is not yet expressed 

by symptoms or signs.

lateral—to the side or on the sides, as in lateral shoot, lateral bud, lateral 

spread, lateral stripes. 

lesion—A localized area of dead or dying tissue caused by a pathogen or 

insect.

life cycle—All stages in the development of an organism from the production of one stage through the production of the same stage again.

longitudinal—Placed or running lengthwise with the axis of the tree bole 

or limb.

M

maturation feeding—A period of feeding by recently pupated, sexually immature adults of some insect species upon specific plants 

or plant parts that is essential for completion of reproductive 

development.

metamorphosis—Development of an insect as it goes through life stages 

from egg to adult.

midge—Adult of a group of small “flies” in the order Diptera.

molt—Process of shedding the exoskeleton between insect instars.

monogamous—Mating with only one individual.

mushroom—A fleshy, fruiting body of a fungus, often with a gilled pore 

surface.

mycelium—The collective mass of vegetative elements or hyphae, of a 

fungus.

mycelial fan—Dense mass of mycelium that takes the form of a thick mat 

that is fan-shaped.

N

necrotic—Dead; usually refers to cells or tissue killed by injury or disease.

necrosis—Localized death of living tissue; usually results in darkening 

of the tissue.
292 USDA Forest Service RMRS-GTR-241. 2010.

needle sheath—The thin, somewhat papery, tubular covering that surrounds the base of a pine needle fascicle.

node—Location on a stem where branches or leaves normally originate.

notch—Abrupt, rounded protuberance of a bark beetle egg gallery wall; 

has no egg niches or larval galleries. This usually marks the point 

of entry to the egg gallery from the outer bark surface and sometimes is used as a nuptial chamber.

nuptial chamber—An open, cave-like area constructed in the inner bark 

beneath the entrance hole by the male of some bark beetle species where mating takes place and from which the egg galleries 

originate. When viewed with the bark removed, nuptial chambers appear as enlarged areas at junctions or at beginnings of egg 

galleries.

nymph—An immature form of an insect that undergoes incomplete metamorphosis; nymphs resemble the adults except for size and wing 

development.

O

obligate parasite—An organism capable of living only as a parasite on a 

living host.

overwinter—To survive through winter; term commonly used when describing the life stage in which an insect passes the winter months; 

e.g., second instar western spruce budworm larvae overwinter.

ovipositor—A tubular or valved structure used by an insect to lay eggs 

(oviposition); usually concealed but sometimes extended far beyond the end of the body.

P

parasite—An organism that lives at the expense of another living organism; usually by invading and subsequently causing disease.

parenchyma cells—The primary tissue of higher plants, composed of 

thin-walled cells and forms the greater part of leaves, roots, the 

pulp of fruit, and the pith of stems.

parent gallery—Tunnel constructed by invading parent beetles where reproduction takes place and eggs are laid.

pathogen—An organism that causes disease in another organism.

pathogenic—Being capable of causing disease.

perennial—An organism that continues to live from year-to year; persisting for 3 or more years.

perithecium, plural perithecia—Small fungal fruiting structure that is 

round or flask-shaped and contains asci.

pheromone—Substance secreted to the outside of an insect’s body that 

serves as a chemical signal between individuals of the same 

species. Pheromones are usually airborne and act as attractants, 

repellants, or alarm signals.
USDA Forest Service RMRS-GTR-241. 2010. 293

phloem—Vascular tissue of higher plants that actively conducts the food 

produced in leaves and green stems through photosynthesis. 

Phloem is the inner bark of a tree.

phoresy—A form of symbiosis in which one organism is carried on the 

body of another larger-bodied organism; the former does not feed 

off of the latter.

photosynthesis—Process in green plants that uses sunlight and carbon 

dioxide to produce food.

pitch—A resinous exudate of various conifers.

pitch tube—A globular mass of resin, boring dust, and frass that forms on 

the bark of pine trees at bark beetle entrance holes.

pitch streamers—Long, thin flows of pitch on the tree bole.

pith—The soft, sponge-like center of stems and branches of woody plants.

pocket rot—A characteristic pattern of rot that occurs in distinct, scattered 

pockets rather than in columns.

pore—The open end of a tube in which certain spores of higher fungi are 

produced.

pore layer or pore surface—The surface of a fruiting body on which the 

pores are found.

posterior—The hind or rear part of the body.

progeny—Offspring or brood.

proleg—The fleshy, unjointed appendages found on the abdomens of caterpillars and some sawfly larvae; also called false legs.

pronotum—Dorsal (top) plate of an insect prothorax.

prothorax—The front-most portion of the three thoracic segments.

pseudostroma, plural pseudostromata—Small mass of vegetative fungal tissue mixed with host tissue, in or on which fruiting bodies 

are produced.

pseudothecium, plural pseudothecia—Tiny, round to flask-shaped fruiting bodies that contain asci and ascospores and are produced by 

certain Ascomycetes. 

punk knot—Soft, decayed branch stubs that usually indicate the presence 

of decay in a tree.

pupa, plural pupae—The inactive transitional stage between the larval 

and adult life stages that occurs in insects with complete metamorphosis. Characterized by a covering formed by the larva as it 

changes into a pupa.

pupal cell—Cavity at the end of the larval gallery in which pupation 

occurs.

pupate—To change from a larva into a pupa; to pass through the pupal 

stage.

pustule—Blisters of an infecting fungus that mature into fruiting 

structures.

pycnidium, plural pycnidia—Very small fruiting structure, flask-shaped 

and hollow, that produces asexual spores.
294 USDA Forest Service RMRS-GTR-241. 2010.

R

radial—In an outward direction from the center of the tree.

resinosis—Reaction of a tree to invasion by pathogens, insects, or abiotic 

injury that results in flow of resin on the outer bark or accumulation of resin within or under bark.

resistant—Characteristic of a plant that is able to prevent infection or 

limit disease.

rhizomorph—A specialized thread or cord-like structure made up of parallel hyphae with a protective covering.

root collar or root crown—The transition zone between the stem and root 

of a tree; located at the ground line.

S

saprobe or saprophyte—An organism that uses dead organic matter as 

food; a saprotroph.

saprot—Decay of dead wood; organism that decays only dead material; 

also saprophyte.

sapwood—The outer conducting layers of wood that contain living cells 

and food reserves.

sclerotium, plural sclerotia—Small, dark, hardened mass of fungal tissue, capable of enduring adverse conditions and later growing 

again.

score—Mark deeply, forming a groove.

secondary—Organisms that are limited to invading hosts predisposed by 

stress or attack by some other more aggressive organism.

seta, plural setae—Hollow, often slender, hair-like projections.

sexual fruiting bodies—Fruiting structures that produce spores 

formed through the process of meiosis, providing for genetic 

recombination.

sign—The manifestation of a causal organism, such as beetle galleries or 

fungal conks. (Contrast with symptom.)

slash—Woody debris such as logs, bark, and branches left after logging 

activities.

species, plural species—A taxonomic grouping or organisms alike in appearance and structure that can mate and produce fertile offspring.

spore—A microscopic reproductive cell capable of dispersing.

sporulate—To release spores.

stage—A distinguishable period of development, as in a stage of decay, a 

spore type or stage of a rust, or an insect life stage.

stoma, plural stomata—Opening in the epidermis (outer, protective layer 

of cells) of a leaf or stem that allows gas exchange.

stroma, plural stromata—Small mass of vegetative fungal tissue, in or 

on which fruiting bodies are produced.

stress cones—A cone crop produced as the result of tree stress; often associated with root diseases. Cones may be produced relatively 
USDA Forest Service RMRS-GTR-241. 2010. 295

early in the tree’s life and are often very numerous and small in 

size with high percentages of nonviable seeds.

stringy rot—Decay that results in the heartwood being reduced to fibrous 

material.

strip attack—An attack by bark beetles that does not completely girdle 

the tree but is concentrated along one side or within a relatively 

confined area on the bole; results in death of only a portion of the 

tree bole.

sunscald—Damage to the cambium of a thin-barked stem caused by overexposure to the heat from sunlight.

suppressed—Trees with crowns overtopped by larger trees that receive no 

direct sunlight from above or from the sides.

symbiotic—Characteristic of a beneficial relationship between two 

organisms of different species living in the same space; each participating organism is a symbiont.

symptom—The expression of disease or injury by the host plant, such as 

chlorosis or bark streaming.

T

target canker—Canker in which the pattern of annual expansion by the 

pathogen and callus production by the host results in concentric 

ridges.

telium, plural telia—One of the many types of fruiting bodies formed 

during the life cycle of rust fungi; produce teliospores.

tendril—Mass of spores in a gelatinous matrix that oozes from a fruiting 

body in a long, curling string.

terminal—The main or primary growing tip of a conifer.

thorax—In insects, the middle body division located behind the head and 

in front of the abdomen. Bears the wings and legs of adult insects.

tolerant—Characteristic of a host that is able to survive infection, often 

with minimal or delayed development of symptoms.

topkill—Death of the upper crown of a tree; usually caused by insects, 

pathogens, or weather events.

transverse—Lying or running at right angles to the axis of the tree bole 

or limb.

tubercle—A small, rounded prominence.

U

uredinium, plural uredinia—One of the many types of fruiting bodies 

formed during the life cycle of rust fungi; produce urediniospores.

V

vascular plant—Higher plant with specialized tissues for conducting water and nutrients.

vascular tissue—The xylem, cambium, and phloem tissues that make up 

the vascular system of higher plants.

vector—An organism that spreads a pathogen.
296 USDA Forest Service RMRS-GTR-241. 2010.

W

webbing—A mat or loose weave of silk strands produced by larvae; typically forming a protective nest in which larvae feed.

white rot—Decay caused by fungi that attack both chief constituents of 

wood, cellulose, and lignin and leave a whitish or light-colored 

residue.

windthrown—Uprooted and blown down by wind; as in windthrown 

trees.

wingspan—Width of wings when extended as if for flight.

witches’ broom—An abnormal proliferation of branches or twigs on a 

single branch.

X

xylem—The woody tissue of the stem, branches, and roots that transports 

water and minerals.

Z

zone line—Narrow, black or brown layers of tough fungal tissue in decayed wood that presumably resist the advance of other fungi.
USDA Forest Service RMRS-GTR-241. 2010. 297

References

1. Allen, E.A.; Morrison, D.J.; Wallis, G.W. 1996. Common tree diseases of 

British Columbia. Natural Resources Canada, Canadian Forest Service. 

178 p.

2. Amman, A.G.; Amman, S.L.; Amman, G.D. 1974. Development of 

Pityophthorus convertus. Environmental Entomology 3(3):562-563.

3. Amman, G.D.; McGregor, M.D.; Dolph, Jr., R.E. 1985. Mountain pine beetle. Forest Insect and Disease Leaflet 2. Washington, DC: U.S. Department 

of Agriculture, Forest Service. 11 p.

4. Anderson, G.W.; Hinds, T.E.; Knutson, D.M. 1977. Decay and discoloration of aspen. Forest Insect and Disease Leaflet 149. Washington, DC: U.S. 

Department of Agriculture, Forest Service. 4 p.

5. Anderson, R.L.; Skilling, D.D. 1955. Oak wilt damage: a survey in central 

Wisconsin. Station Paper 33. St. Paul, MN: U.S. Department of Agriculture, 

Forest Service, North Central Forest Experiment Station. 11 p.

6. Arthur, J.C. 1934. Manual of the Rusts in the United States and Canada. 

Lafayette, IN: Purdue Research Foundation. 438 p.

7. Baker, C.J.; Harrington, T.C. 2001. Ceratocystis fimbriata. Crop Protection 

Compendium. CABI Publishing. Updated information at: http://www.public.iastate.edu/~tcharrin/CABIinfo.html.

8. Baranyay, J.A.; Ziller, W.G. 1972. Broom rusts of conifers in British 

Columbia. Forest Pest Leaflet 48. Victoria, BC: Natural Resources Canada, 

Canadian Forest Service.

9. Barnes, I.; Crouse, P.W.; Wingfield, B.D.; Wingfield, M.J. 2004. Multigene 

phylogenies reveal that red band needle blight is caused by two distinct 

species of Dothistroma, D. septosporum and D. pini. Studies in Mycology 

50:551-565.

10. Barrett, D.K.; Uscuplic, M. 1971. The field distribution of interacting strains of Polyporus schweinitzii and their origin. New Phytologist 

70:581-598.

11. Batzer, H.O.; Morris, R.C. 1978. Forest tent caterpillar. Forest Insect and 

Disease Leaflet 9. Washington, DC: U.S. Department of Agriculture, Forest 

Service. 8 p.

12. Beatty, J.S.; Mathiasen, R.L. 2003. Dwarf mistletoes of ponderosa pine. 

Forest Insect and Disease Leaflet 40. Washington, DC: U.S. Department 

of Agriculture, Forest Service. 8 p. Online: http://www.fs.fed.us/r6/nr/fid/

fidls/fidl40.htm.

13. Beckwith, R.C. 1973. The large aspen tortrix. Forest Insect and Disease 

Leaflet 139. Washington, DC: U.S. Department of Agriculture, Forest 

Service. 5 p.

14. Bedker, P.J.; O’Brien, J.G.; Mielke, M.M. 1995. How to prune trees. NAFR-01-95. Radnor, PA: U.S. Department of Agriculture, Forest Service, 

Northeastern Area, State and Private Forestry. 12 p. Online: http://www.

na.fs.fed.us/Spfo/pubs/howtos/ht_prune/htprune.pdf.
298 USDA Forest Service RMRS-GTR-241. 2010.

15. Berry, F.H. 1985. Anthracnose diseases of eastern hardwoods. Forest Insect 

and Disease Leaflet 133. Washington, DC: U.S. Department of Agriculture, 

Forest Service. 7 p.

16. Blenis, P.V. 2007. Impact of simulated aspen shoot blight on trembling 

aspen. Canadian Journal of Forest Research 37:719-725.

17. Bolander, P., ed. 1999. Dust palliative selection and application guide. 

Project Report. 9977-1207-SDTDC. San Dimas, CA: U.S. Department 

of Agriculture, Forest Service, San Dimas Technology and Development 

Center. 20 p.

18. Boutz, G.A.; Stack, R.W. 1986. Herbicides (air pollution). In: Riffle, 

J.W.; Peterson, G.W., tech. coords. Diseases of trees in the Great Plains. 

Gen. Tech. Rep. GTR-RM-129. Fort Collins, CO: U.S. Department of 

Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment 

Station: 33-35.

19. Boyce, J.S. 1961. Forest Pathology. New York, NY: McGraw Hill Book 

Co. 572 p.

20. Brookhouser, L.W.; Peterson, G.W. 1971. Infection of Austrian, Scots, and 

ponderosa pines by Diplodia pinea. Phytopathology 61:409-414.

21. Burns, K.S.; Schoettle, A.W.; Jacobi, W.R.; Mahalovich, M.F. 2008. 

Options for the management of white pine blister rust in the Rocky 

Mountain Region. Gen. Tech. Rep. RMRS-GTR-206. Fort Collins, CO: 

U.S. Department of Agriculture, Forest Service, Rocky Mountain Research 

Station. 26 p. Online: http://www.fs.fed.us/rm/pubs/rmrs_gtr206.pdf .

22. Cain, R.; Parker, D. 1998. Conifer pests in New Mexico, Rev. Albuquerque, 

NM: U.S. Department of Agriculture, Forest Service, Southwestern Region. 

50 p.

23. Callan, B.E. 1998. Diseases of Populus in British Columbia: a diagnostic manual. Victoria, BC: Natural Resources Canada, Canadian Forestry 

Service, Pacific Forestry Centre. 157 p.

24. Carolin, V.M.; Knopf, J.A.E. 1968. The Pandora moth. Forest Insect and 

Disease Leaflet 114. Washington, DC: U.S. Department of Agriculture, 

Forest Service. 7 p.

25. Childs, T.W.; Kimmey, J.W. 1938. Studies on probable damage by blister 

rust in some representative stands of young western white pine. Journal of 

Agricultural Research 57:557-568.

26. Childs, T.W.; Shea, K.R.; Stewart, J.L. 1971. Elytroderma disease of ponderosa pine. Forest Pest Leaflet 42. U.S. Department of Agriculture, Forest 

Service. 6 p.

27. Ciesla, W.M.; Ragenovich, I.R. 2008. Western tent caterpillar. Forest Insect 

and Disease Leaflet 119. Washington, DC: U.S. Department of Agriculture, 

Forest Service. 8 p.

28. Cole, W.E. 1971. Pine butterfly. Forest Insect and Disease Leaflet 66. 

Washington, DC: U.S. Department of Agriculture, Forest Service. 4 p.
USDA Forest Service RMRS-GTR-241. 2010. 299

29. Costello, S.L. 2005. Wood borers in fire-damaged ponderosa pine forests of the Black Hills, South Dakota. Fort Collins, CO: Colorado State 

University. 71 p. Thesis.

30. Coutts, M.P.; Rishbeth, J. 1977. The formation of wetwood in grand fir. 

European Journal of Forest Pathology 7:13-22.

31. Crane, P.E.; Blenis, P.V.; Hiratsuka, Y. 1994. Black stem galls on aspen 

and their relationship to decay by Phellinus tremulae. Canadian Journal of 

Forest Research 24(11):2240-2243.

32. Cranshaw, W.S. 2004. Garden insects of North America: the ultimate 

guide to backyard bugs. Princeton University Press. 656 p.

33. Cranshaw, W.S. 2005. Cooley spruce galls. Pest Fact Sheet No. 5.534. Fort 

Collins, CO: Colorado State University, Cooperative Extension. 3 p.

34. Cranshaw, W.S. 2008. Oytershell scale. Extension Fact Sheet No. 5.513. 

Fort Collins, CO: Colorado State University, Cooperative Extension. 2 p.

35. Cranshaw, W.S.; Leatherman, D.A.; Jacobi, W.R.; Mannix L. 2000. Insects 

and diseases of woody plants of the central Rockies. Bulletin 506A. Fort 

Collins, CO: Colorado State University, Cooperative Extension. 284 p.

36. Davidson, R.W.; Hinds, T.E.; Hawksworth, F.G. 1959. Decay of aspen 

in Colorado. Station Paper 45. Fort Collins, CO: U.S. Department of 

Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment 

Station. 14 p.

37. DeMars, C.J.; Roettgering, B.H. 1982. The western pine beetle. Forest 

Insect and Disease Leaflet Number 1. Washington, DC: U.S. Department of 

Agriculture, Forest Service. 6 p.

38. Derr, J.F.; Appleton, B.L. 1988. Herbicide injury to trees and shrubs. 

Virginia Beach, VA: Blue Crab Press. 72 p.

39. Dix, M.E.; Pasek, J.E.; Harrell, M.O.; Baxendale, F.P. 1986. Common insect pests of trees in the Great Plains. EC 86-1548. Nebraska Cooperative 

Extension Service. 44 p.

40. Dobson, M.C. 1991. De-icing salt damage to trees and shrubs. Forestry 

Commission Bulletin 101. London. 64 p.

41. Dreistadt, S. 1994. Pests of landscape trees and shrubs. Publication 3359. 

Berkeley, CA: University of California Press. 327 p.

42. Eager, T.J. 1999. Factors affecting the health of pinyon pine trees (Pinus 

edulis) in the pinyon-juniper woodlands of western Colorado. In: Monsen, 

S.B.; Stevens, R., eds, Proceedings: ecology and management of pinyon-juniper communities within the interior West; 1997 September 15-18; Provo, 

UT. Proc. RMRS-P-9. Ogden, UT: U.S. Department of Agriculture, Forest 

Service, Rocky Mountain Research Station: 397-399.

43. Etheridge, D.E.; Craig, H.M. 1976. Factors influencing infection and initiation of decay by the Indian paint fungus (Echinodontium tinctorium) in 

western hemlock. Canadian Journal of Forest Research 6:299-318.

44. Farr, D.F.; Rossman, A.Y. 2009. Fungal Databases. U.S. Department of 

Agriculture, Agricultural Research Service, Systematic Mycology and 
300 USDA Forest Service RMRS-GTR-241. 2010.

Microbiology Laboratory. Online: http://nt.ars-grin.gov/fungaldatabases/. 

[December 18, 2009].

45. Fellin, D.G.; Dewey, J.E. 1986. Western spruce budworm. Forest Insect 

and Disease Leaflet 53. Washington, DC: U.S. Department of Agriculture, 

Forest Service. 10 p.

46. Fettig, C.J.; Allen, K.K.; Borys, R.R.; Christopherson, J.; Dabney, C.P.; 

Eager, T.J.; Gibson, K.E.; Hebertson, E.G.; Long, D.F.; Munson, A.S. 2006. 

Effectiveness of bifenthrin (Onyx) and carbaryl (Sevin SL) for protecting individual, high-value conifers from bark beetle attack (Coleoptera: 

Curculionidae: Scolytinae) in the Western United States. Journal of 

Economic Entomology 99(5):1691-8.

47. French, D.W.; Stienstra, W.C. 1980. Oak wilt. Folder 310. Minneapolis, 

MN: University of Minnesota, Agricultural Extension Service. 11 p.

48. Funk, A. 1981. Parasitic microfungi of western trees. Victoria, BC: Natural 

Resources Canada, Canadian Forestry Service, Pacific Forestry Centre. 

190 p.

49. Funk, A. 1985. Foliar fungi of western trees. Victoria, BC: Natural 

Resources Canada, Canadian Forestry Service, Pacific Forestry Centre. 

159 p.

50. Furniss, R.L.; Carolin, V.M. 1977. Western forest insects. Misc. Publ. 

1339. Washington, DC: U.S. Department of Agriculture, Forest Service. 

654 p.

51. Furniss, R.L.; Malcolm, M.; Kegley, S.J. 2008. Biology of Dendroctonus 

murrayanae (Coleoptera: Curculionidae: Scolytinae) in Idaho and Montana 

and comparative taxonomic notes. Annals of the Entomological Society of 

America 101(6):1010-1016.

52. Garbutt, R. 1992. Western balsam bark beetle. Forest Pest Leaflet. Victoria, 

BC: Natural Resources Canada, Canadian Forestry Service, Pacific Forestry 

Centre. 4 p.

53. Geils, B.W.; Cibrián Tovar, J.; Moody, B., tech. coords. 2002. Mistletoes 

of North American conifers. Gen. Tech. Rep. RMRS-GTR-98. Ogden, UT: 

U.S. Department of Agriculture, Forest Service, Rocky Mountain Research 

Station. 123 p.

54. Gibbs, J.N. 1978. Intercontinental epidemiology of Dutch elm disease. 

Annual Review of Phytopathology. 16:287-307.

55. Gibson, K.; Kegley, S.; Oakes, B. 1997. Western balsam bark beetle activity and flight periodicity in the Northern Region. Report 97-3. Missoula, 

MT: U.S. Department of Agriculture, Forest Service, Cooperative Forestry 

and Forest Health Protection. 5 p.

56. Gilbertson, R.L. 1974. Fungi that decay ponderosa pine. Tucson: 

University of Arizona Press. 197 p.

57. Gilbertson, R.L.; Ryvarden, L. 1986. North American polypores. Volumes 

1 and 2. Oslo, Norway: Fungiflora. 885 p.
USDA Forest Service RMRS-GTR-241. 2010. 301

58. Haack, R.A.; Acciavatti, R.E. 1992. Twolined chestnut borer. Forest Insect 

and Disease Leaflet 168. Washington, DC: U.S. Department of Agriculture, 

Forest Service. 12 p.

59. Hadfield, J.S.; Mathiasen, R.L.; Hawksworth, F.G. 2000. Douglas-fir 

dwarf mistletoe. Forest Insect and Disease Leaflet 54. Washington, DC: 

U.S. Department of Agriculture, Forest Service. 9 p. Online: http://www.

fs.fed.us/r6/nr/fid/fidls/douglasfirdm.pdf.

60. Hagle, S.K.; Gibson, K.E.; Tunnock, S. 2003. Field guide to disease and 

insect pests of northern and central Rocky Mountain conifers. Report R1-

03-08. Missoula, MT: U.S. Department of Agriculture, Forest Service, 

Northern Region. 67 p.

61. Harestad, A.S.; Bunnell, F.L.; Sullivan, T.P.; Andrusiak, L. 1986. Key to 

injury of conifer trees by wildlife in British Columbia. WHR-23. Victoria, 

BC: Natural Resources Canada, Canadian Forestry Service, Pacific Forestry 

Centre. 38 p.

62. Harrell, M.L. 1993. Influence of pine host species on infestation and 

damage by Dioryctria borers in the central Great Plains. Environmental 

Entomology 22(4):781-783.

63. Harrell, M.L. 1996. Life histories and parasitoids of Dioryctria borers of pines in Nebraska. Journal of the Kansas Entomological Society 

69(4):279-284.

64. Haugen, D.A.; Hoebeke, E.R. 2005. Sirex woodwasp—Sirex noctilio F. 

(Hymenoptera: Siricidae). Pest Alert. NA-PR-07-05. Newton Square, PA: 

U.S. Department of Agriculture, Forest Service, Northeastern Area. 2 p.

65. Hawksworth, F.G. 1961. Dwarf mistletoe of ponderosa pine in the 

Southwest. Tech. Bull. No. 1246. Washington, DC: U.S. Department of 

Agriculture, Forest Service. 112 p.

66. Hawksworth, F.G. 1977. The 6-class dwarf mistletoe rating system. Gen. 

Tech. Rep. RM-48. Fort Collins, CO: U.S. Department of Agriculture, 

Forest Service, Rocky Mountain Forest and Range Experiment Station. 7 p.

66a. Hawksworth, F.G.; Dooling, O.J. 1984. Lodgepole pine dwarf mistletoe. 

Forest Insect and Disease Leaflet 18. Washington, DC: U.S. Department of 

Agriculture, Forest Service. 11 p.

67. Hawksworth, F.G.; Geils, B.W. 1990. How long do mistletoe-infected 

ponderosa pines live? Western Journal of Applied Forestry 5(2):47-48.

68. Hawksworth F.G.; Johnson, D.W. 1989. Biology and management of 

dwarf mistletoe in lodgepole pine in the Rocky Mountains. Gen. Tech. Rep. 

RM-169. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, 

Rocky Mountain Forest and Range Experiment Station. 38 p.

69. Hawksworth, F.G.; Scharpf, R.F. 1981. Phoradendron on conifers. Forest 

Insect and Disease Leaflet 164. Washington, DC: U.S. Department of 

Agriculture, Forest Service. 7 p.

70. Hawksworth, F.G.; Wiens, D. 1996. Dwarf mistletoes: biology, pathology and systematics. Agricultural Handbook 709. Washington, DC: U.S. 

Department of Agriculture, Forest Service. 410 p.
302 USDA Forest Service RMRS-GTR-241. 2010.

71. Hedgcock, C.G. 1913. Herpotrichia and Neopeckia on conifers. 

Phytopathology 3:152-158.

72. Hepting, G.E. 1971. Diseases of forest and shade trees of the United 

States. Agricultural Handbook 386. Washington, DC: U.S. Department of 

Agriculture, Forest Service. 658 p.

73. Hessburg, P.F.; Goheen, D.J.; Bega, R.V. 1995. Black stain root disease of 

conifers. Forest Insect and Disease Leaflet 145 (revised). Washington, DC: 

U.S. Department of Agriculture, Forest Service. 9 p. Online: http://www.

fs.fed.us/r6/nr/fid/fidls/fidl-145.pdf.

74. Himelick, E.B.; Neely, D. 1960. Juniper hosts of cedar-apple and cedarhawthorn rust. Plant Disease Reporter 44:109-112.

75. Hinds, T.E. 1963. Extent of decay associated with Fomes igniarius sporophores in Colorado aspen. Res. Note RM-4. Fort Collins, CO: U.S. 

Department of Agriculture, Forest Service, Rocky Mountain Forest and 

Range Experiment Station. 4 p.

76. Hinds, T.E. 1964. Distribution of aspen cankers in Colorado. Plant Disease 

Reporter 48(8):610-614.

77. Hinds, T.E. 1976. Aspen mortality in Rocky Mountain campgrounds. Res. 

Pap. RM-164. Fort Collins, CO: U.S. Department of Agriculture, Forest 

Service, Rocky Mountain Forest and Range Experiment Station. 20 p.

78. Hinds, T.E. 1981. Cryptosphaeria canker and Libertella decay of aspen. 

Phytopathology 71(11):1137-1145.

79. Hinds, T.E. 1985. Diseases. In: DeByle, N.V.; Winokur, R.P., eds. Aspen: 

ecology and management in the western United States. Gen. Tech. Rep. 

RM-119. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, 

Rocky Mountain Forest and Range Experiment Station. 283 p.

80. Hinds, T.E.; Hawksworth, F.G. 1966. Indicators and associated decay of 

Engelmann spruce in Colorado. Res. Pap. RM-25. Fort Collins, CO: U.S. 

Department of Agriculture, Forest Service, Rocky Mountain Forest and 

Range Experiment Station. 15 p.

81. Hinds, T.E.; Hawksworth, F.G.; Davidson, R.W. 1960. Decay of subalpine 

fir in Colorado. Station Paper 51. Fort Collins, CO: U.S. Department of 

Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment 

Station. 13 p.

82. Hinds, T.E.; Wengert, E.M. 1977. Growth and decay losses in Colorado aspen. Res. Pap. RM-193. Fort Collins, CO: U.S. Department of Agriculture, 

Forest Service, Rocky Mountain Forest and Range Experiment Station. 10 

p.

83. Hiratsuka, Y.; Zalasky, H. 1993. Frost and other climate-related damage 

of forest trees in the prairie provinces. Inf. Rep. NOR-X-286. Edmonton, 

Alberta: Natural Resources Canada, Canadian Forestry Service, Northern 

Forestry Centre. 26 p.

84. Holeski, L.M.; Vogelzang, A.; Stanosz, G.; Lindroth, R.L. 2009. Incidence 

of Venturia shoot blight in aspen (Populus tremuloides Michx.) varies 
USDA Forest Service RMRS-GTR-241. 2010. 303

with tree chemistry and genotype. Biochemical Systematics and Ecology 

37:139-145.

85. Holsten, E.H.; Their, R.W.; Munson, A.S.; Gibson, K.E. 1999. The 

spruce beetle. Forest Insect and Disease leaflet 127. Washington, DC: U.S. 

Department of Agriculture, Forest Service. 12 p.

86. Hornibook, E.M. 1950. Estimating defect in mature and over-mature 

stands of three Rocky Mountain conifers. Journal of Forestry 48:408-417.

87. Hunt, R.S. 1982. White pine blister rust control in British Columbia I. 

The possibilities of control by branch removal. The Forestry Chronicle 

59:136-138.

88. Ives, W.G.H.; Wong, H.R. 1988. Tree and shrub insects of the Prairie 

Provinces. Inf. Report NOR-X-292. Edmonton, Alberta: Natural Resources 

Canada, Canadian Forestry Service, Northern Forestry Centre. 327 p.

89. Jacobi, W.R.; Shepperd, W.D. 1991. Fungi associated with sprout mortality in aspen clearcuts in Colorado and Arizona. Research Note RM-513. 

Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky 

Mountain Forest and Range Experiment Station. 5 p.

90. James, R.L.; Gillman, L.S. 1979. Fomes annosus on white fir in Colorado. 

Tech. Rep. R2-17. Golden, CO: U.S. Department of Agriculture, Forest 

Service, Rocky Mountain Region, Forest Insect and Disease Management. 

9 p.

91. James, R.L.; Gillman, L.S. 1980. Incidence and distribution of conifer 

broom rusts in Colorado. Tech. Rep. R2-21. Golden, CO: U.S. Department 

of Agriculture, Forest Service, Rocky Mountain Region, Forest Insect and 

Disease Management. 30 p.

92. James, R.L.; Lister, C.K. 1978. Insect and disease conditions of pinyon 

pine and Utah juniper in Mesa Verde National Park, Colorado. Biological 

Evaluation R2-78-4. Lakewood, CO: U.S. Department of Agriculture, Forest 

Service, Rocky Mountain Region, Forest Insect and Disease Management.

93. Johnson, D.W. 1982. Forest pest management training manual. Lakewood, 

CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain 

Region. 138 p.

94. Johnson, D.W. 1984. An assessment of root diseases in the Rocky 

Mountain Region. Tech. Rep. R2-29. Lakewood, CO: U.S. Department of 

Agriculture, Forest Service, Rocky Mountain Region. 20 p.

95. Johnson, D.W. 1986. Comandra blister rust. Forest Insect and Disease 

Leaflet 62. Washington, DC: U.S. Department of Agriculture, Forest 

Service. 8 p.

96. Johnson, D.W.; Beatty, J.S.; Hinds, T.E. 1995. Cankers on western quaking aspen. Forest Insect and Disease Leaflet 152. Washington, DC: U.S. 

Department of Agriculture, Forest Service. 8 p.

97. Johnson, J.A.; Harrington, T.C.; Engelbrecht, C.J.B. 2005. Phylogeny and 

taxonomy of the North American clade of the Ceratocystis fimbriata complex. Mycologia 97(5):1067-1092.
304 USDA Forest Service RMRS-GTR-241. 2010.

98. Johnson, W.T.; Lyons, H.H. 1988. Insects that feed on trees and shrubs, 2nd

ed. Ithaca, NY: Cornell University Press. 556 p.

99. Jones, J.R.; DeByle, N.V.; Bowers, D.M. 1985. Insects and other invertebrates. In: DeByle, N.V.; Winokur, R.P., eds. Aspen: ecology and management in the western United States. Gen. Tech. Rep. RM-119. Fort Collins, 

CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain 

Forest and Range Experiment Station: 107-114 p.

100. Juzwik, J.; Nishijima, W.T.; Hinds, T.E. 1978. Survey of aspen cankers in 

Colorado. Plant Disease Reporter 62(10):906-910.

101. Kearns, H.S.J.; Jacobi, W.R. 2005. Impacts of black stain root disease in 

recently formed mortality centers in the piñon-juniper woodlands of southwestern Colorado. Canadian Journal of Forest Research 35:461-471.

102. Kegley, S.J. 2006. Western balsam bark beetle, ecology and management. In: U.S. Department of Agriculture, Forest Service. Forest Insect 

and Disease Identification and Management Guide of Northern and Central 

Rocky Mountain Conifer and Hardwood. U.S. Department of Agriculture, 

Forest Service, Forest Health Protection, Region 1 and 4. Online: http://

www.fs.fed.us/r1-r4/spf/fhp/mgt_guide/western_balsambark_beetle/index.

html.

103. Kegley, S.J.; Livingston, R.L.; Gibson, K.E. 1997. Pine engraver, Ips pini 

(Say), in the western United States. Forest Insect and Disease Leaflet 122. 

Washington, DC: U.S. Department of Agriculture, Forest Service. 8 p.

104. Kern, F.D. 1973. A revised taxonomic account of Gymnosporangium. 

University Park, PA: Pennsylvania State University Press. 134 p.

105. Koch, P. 1996. Lodgepole pine in North America. Volume 1. Madison, 

WI: Forest Products Society. 343 p.

106. Kondo, E.; Foudin, A.; Linit, M.; Smith, M.; Bolla, R.; Winter, R.; 

Dropkin, V. 1982a. Pine wilt disease–nematological, entomological, and 

biochemical investigations. Publication SR 282. Columbia, MO: University 

of Missouri Agricultural Experiment Station. 56 p.

107. Kondo, E.S.; Hiratsuka, Y.; Denyer, W.B.G. 1982b. Proceedings of 

the Dutch elm disease workshop and symposium. Winnipeg: Manitoba 

Department Natural Resources. 517 p.

108. Krupinsky, J.M.; Johnson, D.W. 1986. Septoria leaf spots of cottonwood, 

caragana, and maple. In: Riffle, J.W.; Peterson, G.W., tech. coords. Diseases 

of trees in the Great Plains. Gen. Tech. Rep. RM-129. Fort Collins, CO: 

U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest 

and Range Experiment Station. 149 p.

109. Kruse, J.; Ambourn, A.; Zogas, K. 2007. Aspen leaf miner. R10-PR-14. 

Juneau, AK: U.S. Department of Agriculture, Forest Service, Alaska 

Region, State and Private Forestry. 4 p. Online: http:\\www.fs.fed.us/r10/

spf/fhp/leaflets/aspen_leaf_miner.pdf.

110. Landis, T.D.; Helburg, L.B. 1976. Black stain root disease of pinyon pine 

in Colorado. Plant Disease Reporter 60:713-717.
USDA Forest Service RMRS-GTR-241. 2010. 305

111. Lännenpää, A.; Aakala, T.; Kauhanen, H.; Kuuluvainen, T. 2008. Tree 

mortality agents in pristine Norway spruce forests in northern Fennoscandia. 

Silva Fennica 42(2):151-163.

112. Lawrence, W.H.; Kverno, N.B.; Hartwell, H.D. 1961. Guide to wildlife 

feeding injuries on conifers in the Pacific Northwest. Portland, OR: Western 

Forestry and Conservation Association 44 p.

113. Leatherman, D.; Lange, D. 1997. Western cedar bark beetles. Colorado 

State Forest Service. Unnumbered leaflet. 4 p.

114. Lewis, R., Jr. 1978. Influence of infection court, host vigor, and culture 

filtrates on canker production by Botryodiplodia theobromae conidia in 

sycamore. Plant Disease Reporter 62:934-937.

115. Lightle, P.C.; Weiss, M.J. 1974. Dwarf mistletoe of ponderosa pine in 

the Southwest. Forest Insect and Disease Leaflet 19. Washington, DC: U.S. 

Department of Agriculture, Forest Service. 8 p. Online: http://www.fs.fed.

us/r6/nr/fid/fidls/fidl19.htm.

116. Mahalovich, M.F. 2000. Whitebark pine restoration strategy--some genetic considerations. Nutcracker Notes 11:6-9.

117. Malek, R.B.; Appleby, J.E. 1984. Epidemiology of pine wilt in Illinois. 

Plant Disease 68:180-186.

118. Mathiasen, R.L.; Beatty, J.S.; Pronos, J. 2002. Pinyon pine dwarf 

mistletoe. Forest Insect and Disease Leaflet 174. Washington, DC: U.S. 

Department of Agriculture, Forest Service. 7 p. Online: http://www.na.fs.

fed.us/spfo/pubs/fidls/pinyon_mistletoe/ppd_mistletoe.pdf.

119. McDonald, G.I.; Richardson, B.A.; Zambino, P.J.; Klopfenstein, N.B.; 

Kim, M.S. 2006. Pedicularis and Castilleja are natural hosts of Cronartium 

ribicola in North America: a first report. Forest Pathology 36:73–82.

120. McMillin, J.D.; Allen, K.K.; Long, D.F.; Harris, J.L.; Negron, J.F. 2003. 

Effects of western balsam bark beetle on spruce-fir forests of north-central 

Wyoming. Western Journal of Applied Forestry 18(4):259-266.

121. Miller, P.R. 1993. Abiotic Diseases. In: Scharpf, R.F., tech coord. 

Diseases of Pacific Coast Conifers. Agricultural Handbook 521. Albany, 

NY: U.S. Department of Agriculture, Forest Service, Pacific Southwest 

Research Station. 199 p.

122. Moltzan, B.D.; Blenis, P.V.; Hiratsuka, Y. 2001. Effects of spore availability, spore germinability, and shoot susceptibility on gall rust infection of 

pine. Plant Disease 85:1193-1199.

123. Negron, J.; Wilson, J. 2003. Attributes associated with probability of infestation by the pinyon ips, Ips confusus (Coleoptera: Scolytidae) in pinyon 

pine, Pinus edulis. Western North American Naturalist 63(4):440-51.

124. Ostry, M.E.; Wilson, L.F.; McNabb H.S., Jr.; Moore, L.M. 1989. A guide 

to insect, disease and animal pests of poplar. Agricultural Handbook 677. 

Washington, DC: U.S. Department of Agriculture, Forest Service. 118 p.

125. Partridge, A.D.; Miller, D.L. 1974. Major wood decays in the Inland 

Northwest. Natural Resource Series No. 3. Idaho Research Foundation, Inc. 

125 p.
306 USDA Forest Service RMRS-GTR-241. 2010.

126. Peterson, G.W. 1977. Infection, epidemiology, and control of Diplodia 

blight of Austrian, ponderosa, and Scots pines. Phytopathology 67:511-514.

127. Peterson, G.W. 1978. Effective and economical methods for controlling 

Diplodia tip blight. American Nurseryman 147(1):13, 66, 70, 72.

128. Peterson, G.W. 1981. Diplodia blight of pines. Forest Insect and Disease 

Leaflet 161. Washington, DC: U.S. Department of Agriculture, Forest 

Service. 7 p.

129. Peterson, G.W. 1982. Dothistroma needle blight of pines. Forest Insect 

and Disease Leaflet 143. Washington, DC: U.S. Department of Agriculture, 

Forest Service. 6 p.

130. Peterson, R.S. 1960. Western gall rust of hard pines. Forest Pest Leaflet 

50. Washington, DC: U.S. Department of Agriculture, Forest Service. 8 p.

131. Peterson, R.S. 1961. Conifer tumors in the central Rocky Mountains. 

Plant Disease Reporter 45:472-474.

132. Peterson, R.S. 1963. Effects of broom rusts on spruce and fir. Res. 

Paper INT-7. Ogden, UT: U.S. Department of Agriculture, Forest Service, 

Intermountain Experiment Station.

133. Peterson, R.S. 1971. Wave years of infection by gall rust on pines. Plant 

Disease Reporter 55:163-167.

134. Petty, J.L. 1977. Bionomics of two aspen bark beetles. Great Basin 

Naturalist 37(1):105-127.

135. Price, R.A.; Liston, A.; Strauss, S.H. 1998. Phylogeny and systematics of 

Pinus. In: Richardson, D.M., ed. Ecology and Biogeography of Pinus. Cape 

Town, South Africa: Cambridge University Press. 524 p.

136. Rexrode, C.O.; Jones, T.W. 1970. Oak bark beetles--important vectors of 

oak wilt. Journal of Forestry 68:294-297.

137. Riffle, J.W. 1981. Cankers. In: Stipes, R.J.; Campana, R.J., eds. 

Compendium of elm diseases. St. Paul, MN: American Phytopathological 

Society. 96 p.

138. Riffle, J.W.; Conway, K.E. 1986. Phellinus stem decays of hardwoods. 

In: Riffle, J.W.; Peterson, G.W., tech. coords. Diseases of trees in the Great 

Plains. Gen. Tech. Rep. RM-129. Fort Collins, CO: U.S. Department of 

Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment 

Station. 149 p.

139. Riffle, J.W.; Ostrofsky, W.D.; James, R.L. 1981. Fomes fraxinophilus on 

green ash in Nebraska windbreaks. Plant Disease 65:667-669.

140. Riffle, J.W.; Peterson, G.W., tech. coords. 1986. Diseases of trees in the 

Great Plains. Gen. Tech. Rep. RM-129. Fort Collins, CO: U.S. Department of 

Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment 

Station. 149 p.

141. Riffle, J.W.; Sharon, E.M.; Harrell, M.O. 1984. Incidence of Fomes fraxinophilus on green ash in Nebraska woodlands. Plant Disease 68(4):322-324.

142. Riffle, J.W.; Walla, J.A. 1986. Perennial wood-rotting fungi that cause 

stem decays of hardwoods. In: Riffle, J.W.; Peterson, G.W., tech. coords. 

Diseases of trees in the Great Plains. Gen. Tech. Rep. RM-129. Fort Collins, 
USDA Forest Service RMRS-GTR-241. 2010. 307

CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain 

Forest and Range Experiment Station. 149 p.

143. Ross, W.D. 1976. Fungi associated with root diseases of aspen in 

Wyoming. Canadian Journal of Botany 54:734-744.

144. Samman, S.; Schwandt, J.W.; Wilson, J.L. 2003. Managing for healthy 

white pine ecosystems in the United States to reduce the impacts of white 

pine blister rust. Report R1-03-118. Missoula, MT: U.S. Department of 

Agriculture, Forest Service, Northern Region. 10 p.

145. Scharpf, R.F., tech. coord. 1993. Diseases of Pacific Coast conifers. Agricultural Handbook 521. Washington, DC: U.S. Department of 

Agriculture, Forest Service. 199 p.

146. Schipper, A.L.; Anderson, R.L. 1976. How to identify Hypoxylon canker 

of aspen. How To Leaflet. St. Paul, MN: U.S. Department of Agriculture, 

Forest Service, North Central Forest Experiment Station. 4 p.

147. Schmid, J.M. 1981. Spruce beetles in blowdown. Res. Note RM-411. 

Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky 

Mountain Forest and Range Experiment Station. 5 p.

148. Schmid, J.M.; Frye, R.H. 1976. Stand ratings for spruce beetles. Res. 

Note RM-309. Fort Collins, CO: U.S. Department of Agriculture, Forest 

Service, Rocky Mountain Forest and Range Experiment Station. 4 p.

149. Schmid, J.M.; Frye, R.H. 1977. Spruce beetle in the Rockies. Gen. Tech. 

Rep. RM-49. Fort Collins, CO: U.S. Department of Agriculture, Forest 

Service, Rocky Mountain Forest and Range Experiment Station. 38 p. 

150. Schmitz, R.F.; Gibson, K.E. 1996. Douglas-fir beetle. Forest Insect and 

Disease Leaflet 5. Washington, DC: U.S. Department of Agriculture, Forest 

Service. 8 p.

151. Schoettle, A.W.; Sniezko, R.A. 2007. Proactive intervention to sustain high elevation pine ecosystems threatened by white pine blister rust. 

Journal of Forest Research 12(5):327-336. Online: http://www.springerlink.

com/content/9v91t44278w74430/fulltext.pdf .

152. Schwandt, J.W. 2006. Whitebark pine in peril: a case for restoration. 

Report R1-06-28. Missoula, MT: U.S. Department of Agriculture, Forest 

Service, Northern Region, Forest Health Protection. 20 p. Online: http://

www.fs.fed.us/r1-r4/spf/fhp/whitebark_pine/WBPCover_4.htm.

153. Sever, A.L. 2005. Shade tree insect pest management studies: I. Evaluation 

of non-target effects of pesticides on arthropods: II. Biology of two emergent insect pests of oak. Fort Collins, CO: Colorado State University. 57 p. 

Thesis.

154. Shaw, C.G., II; Kile, G.A., eds. 1991. Armillaria root disease. Agricultural 

Handbook No. 691. Washington, DC: U.S. Department of Agriculture, 

Forest Service. 233 p.

155. Shope, P.F. 1943. Some ascomycetous foliage diseases of Colorado conifers. Boulder, CO: University of Colorado Student Services 2:31-43.

156. Shour, M. 2007. Shucking oystershell scale. Iowa State University 

Extension News. Ames, IA: Iowa State University 2 p.
308 USDA Forest Service RMRS-GTR-241. 2010.

157. Simms, H.R. 1967. On the ecology of Herpotrichia nigra. Mycologia 

59:902-909.

158. Sinclair, W.A.; Lyon, H.H. 2005. Diseases of trees and shrubs. 2nd ed. 

Ithaca, NY: Cornell University Press. 659 p.

159. Sinclair, W.A.; Lyon, H.H.; Johnson, W.T. 1987. Diseases of trees and 

shrubs. Ithaca, NY: Cornell University Press. 574 p.

160. Smith, R.H. 1971. Red turpentine beetle. Forest Insect and Disease Leaflet 

55. Washington, DC: U.S. Department of Agriculture, Forest Service. 8 p.

161. Solomon, J.D. 1995. Guide to insect borers in North American broadleaf trees and shrubs. Agricultural Handbook 706. Washington, DC: U.S. 

Department of Agriculture, Forest Service. 747 p. Online: http://www.forestpests.org/borers.

162. Spielman, L.J. 1985. A monograph of Valsa on hardwoods of North 

America. Canadian Journal of Botany 63:1355-1378.

163. Stack, R.W.; Conway, K.E. 1986. Gloeosporium and Gnomonia leaf 

diseases of broadleaf trees. In: Riffle, J.W.; Peterson, G.W., tech. coords. 

Diseases of trees in the Great Plains. Gen. Tech. Rep. RM-129. Fort Collins, 

CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain 

Forest and Range Experiment Station. 149 p.

164. Staley, J.M. 1964. A survey of coniferous foliage diseases (other than 

rusts) in Colorado. Plant Disease Reporter 48(7):562-563.

165. Staley, J.M. 1976. Notes and a key, by host species, to the common foliar 

fungi and pathogens of conifers in the northern, central and southern Rocky 

Mountains, Great Basin, and Arizona and New Mexico. Unpublished document on file at U.S. Department of Agriculture, Forest Service, Rocky 

Mountain Research Station, Fort Collins, CO.

166. Staley, J.M. 1979. Penetration and latency in needlecast fungi. In: 

Proceedings of the 26th Annual meeting of the Western International Forest 

Disease Work Conference: 136-140.

167. Stevens, R.E. 1971. Ponderosa pine tip moth. Forest Insect and Disease 

Leaflet 103. Washington, DC: U.S. Department of Agriculture, Forest 

Service. 5 p.

168. Stewart, J.L. 1965. Fomes annosus found in Nebraska. Plant Disease 

Reporter 49(5):456.

169. Stokland, J.; Kauserud, H. 2004. Phellinus nigrolimitatus—A wooddecomposing fungus highly influenced by forestry. Forest Ecology and 

Management 187:333-343.

170. Sullivan, K. 2003. Decay of living conifers in spruce-fir forests of the 

central Rocky Mountains. LSC-03-04. Golden, CO: U.S. Department 

of Agriculture, Forest Service, Rocky Mountain Region, Renewable 

Resources. 19 p. Online: http://www.fs.fed.us/r2/fhm/bugcrud/decays_lsc03-04.pdf.

171. Taylor, J.E.; Mathiasen, R.L. 1999. Limber pine dwarf mistletoe. Forest 

Insect and Disease Leaflet 171. Washington, DC: U.S. Department of 

Agriculture, Forest Service. 7 p.
USDA Forest Service RMRS-GTR-241. 2010. 309

172. Tisserat, N.A. 2001. Juniper Diseases. Pub. No. C-711. Kansas State 

University Research and Extension. 12 p.

173. True, R.P.; Barnett, H.L.; Dorsey, C.K.; Leach, J.G. 1960. Oak wilt in 

West Virginia. Bull. 448T. Morgantown, WV: West Virginia University 

Agricultural Experiment Station. 119 p.

174. U.S. Department of Agriculture, Forest Service. 1985. Insects of eastern 

forests. Misc. Publ. 1426. Washington, DC: U.S. Department of Agriculture, 

Forest Service. 608 p.

175. U.S. Department of Agriculture, Forest Service. 2003. Forest insect and 

disease identification and management. U.S. Department of Agriculture, 

Forest Service, Northern Region; Idaho Department of Lands, Insect and 

Disease Control; Montana Department of State Lands, Division of Forestry. 

223 p.

176. U.S. Department of Agriculture, Forest Service. January 2000. Ips bark 

beetle management guide. [Online]. U.S. Department of Agriculture, Forest 

Service, Rocky Mountain Region. Online: http://www.fs.fed.us/r2/fhm/

bugcrud/silvips.htm.

177. van der Kamp, B. 1994. Lodgepole pine stem diseases and management 

of stand density in the British Columbia interior. The Forestry Chronicle 

70(6):773-779.

178. Wagener, W.W. 1959. The effect of a western needle fungus (Hypodermella 

medusa Dearn.) on pines and its significance in forest management. Journal 

of Forestry 57:561-564.

179. Wall, R.E. 1971. Variation in decay in aspen stands as affected by their 

clonal growth pattern. Canadian Journal of Forest Research 1:141-146.

180. Walla, J.A. 1984. Incidence of Phellinus punctatus on living woody 

plants in North Dakota. Plant Disease 68:252-253.

181. Walters, J.W.; Hinds, T.E.; Johnson, D.W.; Beatty, J.S. 1982. Effects of 

partial cutting on diseases, mortality and regeneration of Rocky Mountain 

aspen stands. Res. Pap. RM-240. Fort Collins, CO: U.S. Department of 

Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment 

Station. 12 p.

182. Whitney, R.; Fleming, R.; Zhou, K.; Mossa, D. 2002. Relationship of 

root rot to black spruce windfall and mortality following strip clear-cutting. 

Canadian Journal of Forest Research 32(2):283-294.

183. Whitney, R.D. 1995. Root-rotting fungi in white spruce, black spruce, 

and balsam fir in northern Ontario. Canadian Journal of Forest Research 

25(8):1209-1230.

184. Whitney, R.D.; Denyer, W.B.G. 1968. Rates of decay by Coniophora 

puteana and Polyporus tomentosus in living and dying white spruce. Forest 

Science 14(2):122-126.

185. Wickman, B.E.; Mason, R.R; Trestle, G.C. 1981. Douglas-fir tussock moth. Forest Insect and Disease Leaflet 86. Washington, DC: U.S. 

Department of Agriculture, Forest Service. 10 p.
310 USDA Forest Service RMRS-GTR-241. 2010.

186. Wilson, L.F. 1972. Life history and outbreaks of an oak leafroller, 

Archips semiferanus (Lepidoptera: Tortricidae), in Michigan. Great Lakes 

Entomologist 55:71-77.

187. Wingfield, M.J.; Blanchette, R.A.; Nicholls, T.H. 1984. Is the pine wood 

nematode an important pathogen in the United States? Journal of Forestry 

82:232-235.

188. Witcosky, J.J.; Schowalter, T.D.; Hansen, E.M. 1986. Hylastes nigrinus (Coleoptera: Scolytidae), Pissodes fasciatus, and Steremnius carinatus (Coleoptera: Curculionidae) as vectors of black stain root disease of 

Douglas-fir. Environmental Entomology (15):1090-1095.

189. Wollerman, E.H. 1971. Bagworm. Forest Insect and Disease Leaflet 97. 

Washington, DC: U.S. Department of Agriculture, Forest Service. 7 p.

190. Wood, S.L. 1982. The bark and ambrosia beetles of North and Central 

America (Coleoptera: Scolytidae), a taxonomic monograph. Great Basin 

Naturalist Memoir 6. 1359 p.

191. Worrall, J.J.; Egeland, L.; Eager, T.; Mask, R.A.; Johnson, E.W.; 

Kemp, P.A.; Shepperd, W.D. 2008. Rapid mortality of Populus tremuloides in southwestern Colorado, USA. Forest Ecology and Management 

255(3-4):686-696.

192. Worrall, J.J.; Fairweather, M. 2009. Decay and discoloration of aspen. 

Forest Insect and Disease Leaflet 149. Washington, DC: U.S. Department 

of Agriculture, Forest Service. 7 p.

193. Worrall, J.J.; Harrington, T.C.; Blodgett, J.T.; Conklin, D.A.; Fairweather, 

M.L. 2010. Heterobasidion annosum and H. parviporum in the southern 

Rocky Mountains and adjoining states. Plant Disease 94:115-118.

194. Worrall, J.J.; Nakasone, K.K. 2009. Decays of Engelmann spruce and 

subalpine fir in the Rocky Mountains. Forest Insect and Disease Leaflet 150. 

Washington, DC: U.S. Department of Agriculture, Forest Service. 12 p.

195. Worrall, J.J.; Parmeter, J.R. 1982a. Formation and properties of wetwood 

in white fir. Phytopathology 72:1209-1212.

196. Worrall, J.J.; Parmeter, J.R. 1982b. Wetwood formation as a host response in white fir. European Journal of Forest Pathology 12(6):432-441.

197. Worrall, J.J.; Parmeter, J.R. 1983. Inhibition of wood decay fungi by wetwood of white fir. Phytopathology 73:1140-1145.

198. Worrall, J.J.; Sullivan, K.F. 2002. Discoloration of ponderosa pine on 

the San Juan National Forest, 1999-2001. Biological Evaluation R2-02-06. 

Lakewood, CO: U.S. Department of Agriculture, Forest Service, Rocky 

Mountain Region, Forest Health Management. 20 p. Online: http://www.

fs.fed.us/r2/fhm/reports/be_r2-02-06.pdf.

199. Worrall, J.J.; Sullivan, K.F.; Harrington, T.C.; Steimel, J.P. 2004. 

Incidence, host relations and population structure of Armillaria ostoyae in 

Colorado campgrounds. Forest Ecology and Management 192:191-206. 

Online: http://www.fs.fed.us/r2/fhm/reports/ArmillariaCGCO.pdf .

200. Wu, Y.; Johnson, D.W.; Angwin, P.A. 1996. Identification of Armillaria

species in the Rocky Mountain Region. Tech. Rep. R2-58. Lakewood, CO: 
USDA Forest Service RMRS-GTR-241. 2010. 311

U.S. Department of Agriculture, Forest Service, Rocky Mountain Region. 

26 p.

201. Ziller, W.G. 1974. The tree rusts of western Canada. Publication No. 

1329. Victoria, BC: Canadian Forestry Service, Environment Canada. 

272 p.
312 USDA Forest Service RMRS-GTR-241. 2010.

Host-Pest Index

(not a complete pest list for each host)

Alder

StemAlder borer (238)

FoliageCottonwood leaf beetle (196, 201)

Eriophyid mites (263, 264, 267)

Fall webworm (196, 207)

Forest tent caterpillar (196, 209)

Large aspen tortrix (196, 212)

Western tent caterpillar (195, 196, 233)

Ash (green, white, black)

StemAsh bark beetles (156)

Lilac (Ash) borer (238, 253)

Carpenterworm (238, 245)

Emerald ash borer (251)

Stem decays of hardwoods (56)

Wetwood (149)

Branches and TerminalsAsh bark beetles (156)

Oystershell scale (263, 270)

FoliageAnthracnose (110)

Eriophyid mites (263, 264, 267)

Fall webworm (196, 207)

Forest tent caterpillar (196, 209)

Verticillium wilt (79, 80, 89)

Seeds and ConesAspen

RootArmillaria root disease (23, 28, 42, 192)

White mottled rot (23, 24, 42)

StemAbiotic damage (145)

Animal damage (135)

Aspen bark beetles (157)

Aspen trunk rot (21, 24, 44)

Black canker (60-63, 64)

Bronze poplar borer (238, 242)

Brown crumbly rot (23, 46)

Burls, galls, and tumors (134)
USDA Forest Service RMRS-GTR-241. 2010. 313

Cryptosphaeria canker (24, 60-62, 69)

Cytospora canker (60-63, 70, 71, 129)

Hypoxylon canker (61-63, 75)

Oystershell scale (263, 270)

Poplar borer (75, 238, 255)

Sooty bark canker (60-62, 69, 77)

Wetwood (149)

White trunk rot (24, 44)

Branches and TerminalsAnimal damage (135

)

Oystershell scale (263, 270)

Petiolegall aphids (272)

Shepherd’s crook (132)

FoliageAbiotic damage (138

)

Aspen leafminer (196, 197)

Eriophyid mites (263, 264, 267)

Forest tent caterpillar (196, 209)

Ink spot leaf blight (120)

Large aspen tortrix (196, 212)

Marssonina blight (108, 110, 121)

Melampsora rusts (92, 124)

Septoria leaf spot and canker (128)

Shepherd’s crook (132)

Western tent caterpillar (195, 196, 233)

Seeds and ConesEriophyid mites (263, 264, 267

)

Birch (paper birch)

RootArmillaria root disease (23, 28, 42, 192)

StemAbiotic damage (145

)

Animal damage (135)

Stem decays of hardwoods (56)

Chokecherry (and other Prunus)

StemStem decays of hardwoods (56

)

Branches and TerminalsBlack knot of cherry (66

)

FoliageWestern tent caterpillar (195, 196, 233

)

Eastern tent caterpillar (196)

Fall webworm (196, 207)
314 USDA Forest Service RMRS-GTR-241. 2010.

Cottonwood (Plains, narrowleaf)

RootArmillaria root disease (23, 28, 42, 192)

White mottled rot (23, 24, 42)

StemCarpenterworm (238, 245)

Cottonwood borer (236-238, 247)

Cryptosphaeria canker (24, 69)

Poplar borer (75, 238, 255)

Stem decays of hardwoods (56)

Wetwood (149)

Branches and Terminals

Oystershell scale (263, 270)

Petiolegall aphids (272)

FoliageAnimal damage (135)

Aspen leafminer (196, 197)

Cottonwood leaf beetle (196, 201)

Fall webworm (196, 207)

Forest tent caterpillar (196, 209)

Ink spot leaf blight (120)

Marssonina leaf blight (108, 110, 121)

Melampsora rusts (92, 124)

Poplar blackmine beetle (198)

Septoria leaf spot and canker (128)

Western tent caterpillar (195, 196, 233)

Currant (Ribes spp.)

FoliageWhite pine blister rust (92, 93, 105)

Douglas-fir

RootArmillaria root disease (23, 28, 35, 42, 192)

Schweinitzii root and butt rot (23, 36, 40)

StemAbiotic damage (145)

Animal damage (135)

Ambrosia beetles (236, 238, 240)

Brown crumbly rot (23, 46)

Douglas-fir beetle (42, 49, 162, 164)

Douglas-fir engraver beetles (163)

Douglas-fir pole beetle (163)

Flatheaded wood borers (236, 238, 250)

Gray-brown saprot (48)

Red ring rot (24, 38, 51)
USDA Forest Service RMRS-GTR-241. 2010. 315

Roundheaded wood borers (238, 256)

Wood wasps (237, 238, 260)

Branches and TerminalsCytospora canker (73)

Douglas-fir dwarf mistletoe (6, 10)

Douglas-fir secondary beetles (163)

FoliageAbiotic damage (138)

Cooley spruce gall adelgid (263, 265)

Douglas-fir tussock moth (168, 195, 196, 202, 231)

Giant conifer aphids (263, 269)

Melampsora leaf rust (92, 124)

Pine needle scale (263, 274)

Tiger moth (196, 225)

Western spruce budworm (168, 195, 196, 227, 231)

Seeds and ConesConifer seed bugs (263)

Cooley spruce gall adelgid (263, 265)

Elm (American, Siberian, and others)

StemAbiotic damage (145)

Botryodiplodia canker (67)

Carpenterworm (238, 245)

Elm bark beetles (81, 82, 165)

Elm borer (238, 248)

Pigeon tremex (237, 261)

Spider mites (262-264, 278)

Wetwood (149)

FoliageAnthracnose (110)

Bagworm (196, 199)

Dutch elm disease (79, 80, 81, 166, 167)

Elm leaf beetle (196, 204)

Elm leafminer (196)

Eriophyid mites (263, 264, 267)

European elm scale (263, 266)

Fall webworm (196, 207)

Forest tent caterpillar (196, 209)

Verticillium wilt (79, 80, 89)

Fir (white, subalpine, corkbark)

RootAnnosus root disease (23, 25, 35, 38)

Armillaria root disease (23, 28, 42, 192)

Coniophora root and butt rot (35)
316 USDA Forest Service RMRS-GTR-241. 2010.

StemAbiotic damage (145)

Ambrosia beetles (236, 238, 240)

Burls, galls, tumors (134)

Fir engraver beetle (26, 167)

Flatheaded wood borers (236, 238, 250)

Rust-red stringy rot (24, 53)

Roundheaded wood borers (238, 256)

Red heart rot (24, 53)

Red ring rot (24, 38, 51)

Western balsam bark beetle (190)

Wetwood (149)

Wood wasps (237, 238, 260)

Branches and TerminalsFir broom rust (92, 94)

Fir engraver beetle (26, 167)

FoliageBrown felt blight/snow mold (113)

Douglas-fir tussock moth (168, 195, 196, 202, 231)

Giant conifer aphids (263, 269)

Melampsora rusts (92, 124)

Tiger moth (196, 225)

Western spruce budworm (168, 195, 196, 227, 231)

White fir needleminer (196, 214)

Hackberry

Branches and TerminalsEriophyid mites (witches’ broom) (263, 264, 267)

Spider mites (262-264, 278)

FoliageHackberry bud gall psyllid (263)

Hackberry nipplegall maker (263)

Juniper (One-seed, Utah, Rocky Mountain, Eastern redcedar)

RootAnnosus root disease (23, 25, 38)

Armillaria root disease (23, 28, 42, 192)

StemCedar bark beetles (161)

Juniper borers (238, 252)

Juniper twig pruner (252, 280)

Branches and TerminalsCedar bark beetles (161)

Giant conifer aphids (263, 269)

Gymnosporangium rusts (92, 94, 100)
USDA Forest Service RMRS-GTR-241. 2010. 317

Juniper mistletoe (18)

Juniper twig pruner (252, 280)

FoliageBagworm (196, 199

)

Cercospora blight (114)

Giant conifer aphids (263, 269)

Tiger moth (196, 225)

Spider mites (262-264, 278)

Maples (Boxelder, Rocky Mountain)

StemCarpenterworm (238, 245

)

Pigeon tremex (237, 261)

Wetwood (149)

Branches and TerminalsOystershell scale (263, 270

)

FoliageAnthracnose (110

)

Bagworm (196, 199)

Eriophyid mites (263, 264, 267)

Boxelder leafroller (196)

Boxelder leafminer (196)

Fall webworm (196, 207)

Verticillium wilt (79, 80, 89)

Seeds and ConesBoxelder bug (262, 263)

Oak (Gamble, bur)

RootArmillaria root disease (23, 28, 42, 192)

StemAgrilus quercicola (238, 239

)

Botryodiplodia canker (67)

Oak wilt (79-81, 84)

Twolined chestnut borer (258)

Branches and TerminalsAgrilus quercicola (238, 239

)

FoliageAnthracnose (110

)

Bagworm (196, 199)

Forest tent caterpillar (196, 209)

Oak leafroller (196, 216)

Oak wilt (79-81, 84)

Sonoran tent caterpillar (196)

Western tent caterpillar (195, 196, 233)
318 USDA Forest Service RMRS-GTR-241. 2010.

Pines (Pinyon, ponderosa, lodgepole, limber, bristlecone, whitebark, 

Scots, Austrian)

RootArmillaria root disease (23, 28, 35, 42, 192)

Annosus root disease (23, 25, 38)

Black stain root disease (22, 31, 79, 81, 178)

Coniophora root and butt rot (35)

Schweinitzii root and butt rot (23, 36, 40)

StemAbiotic damage (145)

Animal damage (135)

Ambrosia beetles (236, 238, 240)

Brown crumbly rot (23, 46)

Burls, galls, and tumors (134)

Comandra blister rust (92-94, 97)

Dwarf mistletoes (6, 12, 13, 15, 16)

Flatheaded wood borers (236, 238, 250)

Gray-brown saprot (48)

Ips species (132, 169, 174, 177, 180, 181, 194)

Mountain pine beetle (31, 170, 171, 172, 183, 189, 190, 191, 228)

Limber pine engraver (169)

Lodgepole pine beetle (170)

Pine engraver beetles (132, 174, 194)

Pinyon ips (177, 180)

Pinyon twig beetle (179, 190)

Pitch moths (238, 280, 282)

Red ray rot (24, 49)

Red ring rot (24, 38, 51)

Red turpentine beetle (181, 194)

Roundheaded pine beetle (183, 194)

Roundheaded wood borers (238, 256)

Stalactiform rust (92, 97)

Western gall rust (91-93, 103, 134)

Western pine beetle (183, 184, 194)

White pine blister rust (12, 13, 91-94, 105, 169, 170)

Wood wasps (237, 238, 260)

Zimmerman pine moth (282)

Branches and TerminalsComandra blister rust (92-94, 97)

Dwarf mistletoes (6, 12, 13, 15, 16)

Eriophyid mites (263, 264, 267)

Giant conifer aphids (263, 269)

Ips species (132, 169, 174, 177, 180, 181, 194)

Limber pine dwarf mistletoe (12)

Limber pine engraver (169)
USDA Forest Service RMRS-GTR-241. 2010. 319

Lodgepole pine dwarf mistletoe (8, 13)

Lodgepole terminal weevil (285)

Pine engraver beetles (132, 174, 194)

Pine shoot blight (129, 148)

Pine tip moths (280, 281)

Pinyon dwarf mistletoe (15)

Pinyon ips (177)

Pinyon spindle gall midge (263, 264)

Pinyon twig beetle (179)

Pitch moths (238, 280, 282)

Pitch nodule moths (280, 283)

Southwestern dwarf mistletoe (6-9, 14, 16, 263)

Twig beetles (105, 179, 188, 280)

Western gall rust (91-93, 103, 134)

White pine blister rust (12, 13, 91-94, 105, 169, 170)

Zimmerman pine moth (282)

FoliageAbiotic damage (138)

Bagworm (196, 199)

Brown felt blight/snow mold (113)

Brown spot needle blight (125)

Davisomycella needle cast (108, 109, 115)

Elytroderma needle cast (109, 118)

Lophodermella needle cast (109, 115)

Melampsora rusts (92, 124)

Needle casts (107, 109, 110, 115, 118, 126)

Needleminers (196, 214)

Pandora moth (196, 218)

Pine butterfly (196, 220)

Pine needle scale (263, 274)

Pine sawflies (196, 222)

Pine wilt disease (79, 81, 87)

Pinyon needle scale (263, 276)

Red band needle blight (108, 125)

Tiger moth (196, 225)

Western pine budworm (196, 227)

Western pine tussock moth (196, 229)

Serviceberry

FoliageGymnosporangium rust (92, 94, 100)

Spruce (Blue, Engelmann, white)

RootArmillaria root disease (23, 28, 35, 42, 192)

Coniophora root and butt rot (35)
320 USDA Forest Service RMRS-GTR-241. 2010.

Root diseases with white pocket rots (21, 23, 38)

Schweinitzii root and butt rot (23, 36, 40)

StemAbiotic damage (145)

Animal damage (135)

Ambrosia beetles (236, 238, 240)

Blue spruce engraver (159, 188)

Brown crumbly rot (23, 46)

Brown felt blight/snow mold (113)

Burls, galls, and tumors (134)

Cytospora canker (73)

Flatheaded wood borers (236, 238, 250)

Gray-brown saprot (48)

Redheart rot (24, 53)

Red ring rot (24, 38, 51)

Roundheaded wood borers (238, 256)

Spruce engraver beetles (187)

Spruce beetle (159, 171, 172, 184, 188, 191)

Wood wasps (237, 238, 260)

Branches and TerminalsCooley spruce gall adelgid (263, 265)

Blue spruce engraver (159, 188)

Broom rust (92, 93, 94)

Cytospora canker (73)

Giant conifer aphids (263, 269)

Spruce engraver beetles (187)

Twig beetles (188)

White pine weevil (285)

FoliageAbiotic damage (138)

Bagworm (196, 199)

Douglas-fir tussock moth (168, 195, 196, 202, 231)

Pine needle scale (263, 274)

Spider mites (262-264, 278)

Broom rust (92, 93, 94)

Needleminers (196, 214)

Western spruce budworm (168, 195, 196, 227, 231)

Willows (multiple species)

StemCottonwood borer (236-238, 247)

Cytospora canker (60, 61, 63, 70, 71, 129)

Stem decays of hardwoods (56)
USDA Forest Service RMRS-GTR-241. 2010. 321

Branches and TerminalsCytospora canker (60, 61, 63, 70, 71, 129)

Oystershell scale (263, 270)

FoliageBagworm (196, 199)

Cottonwood leaf beetle (196, 201)

Eastern tent caterpillar (196)

Fall webworm (196, 207)

Forest tent caterpillar (196, 209)

Melampsora rusts (92, 124)

Western tent caterpillar (195, 196, 233)
322 USDA Forest Service RMRS-GTR-241. 2010.

General Index

A

Abiotic foliage damage (138)

Abiotic stem damage (145)

Aceria macrorrhyncha (267)

Adelges cooleyi (263, 265)

Agrilus anxius (259)

Agrilus bilineatus (258)

Agrilus liragus (238, 242)

Agrilus quercicola (238, 239)

Agrilus planipennis (251)

Alder borer (238)

Ambrosia beetles (236, 238, 240)

Amylostereum chaillettii (24)

Animal damage (135)

Annosus root disease (23, 25, 38)

Anthracnose (110)

Antler rubbing/scraping (136)

Antrodia serialis (24)

Aphrophora spp. (263)

Apiognomonia errabunda (111, 112)

Apiognomonia quercina (110, 111, 112)

Apiognomonia tiliae (111)

Apiognomonia veneta (111)

Apiosporina morbosa (66)

Aphid (135, 262-265, 269, 270, 272-274, 280)

Arceuthobium americanum (6, 13)

Arceuthobium cyanocarpum (12)

Arceuthobium divaricatum (15)

Arceuthobium douglasii (10)

Arceuthobium spp. (6, 10, 12, 13, 15, 16)

Arceuthobium vaginatum subsp. cryptopodum (16)

Archips semiferana (196, 216)

Armillaria ostoyae (21, 28)

Armillaria solidipes (23, 28)

Armillaria root disease (23, 28, 42, 192)

Artist’s conk (23, 42)

Ash bark beetles (156)

Ash borer (238, 253)

Asian longhorned beetle (256)

Aspen bark beetles (157)

Aspen leafminer (196, 197)

Aspen shoot blight (132)

Aspen trunk rot (21, 24, 44)
USDA Forest Service RMRS-GTR-241. 2010. 323

Atimia huachucae (252, 253)

B

Bagworm (196, 199)

Bark beetle-associated mites (262)

Bastard toadflax (92, 97, 99)

Bear (136, 137)

Bearberry (92, 94)

Bear wipe (113)

Bifusella linearis (109)

Bifusella pini (109)

Bifusella saccata (109)

Black canker (60, 61, 63, 64)

Black knot (66)

Black pine needle scale (263)

Black stain root disease (22, 31, 79, 81, 178)

Black-horned juniper borer (252, 253)

Bleeding Stereum (24, 53, 54)

Blue spruce engraver (159, 188)

Botryodiplodia canker (67)

Botryodiplodia hypodermia (67)

Botryodiplodia theobromae (67)

Botryosphaeria spp. (67)

Boxelder bug (262, 263)

Boxelder leafminer (196)

Boxelder leafroller (196)

Bronze poplar borer (238, 242)

Brown crumbly rot (23, 46)

Brown felt blight (113)

Brown spot needle blight (125)

Browse/browsing (136, 138, 235)

Buprestidae (194, 237-239, 242, 250, 258)

Burls, galls and tumors (134)

Bursaphelenchus xylophilus (79, 87)

C

Callidium texanum (252, 253)

Camponotus spp. (244)

Carpenter ants (244)

Carpenterworm (238, 245)

Castilleja spp. (98, 105)

Cecidomyia spp. (263)

Cedar bark beetles (161)

Cenangium canker (77)

Cerambycidae (183, 194, 238, 247, 248, 252, 253, 255, 256)

Cerastium spp. (94)
324 USDA Forest Service RMRS-GTR-241. 2010.

Ceratocystis canker (64)

Ceratocystis fagacearum (79, 84)

Ceratocystis fimbriata (64)

Ceratocystis populicola (64)

Ceratocystis ulmi (79)

Cercospora blight (114)

Chalcophora virginiensis (250)

Chemical damage (126, 142, 143)

Chionaspis pinifoliae (263, 274)

Choristoneura conflictana (196, 212)

Choristoneura lambertiana (196, 227)

Choristoneura occidentalis (196, 227, 231)

Chrysomela scripta (196, 201)

Chrysomyxa arctostaphyli (92, 94, 96)

Ciborinia whetzelii (120)

Cinara spp. (263, 269)

Cinara pilicornis (269)

Cinara pulverulens (269)

Circinatus root rot (23, 38)

Clear-wing moth (237)

Coleotechnites edulicola (214)

Coleotechnites piceaella (214, 215)

Coleotechnites ponderosae (214)

Coleotechnites sp. (196)

Colomerus vitis (268)

Coloradia doris (218)

Coloradia pandora (196, 218)

Comandra blister rust (92-94, 97)

Comandra livida (98)

Comandra umbellata (97)

Conifer-aspen rust (124)

Conifer-cottonwood rust (124)

Conifer seed bugs (263, 264)

Coniophora puteana (23, 35, 37)

Cooley spruce gall adelgid (263, 265)

Cottonwood borer (236-238, 247)

Cottonwood leaf beetle (196, 201)

Cronartium coleosporioides (92, 97)

Cronartium comandrae (92, 97)

Cronartium ribicola (92, 105)

Cryptoporus (Polyporus) volvatus (48)

Cryptosphaeria canker (24, 60-62, 69)

Cryptosphaeria lignyota (populina) (24, 69, 70)

Cyclaneusma niveum (109)

Cytospora (Leucocytospora) kunzei (73)
USDA Forest Service RMRS-GTR-241. 2010. 325

Cytospora canker (60, 61, 63, 70, 71, 73, 129)

Cytospora chrysosperma (70, 71, 73, 129)

D

Dasychira grisefacta (196, 229)

Dasychira pinicola (229)

Dasychira plagiata (229)

Davisomycella medusa (109, 115-117)

Davisomycella needle cast (108, 116)

Davisomycella ponderosae (109, 116, 117)

Deer (135, 136)

Dendroctonus adjunctus (183, 194)

Dendroctonus brevicomis (183, 193, 194)

Dendroctonus murrayanae (170)

Dendroctonus ponderosae (170-172, 183)

Dendroctonus pseudotsugae (162)

Dendroctonus rufipennis (159, 171, 184, 188)

Dendroctonus valens (181, 194)

Dichomitus squalens (Polyporus anceps) (24, 49-51)

Diffuse canker (60, 62)

Dioryctria ponderosae (282, 283)

Dioryctria zimmermani (282)

Diplodia blight (129, 148)

Diplodia pinea (129, 131)

Diplodia scrobiculata (129)

Discula spp. (111)

Discula quercina (111)

Dothistroma needle blight (125)

Dothistroma pini (109, 125)

Dothistroma septosporum (109, 125)

Douglas-fir beetle (42, 49, 162, 164)

Douglas-fir dwarf mistletoe (6, 10)

Douglas-fir pole and engraver beetles (163)

Douglas-fir tussock moth (168, 195, 196, 202, 231)

Drepanopeziza populorum (122)

Drepanopeziza tremulae (121)

Dryocoetes confusus (190)

Dutch elm disease (79, 80, 81, 166, 167)

E

Echinodontium tinctorium (24, 53)

Elk (135, 136, 242)

Elm bark beetle (81, 165)

Elm borer (238, 248)

Elm leaf beetle (196, 204)

Elytroderma deformans (109, 118)
326 USDA Forest Service RMRS-GTR-241. 2010.

Elytroderma needle cast (109, 118)

Emerald ash borer (251, 252)

Encoelia pruinosa (Cenangium singulare) (77, 78)

Endothenia albolineana (214-216)

Engraver beetles (132, 163, 174, 187, 194)

Entoleuca (Hypoxylon) mammatum (75)

Epinotia meritana (214)

Eriophyes calcercis (268)

Eriophyes celtis (268)

Eriophyes parapopuli (268)

Eriophyid mites (263, 264, 267)

European elm scale (263, 266)

F

Fall webworm (196, 207)

Fir broom rust (92, 94)

Fire damage (145, 149, 163, 251, 257, 261)

Fir engraver (26, 167)

Fir-willow rust (124)

Flammulina populicola (23)

Flatheaded borers (Buprestids) (236, 238, 239, 242, 250, 252)

Fomitiporia (Phellinus) punctata (56-59)

Fomitiporia hartigii (24) 

Fomitopsis (Fomes) pinicola (23, 36, 46-48)

Forest tent caterpillar (196, 209)

Frost cracks (145, 147, 149)

Frost/freeze injury (141, 142, 145-147, 149)

Fusicladium radiosum var. lethiferum (Pollacia radiosum) (132)

G

Gall mites (267)

Ganoderma applanatum (23, 42, 43)

Giant conifer aphid (263, 269)

Gloeosporium ulmicolum (111)

Gnathotrichus (240)

Gnomonia leptostyla (111)

Gossyparia (Eriococcus) spuria (263, 266)

Gray-brown saprot (48)

Gymnosporangium spp. (92, 100, 102)

Gymnosporangium juniperi-virginianae (100)

Gymnosporangium rusts (92, 94, 100)

H

Hackberry nipplegall maker (263)

Hail (74, 130-132, 145, 148, 149)

Halisidota ingens (225)

Heat injury (145, 147, 149)
USDA Forest Service RMRS-GTR-241. 2010. 327

Hemiphacidium planum (109)

Herbicide damage (143, 144)

Herpotrichia juniperi (Herpotrichia nigra) (109, 113)

Heterobasidion annosum (23, 25-27)

Heterobasidion parviporum (23, 25-27)

Hip canker (103, 104)

Horntails (260)

Hylesinus spp. (156)

Hylurgopinus rufipes (81,165)

Hyphantria cunea (196, 207)

Hypoxylon canker (61-63, 75)

I

Indian paint fungus (24, 53-55)

Ink spot (107, 120)

Ips spp. (132, 169, 174, 177, 180, 181, 194)

Ips borealis (187)

Ips confusus (177, 180)

Ips hunteri (159, 160, 187, 188) 

Ips pilifrons (187)

Ips pini (175, 176, 194)

Ips tridens (187)

Ips woodi (169)

Isthmiella (Bifusella) abietis (109)

Isthmiella (Bifusella) crepidiformis (109)

J

Juniper bark beetles (161)

Juniper borers (238, 252)

Juniper mistletoe (18)

Juniper twig pruner (252, 280)

K

Kabatiella apocrypta (111)

L

Large aspen tortrix (196, 212)

Laurilia sulcata (24)

Leaf blight (110, 120-123)

Leaf spot (96, 98, 102, 105, 122-124, 128)

Lecanosticta (Coryneum) cinerea (109)

Lentinellus montanus (23)

Leopard spotting (78)

Lepidosaphes ulmi (263, 270)

Leptographium wageneri (31, 79)

Lettuce root aphid (273)

Leucocytospora nivea (71, 73)
328 USDA Forest Service RMRS-GTR-241. 2010.

Leucostoma niveum (71)

Libertella sp. (69)

Lightning (145-147, 194)

Lilac (ash) borer (238, 253)

Limber pine dwarf mistletoe (12)

Limber pine engraver (169)

Lirula abietis-concoloris (109)

Lirula macrospora (Lophodermium filiforme) (109)

Lodgepole pine beetle (170)

Lodgepole pine dwarf mistletoe (8, 13)

Lodgepole terminal weevil (285)

Longhorned borers (238, 256)

Lophocampa spp. (196) 

Lophocampa ingens (196, 225)

Lophodermella (Hypodermella) arcuata (109, 115, 117)

Lophodermella cerina (109, 115, 117)

Lophodermella concolor (109, 115-117)

Lophodermella montivaga (109, 115-118)

Lophodermella needle cast (115)

Lophodermium decorum (109)

Lophodermium juniperinum (109)

Lophodermium nitens (109)

Lophodermium piceae (109)

Lophomerum (Lophodermium) autumnale (109)

M

Malacosoma californicum (196, 233)

Malacosoma disstria (196, 209)

Marssoniella juglandis (111)

Marssonina brunnea (121-123)

Marssonina leaf blight (108, 110, 121)

Marssonina populi (122)

Matsucoccus acalyptus (263, 276)

Mechanical damage (136, 145, 147-149, 157, 178, 182, 194, 250)

Melampsora epitea (124)

Melampsora medusae (Melampsora albertensis) (92, 124)

Melampsora occidentalis (124)

Melampsora rust (92, 124)

Melampsorella caryophyllacearum (92, 94)

Melanophila spp. (251)

Mites (262-264, 267, 268, 270, 278, 279, 280)

Monochamus spp. (87, 256)

Monochamus clamator (256)

Mountain pine beetle (31, 170, 171, 172, 183, 189, 190, 191, 228)

Mycosphaerella acicola (125)
USDA Forest Service RMRS-GTR-241. 2010. 329

Mycosphaerella pini (109, 125)

Mycosphaerella populicola (128, 129)

Mycosphaerella populorum (128, 129)

N

Nantucket pine tip moth (281)

Needle cast (107-110, 115-118, 120, 126, 265)

Needleminers (196, 214)

Nematode (79, 87-89, 256)

Neodiprion autumnalis (223, 224)

Neodiprion gillettei (223)

Neodiprion spp. (196, 222, 223)

Neopeckia coulteri (109, 113)

Neophasia menapia (196, 220)

Nuclaspis californica (263)

O

Oak bark beetle (84, 86)

Oak leafroller (196, 216)

Oak wilt (79-81, 84)

Oligonychus ununguis (279)

Onnia leporina (23, 38)

Onnia tomentosa (23, 37-39)

Ophiostoma spp. (81, 82)

Ophiostoma novo-ulmi (81, 83, 167)

Orgyia pseudotsugata (196, 202)

Oystershell scale (263, 270)

P

Pachypsylla celtidismamma (263)

Pandora moth (196, 218)

Paraorgyia grisefacta (229)

Pedicularis spp. (105)

Pemphigus spp. (263, 272-274)

Pemphigus bursarius (273)

Pemphigus populitransversus (272)

Pemphigus populivenae (273)

Peniophora polygonia (24, 45)

Perenniporia fraxinophila (56-59)

Peridermium harknessii (91, 92, 103)

Petiolegall aphids (272)

Phaenops gentilis (250)

Phaeocryptopus gaeumannii (109)

Phaeocryptopus nudus (Adelopus balsamicola) (109)

Phaeolus schweinitzii (23, 36, 40-42)

Phellinidium ferrugineofuscum (24)

Phellinus igniarius (56, 57, 59)
330 USDA Forest Service RMRS-GTR-241. 2010.

Phellinus tremulae (24, 44-46, 51)

Phellopilus nigrolimitatus (23, 38, 40, 53)

Phlebiopsis gigantea (28)

Phloeosinus spp. (161)

Pholiota alnicola (23)

Pholiota squarrosa (scaly Pholiota) (23)

Phoradendron juniperinum(18)

Phyllocnistis populiella (196, 197)

Pigeon tremex (237, 261)

Pine butterfly (196, 220)

Pine needle scale (263, 274)

Pine sawflies (195, 196, 222)

Pine sawyer (87-89, 256) 

Pine shoot blight (129, 148)

Pine tip moth (280, 281)

Pine wilt disease (79, 81, 87, 257)

Pine wood nematode (87)

Pinyon dwarf mistletoe (15)

Pinyon Ips (177)

Pinyon needleminer (214, 215)

Pinyon needle scale (263, 276)

Pinyon pitch mass borer (280, 282, 283)

Pinyon pitch nodule moth (283, 284)

Pinyon spindle gall midge (263, 264)

Pinyon twig beetle (179, 190)

Pinyonia edulicola (263)

Pissodes strobi (285)

Pissodes terminalis (285)

Pitch midge (263)

Pitch moths (238, 280, 282)

Pitch nodule moths (280, 283) 

Pityogenes spp. (188)

Pityophthorus spp. (179, 188)

Platypus spp. (240, 241)

Platypus wilsoni (241)

Plectrodera scalator (238, 247)

Pleurotus populinus (23)

Podosesia syringae (238, 253)

Ponderosa pine needleminer (214, 215)

Ponderosa pitch nodule moth (283)

Poplar blackmine beetle (198)

Poplar borer (75, 238, 255)

Poplar petiolegall aphid (272, 273)

Porcupine (9, 100, 135-137)

Porodaedalea (Phellinus) pini (24, 38, 51-54, 96)
USDA Forest Service RMRS-GTR-241. 2010. 331

Pouch fungus (48, 49)

Powdery juniper aphid (269)

Prionoxystus robiniae (238, 245)

Procryphalus mucronatus (157-159)

Pseudocercospora juniper (Cercospora sequoia var. juniperi) (114)

Pseudohylesinus nebulosus (163, 164)

Pseudopityophthorus spp. (84, 86)

Punk knot (24, 52-54)

Pyrrhalta luteola (204)

R

Red band needle blight (108, 125) 

Red belt (weather damage) (140, 141)

Red-belt fungus (36, 46)

Red-brown cubical root and butt rot (Schweinitzii) (40)

Red heart rot (24, 53)

Red ray rot (24, 49)

Red ring rot (24, 38, 51)

Red root rot (23, 38-40)

Red turpentine beetle (181, 194)

Resin midge (263)

Retinia (Petrova) arizonensis (283)

Retinia (Petrova) metallica (283)

Rhabdocline pseudotsugae (109)

Rhizosphaera kalkhoffii (109)

Rhyacionia spp. (281)

Rhyacionia bushnelli (281)

Rhyacionia frustrana (281)

Rhyacionia neomexicana (281)

Ribes-willow rust (124)

Roundheaded borers (87, 238, 250, 252, 253, 255, 256)

Roundheaded pine beetle (183, 194)

Rust-red stringy rot (24, 53)

Rusty knot (54, 56)

S

Salt (142-144)

Saperda calcarata (238, 242, 255)

Saperda obliqua (238)

Saperda tridentate (238, 248)

Saprot (46, 48, 49)

Sapsucker (46, 136, 137)

Sarcophaga aldrichi (211)

Schweinitzii root and butt rot (23, 36, 40)

Scirrhia (Systremma) acicola (125, 126)

Scirrhia pini (125)
332 USDA Forest Service RMRS-GTR-241. 2010.

Scolytus monticolae (164)

Scolytus multistriatus (81, 165)

Scolytus schevyrewi (81, 165)

Scolytus ventralis (26, 167)

Septoria leaf spot and canker (128)

Septoria musiva (128)

Septoria populicola (128)

Shepherd’s crook (132, 280)

Silver spotted tiger moth (225)

Sirex juvencus (260)

Sirex noctilio (261)

Siricidae (237, 238, 260)

Sistotrema raduloides (23)

Snake canker (69)

Snow mold (113)

Sooty-bark canker (60-62, 69, 77)

Southwestern dwarf mistletoe (6-9, 14, 16, 263)

Southwestern pine tip moth (281)

Sphaeropsis shoot blight (129)

Sphaeropsis sapinea (129)

Spider mites (262-264, 278)

Spittlebugs (263, 264)

Spruce beetle (159, 171, 172, 184, 188, 191)

Spruce broom rust (92, 93, 94)

Spruce engraver beetles (187)

Spruce needleminers (214, 215)

Spruce shoot aphid (269)

Spruce spider mite (279)

Stalactiform rust (92, 97)

Stegophora ulmea (111)

Stellaria spp. (94)

Stem decays of hardwoods in the plains (56)

Stereum (Haematostereum) sanguinolentum (24, 53)

Striped ambrosia beetle (236, 240)

Stroma/stromata (61, 62, 74-76, 122)

Styloxus bicolor (252, 253)

Sugar pine tortrix (228)

Sugarbeet root aphid (273)

Sunscald (46, 145-147, 149)

T

Target canker (60, 62, 64, 72, 103)

Termite (245)

Thyridopteryx ephemeraeformis (196, 199)

Tiger Moth (196, 225)
USDA Forest Service RMRS-GTR-241. 2010. 333

Trypodendron spp. (240)

Trypodendron lineatum (240)

Trypophloeus populi (157-159)

Tumors (134)

Twig beetles (105, 179-181, 188, 280)

Twolined chestnut borer (258)

U

Urocerus gigas (260)

V

Valsa (Leucostoma) kunzei (73)

Valsa sordida (71, 72)

Veluticeps abietina/fimbriata (24)

Venturia spp. (132)

Venturia moreletii (132)

Venturia populina (132)

Venturia tremulae var. grandidentatae (132)

Verticillium albo-atrum (79, 89)

Verticillium dalhiae (79, 89)

Verticillium wilt (79, 80, 89)

Vesiculomyces citrinus (23)

Virgella (Hypoderma) robusta (109)

Vole (137)

W

Western balsam bark beetle (190)

Western gall rust (91-93, 103, 134)

Western pine beetle (183, 184, 193)

Western pine budworm (196, 227)

Western pine tip moth (281)

Western pine tussock moth (196, 229)

Western spruce budworm (168, 195, 196, 227, 231)

Western tent caterpillar (195, 196, 233)

Wetwood (149)

White fir needleminer (214-216)

White mottled rot (23, 24, 42)

White pine blister rust (12, 13, 91-94, 105, 169, 170)

White pine weevil (285)

White trunk rot (24, 44, 45)

White-headed ambrosia beetle (241)

Willow rust (124)

Wind damage (148, 237)

Winter injury/winter desiccation (72, 126, 139, 140)

Woodpecker (45, 46, 173, 186, 244, 247, 256, 260)

Wood wasps (237, 238, 260)
334 USDA Forest Service RMRS-GTR-241. 2010.

X

Xanthogaleruca luteola (196, 204)

Xyleborus spp. (240)

Z

Zadiprion rohweri (223, 224)

Zadiprion spp. (196, 222, 223)

Zadiprion townsendi (224)

Zeugophora scutellaris (198)

Zimmerman pine moth (282, 283)
USDA Forest Service RMRS-GTR-241. 2010. 335

Instructions for Submitting 

Insect and Disease Specimens 

for Identification

Proper collection and shipping of specimens often make the difference between identifiable specimens and those no longer identifiable when they reach 

the specialists. It is important to follow the procedures listed below to ensure 

accurate and timely identification.

Collection

● Collect adequate material. Failure to identify a specimen is often a 

result of insufficient material to indicate the cause of damage. 

● Provide as much information as possible.

○ Describe appearance of affected tree—include species, age, 

and setting (most important item should be listed first).

○ Who submitted the specimen.

○ Date: when the species was collected.

○ Location: legal description or the county, city, road, and address if available.

Any information you think might be related to the problem, such as the 

number of trees affected, any human activity that may be related, weather, or 

other environmental conditions. Give your opinion concerning the identity of 

the problem.

Shipment

● Plant materials such as wood, bark, foliage, roots, or conks should be 

wrapped in paper bags or newspapers and mailed in a box without 

any plastic. Plastic wrappings induce mold.

● Insects, except adult moths and butterflies, should be sent in leakproof vials or bottles with 70 percent isopropyl (rubbing) alcohol. 

Place the vial or bottle in a box for mailing.

● Moths and butterflies can be killed by putting them in a jar in an 

oven at 140 °F for 10 minutes or by putting them in a freezer for 

15 minutes. Place the specimen between folds of a paper to keep the 

wings flat and pack in a box for mailing.

● Mail specimens as soon as possible to prevent drying of foliage or 

insects or deterioration of conks.
336 USDA Forest Service RMRS-GTR-241. 2010.

Technical Assistance Sources

Regional Office Staff

USDA Forest Service

Forest Health Management

740 Simms St.

Golden, CO 80401

(303) 275-5061 

Rapid City Service Center

USDA Forest Service

Rapid City Service Center

8221 S. Highway 16

Rapid City, SD 57702

(605) 716-2781

Lakewood Service Center

USDA Forest Service 

Lakewood Service Center

740 Simms St.

Golden, CO 80401

Phone: (303) 236-9541 

Fax: (303) 236-9542

Gunnison Service Center

USDA Forest Service

Gunnison Service Center

216 N. Colorado St.

Gunnison CO 81230 

(970) 642-1133

In Colorado:

Colorado State Forest Service 

Colorado State University 

5060 Campus Delivery

Fort Collins, CO 80523-5060

Phone: (970) 491-7282

Fax: (970) 491-7737 

In Kansas:

Kansas Forest Service 

2610 Claflin Road

Manhatten, KS 66502

Phone: (785) 532-3309 

Fax: (785) 532-3305

In Nebraska:

Nebraska Forest Service

University of Nebraska–Lincoln

PO Box 830815

Lincoln, NE 68501

Phone: (402) 472-6635 

Fax: (402) 472-2964 

In South Dakota:

South Dakota Dept of Agriculture

Resource Conservation and Forestry 

Division

3305 1⁄2 West South Street

Rapid City, SD 57702

Phone: (605) 394-2663

Fax: (605) 394-2549

In Wyoming:

Wyoming State Forestry Division 

1100 West 22nd St. 

Cheyenne, WY 82002 

Phone: (307) 777-5495

Fax: (307) 777-5986

State and Private Lands:

Federal Lands:

Federal Recycling Program Printed on Recycled Paper

Rocky

 Mountain

 Research Station

The Rocky Mountain Research Station develops scientific information 

and technology to improve management, protection, and use of the 

forests and rangelands. Research is designed to meet the needs of 

the National Forest managers, Federal and State agencies, public and 

private organizations, academic institutions, industry, and individuals. 

Studies accelerate solutions to problems involving ecosystems, range, 

forests, water, recreation, fire, resource inventory, land reclamation, 

community sustainability, forest engineering technology, multiple use 

economics, wildlife and fish habitat, and forest insects and diseases. 

Studies are conducted cooperatively, and applications may be found 

worldwide.

Station Headquarters

Rocky Mountain Research Station

240 W Prospect Road

Fort Collins, CO 80526

(970) 498-1100

Research Locations

Reno, Nevada

Albuquerque, New Mexico

Rapid City, South Dakota

Logan, Utah

Ogden, Utah

Provo, Utah

The U.S. Department of Agriculture (USDA) prohibits discrimination in all its 

programs and activities on the basis of race, color, national origin, age, disability, 

and where applicable, sex, marital status, familial status, parental status, religion, 

sexual orientation, genetic information, political beliefs, reprisal, or because all 

or part of an individual’s income is derived from any public assistance program. 

(Not all prohibited bases apply to all programs.) Persons with disabilities who 

require alternative means for communication of program information (Braille, 

large print, audiotape, etc.) should contact USDA’s TARGET Center at (202) 

720-2600 (voice and TDD). To file a complaint of discrimination, write to USDA, 

Director, Office of Civil Rights, 1400 Independence Avenue, S.W., Washington, 

DC 20250-9410, or call (800) 795-3272 (voice) or (202) 720-6382 (TDD). USDA 

is an equal opportunity provider and employer.

Flagstaff, Arizona

Fort Collins, Colorado

Boise, Idaho

Moscow, Idaho

Bozeman, Montana

Missoula, Montana