Integrating Chemical and Biological Control

of the Hemlock Woolly Adelgid:

A Resource Manager’s Guide

Albert E. Mayfield III, Scott M. Salom, Kenton Sumpter, 

Tom McAvoy, Noel F. Schneeberger, and Rusty Rhea

United States Department of Agriculture

Technology

Transfer Integrated

Pest Management

Forest Health Assessment

and Applied Sciences Team

FHAAST-2018-04

January 2020
Acknowledgments

The following individuals are sincerely thanked for providing critical reviews of the initial draft of the 

manuscript: Elizabeth McCarty, University of Georgia; Rachael Dickson, USDA Forest Service; Robert Jetton, 

Camcore, North Carolina State University; Brad Onken, USDA Forest Service (retired); Richard Reardon, 

USDA Forest Service (retired); Tim Tomon, Pennsylvania Bureau of Forestry; Biff Thompson, Maryland 

Department of Agriculture; Margot Wallston, Hemlock Restoration Initiative; Jesse Webster, Great Smoky 

Mountains National Park. We extend our gratitude to Richard Reardon (FHAAST) for producing this guide; 

the layout was designed by Rachel L. Winston, MIA Consulting.

Cover Photos: (a) making visual crown rating estimates of hemlock health; 

(b) hemlock branch with a high density of HWA; (c) adult Laricobius

beetle; (d) tablet formulation delivery method for treating hemlock trees 

with imidacloprid (Credits: a. A. Mayfield, USDA Forest Service; b. S. 

Salom, Virginia Tech; c,d. Elizabeth McCarty, University of Georgia, 

Bugwood.org)

How to cite this publication: 

Mayfield, A.E., III; S.M. Salom; K. Sumpter; T. McAvoy; N.F. Schneeberger; R. Rhea. 2020. Integrating chemical 

and biological control of the hemlock woolly adelgid: a resource manager’s guide. FHAAST-2018-04. USDA 

Forest Service, Forest Health Assessment and Applied Sciences Team, Morgantown, West Virginia.

This publication is available online at: 

http://hwa.ento.vt.edu/hwa/index.php/publications-newsletters/

https://www.fs.fed.us/foresthealth/technology/pdfs/FHAAST-2018-04_Chem_and_Biocontrol_HWA_

Mgrs_Guide.pdf

https://bugwoodcloud.org/resource/pdf/hwa-managerguide.pdf

For additional copies of this publication, contact: 

Denise Binion FHAAST, USDA Forest Service, 180 Canfield Street, Morgantown, WV 26505

304.285.1552; denise.binion@usda.gov

a

b

c

d

Federal Recycling Program

Printed on Recycled Paper
Integrating Chemical and Biological Control

of the Hemlock Woolly Adelgid:

A Resource Manager’s Guide

Albert E. Mayfield III, Southern Research Station, USDA Forest Service, Asheville, NC; 

albert.e.mayfield@usda.gov

Scott M. Salom, Department of Entomology, Virginia Tech, Blacksburg, VA; 

salom@vt.edu

Kenton Sumpter, West Virginia Department of Environmental Protection, Morgantown, WV; 

klsumpter0@gmail.com

Tom McAvoy, Department of Entomology, Virginia Tech, Blacksburg, VA (retired); 

tmcavoy@vt.edu

Noel F. Schneeberger, Forest Health and Economics, Eastern Region, USDA Forest 

Service, Coatesville, PA (retired); noelschneeberger@gmail.com

Rusty Rhea, Forest Health Protection, Southern Region, USDA Forest Service, 

Asheville, NC; rusty.rhea@usda.gov

Abstract

A non-native invasive insect, the hemlock woolly adelgid (HWA), threatens the ability of natural resource 

managers to maintain eastern and Carolina hemlocks as critical components of unique forest ecosystems in 

eastern North America. Although substantial progress has been made in both chemical and biological control 

of HWA, neither of these tactics applied alone are expected to provide adequate control of HWA throughout 

its introduced range. This guide presents a methodological strategy for integrating biological and chemical 

control together in the same forest stands. The goal of the strategy is to prolong hemlock health on certain 

hemlock trees through temporary insecticide protection, while simultaneously establishing predators on 

nearby untreated trees. Temporarily-protected hemlocks are expected to eventually support predators after 

their chemical treatment wears off. Guidelines for site selection, treatment timing, spatial considerations, 

monitoring, and assessment are included. The guide is intended as a starting point for a more sustainable 

approach to HWA management that reduces the amount of insecticide applied, and that can be integrated 

with additional management tools as they are developed.
INTEGRATING CHEMICAL AND BIOLOGICAL CONTROL OF THE HEMLOCK WOOLLY ADELGID

Table of Contents

INTRODUCTION ............................................................................................................ 1

Chemical Control of HWA: Benefits and Limitations............................................ 4

Biological Control of HWA: Benefits and Current Status ..................................... 6

Why Integrate Chemical and Biological Control? ................................................ 7

IMPLEMENTING AN INTEGRATED STRATEGY .......................................................... 9

Where to Apply ........................................................................................................ 9

Ownership ........................................................................................................................9

Stand Size and Hemlock Spacing....................................................................................9

HWA Population ...............................................................................................................9

Hemlock Health ..............................................................................................................10

Crown Structure .............................................................................................................10

Other Factors..................................................................................................................10

Chemical Treatments............................................................................................. 11

Spatial Considerations ..................................................................................................12

Variations ........................................................................................................................13

Timing Considerations....................................................................................................15

Biological Control Treatments .............................................................................. 16

Release Timing ...............................................................................................................17

Release Method and Tree Selection...............................................................................17

Release Numbers ...........................................................................................................19

MONITORING AND MAINTENANCE .......................................................................... 20

Beat Sheet Sampling ............................................................................................. 20

Larval Sampling...................................................................................................... 21

Augmentative Predator Releases......................................................................... 22

Augmentative Chemical Treatments .................................................................... 22

ASSESSING THE STRATEGY...................................................................................... 23

Hemlock Health ..................................................................................................... 23

Visual Crown Ratings......................................................................................................23

Branch Tip Sampling ......................................................................................................24

HWA Population Sampling .................................................................................... 26

HWA per cm and HWA Index .........................................................................................26

Percent Trees and Percent Branches Infested ...............................................................26

CONCLUSION .............................................................................................................. 27

GLOSSARY................................................................................................................... 28

REFERENCES .............................................................................................................. 29

APPENDIX .................................................................................................................... 34

Crown Density-Foliage Transparency Card for Rating Tree Health .................. 34
INTEGRATING CHEMICAL AND BIOLOGICAL CONTROL OF THE HEMLOCK WOOLLY ADELGID

INTRODUCTION

Eastern hemlock (Tsuga canadensis) is a highly valued, ecologically important native tree species widely 

distributed throughout the eastern United States (U.S.) and Canada. It is long-living, slow growing, highly 

shade-tolerant, and adaptable to growth under many types of site conditions (Godman and Lancaster 

1990). Eastern hemlock often occurs as a component of mixed hardwood forests (Figure 1a) or as the 

dominant conifer in the later stages of ecological succession. It is also considered a foundation species in 

critical riparian habitats (Figure 1b) and other systems where it has a strong influence on biodiversity and 

ecological processes (Ellison et al. 2005). Another hemlock species found in the eastern U.S. is the relatively 

rare Carolina hemlock (Tsuga caroliniana), which is limited to small pockets of forest in the southern 

Appalachian Mountains (Jetton et al. 2008).

The hemlock woolly adelgid 

(Adelges tsugae, or HWA) is found on 

hemlock species (Tsuga spp.) worldwide 

but is not native to eastern North 

America (Havill and Foottit 2007). In 

its native ranges, HWA rarely reaches 

population levels that are injurious to 

hemlocks, because it is likely suppressed 

through a combination of evolved 

host resistance and a complex of native 

predators (Havill et al. 2006). HWA was 

discovered in the eastern U.S. in the early 

1950s near Richmond, VA, and the origin 

of this population was traced back to the 

southern region of the Japanese island of 

Honshu, near the city of Osaka (Havill et 

al. 2006, Havill et al. 2016a). Since then, 

HWA has become a serious pest on both 

eastern and Carolina hemlocks (Havill et 

al. 2016b). The range of HWA continues 

to expand, and it has become established 

throughout much of the eastern U.S. 

from Maine to Georgia, as far west as 

Michigan, and as far north as Nova Scotia 

(Figure 2). Since the 1980s, HWA has 

caused extensive decline and mortality of 

eastern and Carolina hemlocks (Orwig et 

al. 2002, Vose et al. 2013; Figure 3). HWA 

is capable of killing hemlocks of any age 

and size class.

a

b

Figure 1. Eastern hemlock grows in a variety of habitats, including 

mixed confer-hardwood stands (a) and along stream corridors (b). 

(Credits: a. A. Mayfield, USDA Forest Service; b. N. Schneeberger, 

USDA Forest Service).

INTRODUCTION 1
INTEGRATING CHEMICAL AND BIOLOGICAL CONTROL OF THE HEMLOCK WOOLLY ADELGID

Figure 2. Distribution of the hemlock woolly adelgid in eastern North America as of 2018. (Maps archived at http://hiro.

ento.vt.edu/hwa/index.php/distribution-maps/)

2 INTRODUCTION

Figure 3. Eastern hemlocks killed by the hemlock woolly adelgid in the southern Appalachian Mountains. (Credit: D. 

Casey, USDA Forest Service)
INTEGRATING CHEMICAL AND BIOLOGICAL CONTROL OF THE HEMLOCK WOOLLY ADELGID

In North America, HWA has two 

successive, asexual generations. The progrediens 

generation is present in the spring, and the 

sistens generation is present from summer to 

early spring of the following year (Havill et al. 

2016b) (Figures 4, 5). HWA feeds by inserting 

its mouthparts into twig tissues near the base 

of needles (Oten et al. 2014). This results in the 

depletion of plant sugars, drying and loss of 

foliage, and reduction of new growth (MillerPierce et al. 2010, Domec et al. 2013, GondaKing et al. 2014). During most of the summer, 

HWA is in a dormant stage and does not begin 

feeding again until late September or October.

The health of infested hemlock stands 

is largely dependent on the density of HWA 

(McClure 1991). Following initially heavy 

infestations on healthy hemlocks, trees 

experience dieback, and the HWA population 

declines due to poor host quality and a lack of 

new shoots on which to settle. This decrease in 

the HWA population allows trees to partially 

recover and resume new shoot growth. However, 

Figure 4. The life cycle of the hemlock woolly adelgid (HWA) on hemlock in North America. The sistens generation 

develops from summer through winter, and the progrediens generation develops in the spring. In both generations, 

eggs give rise to mobile crawlers, which settle on hemlock twigs and progress through three more nymph stages 

before becoming adult females. In North America, winged adults in the spring do not reproduce and the life cycle 

on hemlock is driven entirely by wingless females. (Credit: V. D’Amico and N. Havill, USDA Forest Service)

INTRODUCTION 3

Figure 5. A springtime photograph of hemlock woolly 

adelgid taken through a 10x hand lens, showing 

the large, white, cottony ovisacs of the sistens 

generation, and the smaller, more numerous nymphs 

of the recently-hatched progrediens generation. 

(Credit: Bryan Mudder, USDA Forest Service)
INTEGRATING CHEMICAL AND BIOLOGICAL CONTROL OF THE HEMLOCK WOOLLY ADELGID

these new shoots usually become infested by HWA, and the cycle of decline continues (McClure 1991). The 

rate at which this process occurs is variable. Some trees die in as few as four years, whereas others may 

survive for 15 years or more (Havill et al. 2016b). Certain abiotic factors, such as sudden changes or extremes 

in temperature, can negatively affect HWA, prolonging stand survival and slowing HWA range expansion 

(Parker et al. 1998, 1999; Skinner et al. 2003; Paradis et al. 2008; McAvoy et al. 2017b; Mech et al. 2018). 

Chemical Control of HWA: Benefits and Limitations

Chemical insecticides are effective against HWA and have been used widely in urban and landscape settings, 

for highly valued trees in campgrounds and recreation areas, and in managed forests. This has included 

large-scale chemical treatment efforts in certain state and national parks and forests in which hundreds 

of thousands of trees have been treated. Although insecticide treatment of hemlocks has been a critical 

means by which hemlock mortality has been reduced, it is applied on an individual tree basis and is thereby 

relatively cost- and labor-intensive (Vose et al. 2013). 

The most commonly used class of insecticides for control of HWA has been neonicotinoids. 

Neonicotinoids are neurotoxic compounds with relatively low mammalian toxicity that are widely used 

in agriculture and other pest applications worldwide (Durkin 2016). Recent research indicates that these 

compounds are persistent in ecosystems and have negative non-target effects on beneficial organisms, 

including pollinators (Hladik et al. 2018, Calvo-Agudo et al. 2019). Compared with typical agricultural 

applications, however, application of neonicotinoid insecticides for HWA management greatly limits 

potential exposure to non-target organisms. The insecticides are applied directly to the trunk or soil 

immediately around the base of the tree, and are distributed systemically through the tree’s internal vascular 

system to fine branch tissues where the adelgids feed. Furthermore, hemlock is wind pollinated and does 

not produce flowers that would be visited by pollinating insects. Because of their value in reducing the 

impact of non-native forest pests, state forestry agencies in the U.S. have advocated for the continued use of 

neonicotinoid insecticides in forestry (Southern Group of State Foresters 2017). 

Neonicotinoid insecticides should be used carefully and judiciously in HWA management. This guide 

recognizes that application of insecticides is not a stand-alone, long-term strategy for HWA, and stand-level, 

pre-emptive insecticide treatments are not recommended. Rather, a goal of this guide is to use insecticides 

conservatively and strategically in a way that ultimately reduces the amounts applied on the landscape. 

Human health and ecological risk assessments associated with USDA Forest Service uses of neonicotinoids 

have been completed and are available online (Durkin 2009, 2016). 

One of the early insecticides in the neonicotinoid class, imidacloprid, was evaluated and found to be 

highly effective against HWA (Steward and Horner 1994, Cowles et al. 2006). Not only is the imidacloprid 

compound directly lethal to HWA, secondary metabolites, such as olefin, persist in the plant tissue longer 

and are more toxic to the adelgids than imidacloprid (Coots et al. 2013, Benton et al. 2015). The most 

common method of application for imidacloprid involves soil treatments, including soil drench, soil 

injection (Steward et al. 1998), and placement of subsurface tablets (Figure 6). Soil treatments result in 

better distribution of imidacloprid within the tree as compared to stem injection (Dilling et al. 2010; Figure 

6), but stem injection can be useful when proximity to water or other site limitations preclude the use of soil 

application. 

Soil-applied imidacloprid moves slowly from the roots to the top of the tree, may take 3–12 months to 

reach all of the canopy (Coots et al. 2013, Joseph et al. 2011a), and typically protects hemlocks from HWA for 

4 INTRODUCTION
INTEGRATING CHEMICAL AND BIOLOGICAL CONTROL OF THE HEMLOCK WOOLLY ADELGID

a

b

c

INTRODUCTION 5

d

Figure 6. Common delivery methods of treating hemlock trees with imidacloprid. (a) soil drench; (b) soil injection; 

(c) tablet formulation, and (d) stem injection. (Credits: a,b. Great Smoky Mountains National Park Resource 

Management, USDI National Park Service, Bugwood.org; c. Elizabeth McCarty, University of Georgia, Bugwood.

org; d. N. Schneeberger, USDA Forest Service)
INTEGRATING CHEMICAL AND BIOLOGICAL CONTROL OF THE HEMLOCK WOOLLY ADELGID

a period of 4–6 years (Silcox 2002, Benton et al. 2016b). Traces of imidacloprid and its metabolites have been 

detected in hemlock tissue up to 7 years post-treatment (Benton et al. 2015, Mayfield et al. 2015). Description 

of an optimized dosage application based on size of the trees is presented in Benton and Cowles (2017).

An aquatic assessment was conducted with 10 southern Appalachian streams associated with 

chemical treatments for HWA in riparian areas. Although imidacloprid was detected in adjacent waterways, 

sensitive aquatic invertebrate communities were just as diverse and healthy as communities in areas were 

no insecticide was used (Benton et al. 2016a, Benton et al. 2017). In addition, no secondary insecticidal 

metabolites were recovered from these streams (Benton et al. 2016a). 

A different neonicotinoid product that is more water-soluble and faster-acting than imidacloprid is 

dinotefuran (Durkin 2009). For treatment of HWA, it can be applied as a basal bark spray, soil injection, or 

soil drench (Valent 2014) and can suppress HWA populations in as little as one month after treatment (Joseph 

et al. 2011a). However, the efficacy of dinotefuran persists for only 1–2 years. Recent recommendations have 

included tank mixing both imidacloprid and dinotefuran to obtain the best of both products: quick uptake 

with longer-lasting efficacy (Whitmore 2014, McCullough 2017).

Biological Control of HWA: Benefits and Current Status

Natural enemies of HWA are comprised principally of host-specific and generalist predatory insects. Several 

host-specific predators from the native range of HWA have been studied and released as biological control 

agents since the 1990s (Onken and Reardon 2011, Mausel and Salom 2013). Sasajiscymnus tsugae, a ladybird 

beetle from Japan, was the initial focus of this effort (Cheah and McClure 1998). Millions of beetles were 

subsequently reared by several labs and released throughout the eastern U.S. However, difficulty in recovering 

the beetle consistently (Hakeem et al. 2010, 2013) led to increased emphasis on other specialist predators. 

Laricobius beetles from the family Derodontidae are adelgid specialists (Leschen 2011), and several 

geographically separated species are predators of HWA. Two species currently being reared, collected, and 

released are L. nigrinus from western North America and L. osakensis from Japan. Laricobius nigrinus was 

first released in 2003 and was reported to be established following an extensive assessment of 22 release sites 

(Mausel et al. 2010). The number of lab-reared and field-collected L. nigrinus beetles released is approaching 

400,000. These beetles have been released at hundreds of locations, and are now being collected from selected 

field sites in the eastern and western U.S. for redistribution to new sites in the eastern U.S. (Virginia Tech 2020). 

Laricobius osakensis was first released in 2012 (Mooneyham et al. 2016), has been released at more 

than 60 locations, and is established at several sites (Toland et al. 2018). This species was discovered in 

2005 in southern Japan (Montgomery et al. 2011). Since HWA in the eastern U.S. is descended from HWA 

in southern Japan (Havill et al. 2006, 2016a), it is likely that this predator is well adapted to this strain of 

adelgid. How it compares in establishment and impact with L. nigrinus will take time to determine.

Both Laricobius species are active as adults (Figure 7a) in the fall and winter, where they feed on 

developing sistens. Female Laricobius beetles lay eggs in sistens ovisacs around the time progrediens eggs 

are being laid in early spring. Upon hatching, Laricobius larvae (Figure 7b) feed on HWA eggs in the ovisac 

and early stage progrediens nymphs. After mature Laricobius larvae finish feeding on their prey, they drop to 

the soil to pupate. Thus, later stage progrediens nymphs and adults, and sistens eggs, are free from significant 

predation in the late spring when Laricobius beetles are pupating. 

6 INTRODUCTION
INTEGRATING CHEMICAL AND BIOLOGICAL CONTROL OF THE HEMLOCK WOOLLY ADELGID

a b

Figure 7. Laricobius nigrinus. (a) adult on a hemlock twig; (b) larva within an HWA ovisac. (Credits: a. Bryan Mudder, 

USDA Forest Service; b. David Mausel, Virginia Polytechnic Institute and State University, Bugwood.org)

INTRODUCTION 7

An additional biological control agent that would feed on HWA during this late spring season could 

greatly enhance current efforts to control HWA populations. Leucopis silver flies are adelgid-specialist 

predators active during this critical time. Two Leucopis species are closely associated with HWA in the 

western U.S. (Kohler et al. 2008, 2016). Since 2015, these predatory flies have been collected and shipped 

to the eastern U.S. for caged release studies (Motley et al. 2017). The goal is that over time, with continued 

releases, these western silver flies will establish and become an important biocontrol agent for the late spring 

life stages of HWA. 

Why Integrate Chemical and Biological Control? 

The USDA Forest Service and its management partners are trying to achieve an overall integrated pest 

management (IPM) strategy for addressing the HWA problem in eastern North America. An end goal is 

to retain, in a sustainable way, the unique characteristics that hemlocks provide in an ecosystem. Despite 

substantial developments in both chemical and biological control, neither of these options is likely to solve the 

HWA problem alone. This is because 1) it is neither practical nor ecologically responsible to chemically treat 

all the hemlocks in a stand into the indefinite future, and 2) patterns of hemlock decline and mortality suggest 

that biological control agents alone cannot act quickly enough to save all the hemlocks in many locations. It 

takes years for biological control populations to build up, and often trees cannot survive the wait. 

With this in mind, we are proposing a strategy that combines both chemical and biological control 

tactics in the same stand. The goal is to prolong and improve hemlock health on certain hemlock trees through 

temporary insecticide protection, while simultaneously establishing predators on nearby untreated trees, or 

unprotected portions of trees treated with low rates of insecticide (Joseph et al. 2011b, Eisenback et al. 2014, 

Mayfield et al. 2015, Sumpter et al. 2018). In concept, this approach will allow predator populations to increase 

on the untreated trees, and eventually disperse onto the temporarily-treated trees after the insecticide protection 

wears off. Those previously-protected hemlocks should then have better health and potentially greater longevity 

than hemlocks that have never been chemically treated. The previously-protected trees should also be a better 

source of prey because they have more new shoots for HWA to infest (Mayfield et al. 2015).
INTEGRATING CHEMICAL AND BIOLOGICAL CONTROL OF THE HEMLOCK WOOLLY ADELGID

Exposure of predatory insects to systemic insecticides should be limited because 1) the insecticides 

are delivered primarily within the plant tissue, and 2) chemically-protected branches will not support HWA 

and thus will not be attractive to predators. Exceptions may occur with soil-applied insecticides, because 

Laricobius beetles spend part of their spring and summer in the soil. Thus, a proportion of the Laricobius 

population could become exposed to the product when they drop to the ground near the base of treated 

trees. Another exception may occur if trees are chemically treated after predatory beetles are already present 

in the stand, and insecticide is actively being consumed by adelgids when predators are feeding on them. 

In the IPM strategy outlined below, guidelines are offered to help minimize these kinds of exposures and 

ensure that most of the predator population does not encounter the insecticide. 

The IPM strategy presented herein does not propose the complete prevention of hemlock decline 

and mortality. Realistically, land managers should be prepared to accept some impact from HWA without 

losing all their hemlock trees. The intent is a strategic use of insecticide application that facilitates predator 

establishment and growth, hopefully reducing and maintaining HWA populations below unacceptable 

levels. Current research is attempting to determine what impact predators have on HWA populations and 

tree health in stands where they are established. A recent multi-year, multi-site field study demonstrated that 

L. nigrinus has a significant impact on ovisacs of the winter sistens generation (Jubb et al. 2020). However, 

there is also evidence that densities of the progrediens generation rebound following Laricobius predation 

(Crandall et al. in review). As work continues on assessing the contribution of biological control, the 

proposed strategy is offered as a starting point for managing hemlock stands and HWA in a more integrated, 

sustainable way.

8 INTRODUCTION
INTEGRATING CHEMICAL AND BIOLOGICAL CONTROL OF THE HEMLOCK WOOLLY ADELGID

IMPLEMENTING AN INTEGRATED STRATEGY

Where to Apply 

Hemlock trees are found in diverse settings, ranging from large expansive natural stands to small yards and 

parks. Due to the limited availability of predators for release, distribution of biological control agents has 

been concentrated on public lands that typically support natural forests of either pure hemlock or hemlocks 

mixed with other tree species. Additionally, high-value trees growing in recreational areas or preserves have 

been targeted as well. 

The following guidelines should be considered when choosing a site to integrate biological and 

chemical control of HWA. Most of the thresholds mentioned below are not derived from specific research 

trials, but are recommendations based on manager experiences and current knowledge about HWA, their 

predators, and their interaction with hemlock ecosystems. Additional site selection criteria can be found on 

the HWA Predator Database (Virginia Tech 2020). 

Ownership

The owners or managers of the property should have a long-term commitment to managing the forest 

in a way that is consistent with the IPM strategy. Often this requires public ownership or lands with 

conservation status. Incompatible management practices may include excessive hemlock removal or 

prescribed fire treatments that might kill aestivating Laricobius beetles and/or hemlocks.

Stand Size and Hemlock Spacing

Sites should be large enough, and with a sufficient hemlock component, to support predator population 

increase and dispersal. Ideally, stands should be at least 4 ha (10 ac) in size with hemlock comprising 

at least 20% of the basal area. Try to avoid stands in which the average hemlock spacing is excessively 

wide (>30 m or >100 ft) such that predators cannot readily find each other or their prey. 

HWA Population

A population of HWA must be present in order to support predators and to warrant use of insecticides. 

If HWA has only recently arrived to the stand and adelgid densities are very low, the prey population 

may not yet be large enough to achieve predator establishment. As a lower threshold for implementing 

an IPM strategy, we recommend that at least 20% of the hemlock trees in the stand be infested with 

HWA. A statistically reliable estimate of the percentage of infested trees can be determined using the 

methods and data sheet provided in Costa and Onken (2006). Trees onto which predators will be 

released should have at least some branches with a high density of HWA ovisacs (Figure 8). 

IMPLEMENTING AN INTEGRATED STRATEGY 9
INTEGRATING CHEMICAL AND BIOLOGICAL CONTROL OF THE HEMLOCK WOOLLY ADELGID

Figure 8. A healthy, hemlock branch with a high density of HWA. Such branches are desired locations for 

releasing adult predator beetles. (Credit: S. Salom, Virginia Tech)

10 IMPLEMENTING AN INTEGRATED STRATEGY

Hemlock Health

Choose stands in which the majority of hemlocks are in good health. The trees should have abundant 

new shoot growth necessary to sustain HWA as food for predators (Figure 8), and should possess 

sufficient vigor to survive infestations for several years. Such trees may include hemlocks that 1) are 

in the early stages of infestation (first 2–3 years) with little to no dieback or decline apparent; 2) have 

rebounded well from a first wave of HWA infestation; or 3) are just coming out of chemical protection. 

Most of the hemlocks should have a crown density rating 60% or above and a foliage transparency 

rating of 45% or lower (see “Hemlock Health” in Chapter 4: Assessing the Strategy). 

Crown Structure

At least some of the hemlocks within or on the edges of the stand should have lower branches that 

extend near the ground and can be reached by hand. These trees will typically have a live crown ratio 

greater than 80%. Having branches that can be reached by hand from the ground greatly facilitates the 

assessment of HWA populations, the release of predator beetles onto branches, and future monitoring 

of both HWA and predators. However, not all hemlock trees in the stand need to have this characteristic.

Other Factors

Choose sites that have a consistent, organic duff layer over the mineral soil and beneath the drip lines 

of the hemlock trees that is at least an average depth of 2.5 cm (1.0 in). This organic layer is habitat for 

Laricobius larvae in the spring and adults in the summer. 
INTEGRATING CHEMICAL AND BIOLOGICAL CONTROL OF THE HEMLOCK WOOLLY ADELGID

Chemical Treatments

A proposed strategy for integrating chemical and biological control in the same stand is to create different 

“classes” of hemlock trees on the landscape. The classes are created by varying the level and duration of 

insecticide protection among hemlocks. The level of chemical protection can be manipulated by varying the 

amount of insecticide applied (e.g., full rate, reduced rate, none), whereas the duration of protection can be 

manipulated by the frequency of insecticide re-application or the type of insecticide used. In this strategy, 

some trees remain untreated to harbor HWA and support early predator establishment and population 

growth. Other trees are chemically treated to temporarily preserve crown health, but eventually support 

prey after predator populations have the opportunity to establish and increase in the stand. If resources 

permit, a limited number of high-value trees are treated perpetually.

Class 4: Perpetual, full protection 

Class 4 trees are hemlocks that receive the full rate of the insecticide and are retreated on an interval that 

maintains protection from HWA for as long as possible. If using imidacloprid, trees are retreated approximately 

every 5–7 years or when chemical protection wears off (based on observance of HWA on the branches). No 

predators are released on these trees. Class 4 is expensive to maintain and thus might comprise only 1–10% 

of the hemlocks in the stand. It should be reserved for hemlocks with the highest ecological, economic, or 

aesthetic value (e.g., very large trees, trees valued for seed production, trees adjacent to recreational sites, etc.) 

(Figure 9). 

Class 3: Temporary, full protection 

Class 3 trees receive a one-time treatment of imidacloprid at the full rate and are not retreated. These are trees 

for which good crown health is being maintained for 5–7 years while predator populations are increasing on 

the Class 1 and 2 trees (described below). When Class 3 trees lose chemical protection, both HWA and their 

predators are permitted to colonize these trees. Managers may consider designating 10–20% of the hemlocks 

in the stand as Class 3 trees. 

Class 2: Temporary, partial protection

Class 2 trees receive a one-time imidacloprid treatment, but at a reduced rate (e.g., 25–50% of the full rate). 

Based on previous experience (Joseph et al. 2011b, Eisenback et al. 2014, Mayfield et al. 2015), HWA will 

colonize these trees, or portions thereof, within 1–4 years. To achieve partial tree protection through a reduced 

rate treatment, reduce the number of fluid ounces, injection pumps, or tablets applied (depending on the 

product and delivery method used) to approximately 25–50% of that recommended for a given tree diameter 

in Benton and Cowles (2017). Like Class 3 trees, Class 2 trees represent hemlocks for which future crown 

health is being maintained while predator populations increase on other trees, but to a lesser degree and for a 

shorter time. Managers may consider designating 10–20% of hemlocks in the stand as Class 2 trees. 

Class 1: No protection

Class 1 hemlocks receive no insecticide treatment. They are infested with HWA or are expected to be infested in 

the very near future. As such, Class 1 trees represent hemlocks on which predators will be released or to which 

they will initially disperse. Early predator populations will establish and begin to increase in number on Class 

1 trees before prey becomes available on Class 2 and 3 trees. Managers may consider designating a moderate to 

high percentage (≥50%) of hemlocks in the stand as Class 1 trees.

IMPLEMENTING AN INTEGRATED STRATEGY 11

CLASS 4 CLASS 3 CLASS 2 CLASS 1
INTEGRATING CHEMICAL AND BIOLOGICAL CONTROL OF THE HEMLOCK WOOLLY ADELGID

a b

Figure 9. Large hemlocks with high aesthetic, recreational, and ecological value are good candidates for perpetual 

chemical protection (Class 4). (Credits: A. Mayfield, USDA Forest Service)

12 IMPLEMENTING AN INTEGRATED STRATEGY

Figures 10 and 11 provide a schematic of how the classes might be arranged on a hypothetical landscape 

to support both predator proliferation and hemlock health. The number of classes and their arrangement 

could vary depending on the objectives of the resource manager and the local conditions. For determining 

the “full rate” dosage for imidacloprid (see Classes 3 and 4 above), we recommend using the diameteroptimized tables and instructions presented in “Optimized Insecticide Dosage for Hemlock Woolly Adelgid 

Control in Hemlock Trees” (Benton and Cowles 2017). 

Spatial Considerations 

The treatment classes described above should be arranged on the landscape to facilitate the dispersal 

of predators onto newly infested trees over time. Thus, the Class 1 trees onto which predators are 

released should be in close proximity (within 30 m) to other Class 1 trees, where prey is expected 

to be available during the first few years (Figures 10, 11). These Class 1 trees should also be in close 

proximity to Class 2 trees, which will begin to support HWA within 1–4 years. In turn, Class 2 trees 

should be located near Class 3 trees, which should begin to support prey after 5–7 years.

Two different hypothetical scenarios for arranging Class 1–4 hemlock treatments on the 

landscape are shown in Figures 10 and 11. In Figure 10, the chemical treatment classes are applied 

in a mosaic pattern among hemlocks on both sides of a stream. The highest value trees (Class 4) 

are scattered throughout the stand. The Class 1 trees, chosen for predator release, are immediately 

adjacent to other Class 1 trees onto which predators can disperse. Class 2 and 3 trees are intermixed to 

allow predator dispersal from Class 1 to 2, and 2 to 3, over time. Class 4 trees do not support predators 

because they are not infested with HWA. The design illustrated in Figure 10 also favors conservation 

of hemlock genetic diversity because the Class 4 trees are widely spaced and thus are less likely to be 

closely related (Jetton et al. 2013). 
INTEGRATING CHEMICAL AND BIOLOGICAL CONTROL OF THE HEMLOCK WOOLLY ADELGID

Figure 10. A hypothetical, integrated chemical-biological control scenario. In this spatial arrangement, treated 

trees (classes 2–4) are scattered among untreated trees (Class 1). Predators are released on Class 1 trees, 

spread initially to other Class 1 trees, and eventually colonize Class 2 and 3 trees when the chemical protection 

wears off (in 1–4 years for Class 2; 5–7 years for Class 3). High-value class 4 trees remain perpetually protected.

IMPLEMENTING AN INTEGRATED STRATEGY 13

 In contrast, the chemical treatment classes in Figure 11 are arranged as a spatial gradient in 

which predators are expected to disperse in the direction of a recreational area over time. The highest 

value trees are those next to the recreational area and are treated as Class 4 for perpetual protection. 

Beetles are released on Class 1 trees at the margins of the stand and initially disperse to other Class 

1 trees. Established predators are expected to move towards the recreational area as prey becomes 

available first on Class 2 and then on Class 3 trees. Again, predators are not expected on Class 4 trees 

because their prey is absent. A slight variation on this design would be to create a “core” area of Class 4 

trees that are surrounded by concentric circular groupings of Class 3, 2, and 1 trees radiating outward. 

Variations

These are hypothetical scenarios that could be modified to fit different stand conditions, available 

resources, and manager objectives. For example, the percentage of untreated (Class 1) trees in the 

stand could be increased or decreased. In the examples above, we presented the number of untreated 

trees as a moderately high percentage (about 50% in Figures 9 and 10). One reason for this is that 

chemical treatment is expensive to maintain, and is usually viewed as a stopgap measure against HWA 

until other more sustainable controls (such as biological control) are developed and optimized. Thus, 

a strategy that minimizes the amount of insecticide needed to sustain a hemlock stand is a desirable 

goal when integrating chemical and biological control. 
INTEGRATING CHEMICAL AND BIOLOGICAL CONTROL OF THE HEMLOCK WOOLLY ADELGID

Figure 11. Another hypothetical, integrated chemical-biological control scenario. In this spatial arrangement, the 

highest-value trees are located adjacent to a recreation area. Treatment classes are therefore arranged in a 

spatial gradient that will allow predators to first become established on Class 1 (untreated) trees, and then 

move directionally toward the recreation area over time as the Class 2 and 3 trees lose protection. Class 4 

trees remain perpetually protected.

14 IMPLEMENTING AN INTEGRATED STRATEGY

At the same time, it is realistic to assume that some, or even many, of the Class 1 trees are going 

to decline in health or die from HWA infestation. In order to provide an initial abundance of prey, the 

health of Class 1 trees is sacrificed so that the predator population may build and ultimately colonize 

Class 2 and 3 trees (and even Class 4 trees, if chemical treatments are eventually discontinued) in the 

longer term. If resources allow, managers may decide that they wish to chemically protect more than 

50% of the trees in a stand to help minimize early hemlock losses. 

Managers should consider a diversity of canopy positions when choosing trees to receive chemical 

treatment. As noted above, large hemlock trees may be good candidates for perpetual chemical protection 

(Class 4) given their high ecological or aesthetic value. However, depending on stand composition, it may 

be beneficial to include some mid-story or understory trees in the chemically-treated classes. This could 

serve to promote stand structural diversity and the presence of canopy-dominant hemlocks in the future. 

Variations on the classes presented above are also possible. For example, some Class 3 trees 

could be re-treated more than once to extend foliage health further into the future. A very simple 
INTEGRATING CHEMICAL AND BIOLOGICAL CONTROL OF THE HEMLOCK WOOLLY ADELGID

variation would be to use just two classes of trees, such as Class 1 (untreated) and Class 4 (perpetually 

treated). Which variation managers choose may depend on factors such as available operating budget, 

stand size and composition, existing health of the trees, and the degree to which climatic or other 

environmental conditions affect HWA populations. 

As an alternative to using a reduced rate of imidacloprid to create Class 2 trees, managers could 

use the label rate of a shorter-duration insecticide such as dinotefuran. Applying dinotefuran at the 

prescribed label rate (Valent 2014) is expected to suppress adelgid populations within 1–2 months 

of application, with efficacy lasting for 1–2 years (Joseph et al. 2011a). Due to this shorter period of 

efficacy, Class 2 trees created using dinotefuran may support predators and prey sooner than Class 2 

trees created using a reduced rate of imidacloprid. 

Regardless of the number of classes used or trees treated, the amount of imidacloprid applied 

should not exceed 0.45 kg of active ingredient per ha (0.4 lbs per ac) per year (Durkin 2016, Bayer 2019). 

Thresholds for this per-acre limit, in terms of fluid ounces applied, number of soil injection pumps 

applied, or tablets applied, is provided in Benton and Cowles (2017). Similarly, dinotefuran application 

should not exceed 0.6 kg of active ingredient per ha (0.54 lbs per ac) per year. Some pesticide products 

are restricted use and can only be applied by a certified and licensed applicator, and then only under 

specific conditions. Managers should ensure that applications are being performed within the limits 

and requirements of the product label(s), and should consult with their state cooperative extension 

service with specific questions regarding insecticide use and applicator certification requirements. 

Caution should be applied when treating hemlocks in riparian areas, and soil applications should not 

be made within 3 m (10 ft) of a stream channel, lake, pond, or wetland. For additional guidelines on 

imidacloprid application see Benton and Cowles (2017).

There is a growing body of evidence to suggest that elevated levels of sunlight, and associated 

elevated temperatures, have negative effects on adelgids and beneficial effects on infested hemlock 

trees (Sussky and Elkinton 2015, Hickin and Preisser 2015, Brantley et al. 2017, McAvoy et al. 2017a). 

Recent and current research is exploring the potential use of silvicultural treatments, such as thinning 

and gap creation, to enhance hemlock resilience in the presence of HWA (Mayfield et al. 2019, Miniat 

et al. 2020). However, management recommendations based on this research are still in development. 

As specific silvicultural prescriptions become available, these could be integrated strategically with 

the chemical treatment classes described above to improve the health and longevity of hemlocks that 

are not chemically protected. In the meantime, it is worth noting that hemlocks located in small 

canopy gaps, along forest edges, or in other environments with elevated light levels may be good Class 

1 candidate trees for predator releases, provided they are infested with HWA at the time of release 

(Figure 12).

Timing Considerations

Consideration should be given to the timing of chemical treatments relative to the timing of predator 

releases. In experimental settings, L. nigrinus adults can experience lethal or negative sublethal effects 

from eating adelgids on branches recently treated with imidacloprid (Eisenback et al. 2010). Although 

predators should be released onto untreated (Class 1) hemlock trees, it is expected that predators will 

disperse to other hemlocks nearby. If nearby hemlocks have been recently treated, they may harbor 

poisonous HWA prey, because the insecticide may still be moving into infested branches. 

IMPLEMENTING AN INTEGRATED STRATEGY 15
INTEGRATING CHEMICAL AND BIOLOGICAL CONTROL OF THE HEMLOCK WOOLLY ADELGID

Figure 12. Hemlocks located in canopy gaps, along forest edges, or other environments with elevated sunlight 

may be good trees on which to release predators in years when they are infested with HWA. (Credit: A. 

Mayfield, USDA Forest Service)

16 IMPLEMENTING AN INTEGRATED STRATEGY

The amount of time required for insecticides to be distributed though hemlock trees and 

eliminate adelgids on the branches can vary depending on the type of insecticide used, its formulation 

and delivery method, the time of year, and site-specific conditions (e.g., soil organic matter, moisture 

conditions, temperature) (Dilling et al. 2010, Joseph et al. 2011a, Faulkenberry et al. 2012, Coots et al. 

2013). Table 1 presents a conservative guide for minimizing the possibility that Laricobius beetles will 

encounter poisoned adelgids when they are released in stands where imidacloprid or dinotefuran has 

been recently applied. When hemlock trees have been treated with a liquid or tablet formulation of 

imidacloprid, consider delaying release of Laricobius beetles in the same stand for one year, to allow 

time for the insecticide to eliminate adelgids on the treated trees. When applying dinotefuran, which 

is more water soluble and faster-acting than imidacloprid (Joseph et al. 2011a, Faulkenberry et al. 

2012), consider delaying the release of adult Laricobius in the stand for at least 3 months (Table 1).

Biological Control Treatments

Laricobius beetles for release in eastern North America are typically obtained either from rearing laboratories 

or from field insectaries where they are collected and redistributed to other areas (Salom et al. 2011). In either 

case, adult beetles are typically transported to the field in small, ventilated jars or vials containing clippings 

of hemlock foliage, shredded paper, or some other substrate to which the beetles can cling. Successful 

establishment of both L. nigrinus and L. osakensis from releases of adults is well documented (Mausel et 

al. 2010, Mayfield et al. 2015, Toland et al. 2018). It is also possible to release Laricobius in the egg or larval 

stage, but the degree of establishment success using this method is not well understood or documented. 
INTEGRATING CHEMICAL AND BIOLOGICAL CONTROL OF THE HEMLOCK WOOLLY ADELGID

Table 1. Timing guidelines for release of adult Laricobius beetles following applications of imidacloprid or 

dinotefuran insecticides to hemlock trees, by season of insecticide treatment.

SEASON IMIDACLOPRID

Applied to the soil (as liquid or tablet)

DINOTEFURAN

Applied to the soil or bark (as liquid)

Fall Fall 

Treatment

Fall 

Treatment

Winter Delay 

release

Winter 

Treatment

Delay 

release

Winter 

Treatment

Springa Delay 

release

Delay 

release

Spring 

Treatment

OK to 

release

Delay 

release

Spring 

Treatment

Summerb N/A N/A N/A Summer 

Treatment N/A N/A N/A Summer Treatment

Fall OK to 

release

Delay 

release

Delay 

release

Delay 

release

OK to 

release

OK to 

release

OK to 

release

Delay 

release

Winter OK to 

release

OK to 

release

Delay 

release

Delay 

release

OK to 

release

OK to 

release

OK to 

release

OK to 

release

Springa OK to 

release

OK to 

release

OK to 

release

Delay 

release

OK to 

release

OK to 

release

OK to 

release

OK to 

release

Summerb N/A/ N/A N/A N/A N/A N/A N/A N/A

Fall OK to 

release

OK to 

release

OK to 

release

OK to 

release

OK to 

release

OK to 

release

OK to 

release

OK to 

release

IMPLEMENTING AN INTEGRATED STRATEGY 17

aSpring releases of adult Laricobius shoud be conducted as early as possible. Releases in late spring are not 

recommended because females may have already laid their eggs and adults are nearing the end of their lives.

bSummer releases of adult Laricobius are not applicable (N/A) because the adults are inactive at this time of year.

Release Timing

Laricobius adults can be released in the fall, winter, or early spring when the HWA sistens generation is 

present. A study of 22 L. nigrinus release sites found that the timing of release (made primarily between 

October and March) had no significant effect on the probability of establishment success (Mausel et 

al. 2010). Laboratory-reared beetles should be released as soon as possible after they emerge, so that 

labs do not have to store and feed beetles for extended periods of time under artificial conditions. 

Similarly, beetles reared in field insectaries should be redistributed and released as soon as possible 

after they are collected. Although adult Laricobius may be available in the late spring (late April–May), 

releases at this time are not ideal, because females may have already laid their eggs, and the adults are 

nearing the end of their lives. 

Release Method and Tree Selection

Laricobius adults should be released on healthy, untreated (Class 1) hemlock branches that have 

an abundance of HWA. Beetles may be released by gently lifting the foliage, wood shavings, or 

shredded paper (to which beetles will cling) from the transport container and placing this material 

on infested hemlock trees in the field (Figure 13a,b). Alternatively, open containers may be held or 

attached beneath hemlock branches for several minutes to allow Laricobius adults to disperse from the 

container on their own (Figure 13c,d). 
INTEGRATING CHEMICAL AND BIOLOGICAL CONTROL OF THE HEMLOCK WOOLLY ADELGID

a b

c

18 IMPLEMENTING AN INTEGRATED STRATEGY

d

Figure 13. Various methods for releasing adult Laricobius beetles. (a) hemlock foliage with Laricobius beetles 

clinging to it is lifted from the transport container for placement on hemlock trees in the field; (b) vials 

containing beetles on shredded paper are emptied onto the branches of infested trees; (c,d) open tubs 

containing beetles and excelsior are attached beneath hemlock branches to allow beetles to disperse 

naturally onto the tree. (Credits: a. USDA Forest Service; b. A. Mayfield, USDA Forest Service; c,d. Maine 

Forest Service)

Laricobius larvae drop to the soil in the spring and adults spend the summer in organic duff 

layer. Therefore, when selecting release trees, avoid hemlocks that overhang roads, water, managed 

turf, or other terrain that would be inhospitable for Laricobius larvae and aestivating adults. 

Although releases during very cold weather (<25°F or −4°C) should generally be avoided, 

sometimes beetles are available during such conditions, and it is not possible to hold them for later 

release. In such cases, consider releasing beetles into the organic duff layer or a pocket of leaf litter 

beneath the hemlock canopy, which provides a more insulated environment. 
INTEGRATING CHEMICAL AND BIOLOGICAL CONTROL OF THE HEMLOCK WOOLLY ADELGID

Release Numbers

The number of beetles released at a site is usually determined by the number of beetles available from 

a laboratory or from field collections at other locations. Release sizes typically range from 200 to 

2000 beetles per site per date, depending on the size of the site and the number of predators available. 

Research has shown that establishment success improves as more beetles are released (Mausel et al. 

2010).

Within a site, consider releasing beetles in groups of about 50 to 100 per tree to help increase 

the probability that beetles will be able to find each other for mating. This practice can be facilitated if 

beetles are packaged in vials containing groups of this size, and the entire contents of a vial is placed 

on one tree. If beetles are packaged in larger containers of several hundred beetles or more, try to 

divide the contents of each container relatively evenly among several release trees. 

Additional predator release criteria can be found on the HWA Predator Database (Virginia Tech 

2020).

IMPLEMENTING AN INTEGRATED STRATEGY 19
INTEGRATING CHEMICAL AND BIOLOGICAL CONTROL OF THE HEMLOCK WOOLLY ADELGID

MONITORING AND MAINTENANCE

After the initial predator release, the site should be monitored for predator establishment. Monitoring for 

establishment could begin as early as one year post-release, but in practice it is probably better to wait until 

the second year. This allows more time for the new predator population to increase to detectable levels.

Beat Sheet Sampling

Monitoring for establishment of Laricobius can be accomplished by sampling for adults, larvae, or both. 

Sampling for adults can be accomplished by the use of a beat sheet in the fall, winter, or early spring when 

HWA are visible on the branches. The observer holds the beat sheet directly under an infested hemlock and 

firmly taps the branch up to 10 times with a telescoping walking stick, a piece of PVC pipe, or some other 

type of durable rod (Figure 14). 

Branches without HWA are unlikely to yield Laricobius, so the best branches to sample are those with 

an abundance of HWA. If present, Laricobius adults will typically be dislodged from the branch and fall onto 

the sheet. Here, they can be examined with a hand lens to confirm that they are Laricobius and returned 

to the tree using a fine-tipped paint brush or by gently lifting them from the beat sheet with a moistened 

fingertip. Alternatively, beetles can be brushed or aspirated into a vial, preserved in 95–100% ethanol, and 

saved for microscopic or genetic identification. 

Figure 14. Sampling for Laricobius adults using a beat sheet. (Credit: A. Mayfield, USDA Forest Service)

20 MONITORING AND MAINTENANCE
INTEGRATING CHEMICAL AND BIOLOGICAL CONTROL OF THE HEMLOCK WOOLLY ADELGID

Larval Sampling

Larval sampling is a more reliable monitoring 

method for detecting beetle presence and 

establishment. Laricobius larvae can be targeted 

specifically by clipping HWA-infested hemlock 

branches about 25 cm (10 in) in length during 

March or April. Optimal dates will vary with 

latitude and elevation, but sampling should occur 

when overwintering HWA adults are in peak egg 

production, and Laricobius larvae are feeding on 

the eggs. The cut ends of infested hemlock branches 

can be pressed into blocks of hydrated floral foam 

(soaked in tap water for at least 5 minutes) that 

are wrapped in laboratory film or other plastic 

wrap (Figure 15). These hemlock “bouquets” can 

then be placed into open-topped plastic buckets 

and transferred to a cool indoor room or a cool, 

sheltered outdoor space. Observers should check 

every 2–3 days for Laricobius larvae in the bottom 

of the bucket over a period of 4–6 weeks (Figure 

16). Larvae can be identified as Laricobius species 

using a microscope, and transferred to vials of 

95–100% alcohol for specimen preservation or 

genetic identification later if desired. Additional 

information on predator sampling and 

identification is posted on the HWA Predator 

Database (Virginia Tech 2020). 

a b

Figure 15. Hemlock branches clipped to sample for Laricobius larvae. At left (a), the cut ends of infested branch tips 

are pressed into a block of hydrated floral foam wrapped in laboratory film. At right (b), the hemlock “bouquets” 

are placed into plastic buckets to monitor for Laricobius larval drop. (Credits: A. Mayfield, USDA Forest Service)

MONITORING AND MAINTENANCE 21

Figure 16. Several Laricobius larvae that have dropped 

from hemlock foliage into a collection container. Notice 

that the larvae near the pencil tip are coated with white 

wax from the HWA ovisac in which they were feeding.

(Credit: A. Mayfield, USDA Forest Service)
INTEGRATING CHEMICAL AND BIOLOGICAL CONTROL OF THE HEMLOCK WOOLLY ADELGID

Augmentative Predator Releases

After chemical and biological control treatments have been applied at a site, managers may wish to augment 

their efforts with additional treatments in subsequent years. If predators are available, managers should 

consider augmenting the initial release with another release in the second year to increase the chances of 

successful establishment and enhance Laricobius population growth. 

Furthermore, environmental conditions that dramatically reduce HWA populations may also reduce 

the predator population, hindering or even preventing Laricobius establishment. For example, cold weather 

associated with the winter “polar vortex” events of 2013 and 2014 likely caused a crash in HWA populations 

at two hemlock Appalachian research sites. Subsequently, no Laricobius beetles were recovered after these 

events, even though predators had been previously recovered at these sites post-release (Sumpter et al. 2018). 

In similar cases, augmentative beetle releases may be necessary, but should be delayed until adelgid prey are 

present again in the stand. Regular monitoring of the predator population using the methods described 

above will help determine whether augmentative beetle releases are warranted.

Augmentative Chemical Treatments

Managers may also wish to augment the site with additional chemical treatments if the health of some 

untreated (Class 1) or temporarily-protected (Class 2 and 3) hemlocks is declining at an unacceptable rate. If 

Laricobius beetles are already present in the stand, however, care should be taken to minimize the potential 

exposure of predators to the insecticide. In addition to the timing consideration presented earlier in Table 1, 

consider the following guidelines for augmentative chemical treatments when Laricobius is already present 

in the stand: 

Delay chemical treatment of trees until after Laricobius has finished feeding and has dropped to 

the soil in April and May. For this application (May–Jun), consider using dinotefuran (either alone 

or in combination with imidacloprid, depending on the duration of protection desired) to achieve 

faster delivery of insecticide to the foliage, so that adelgids are killed before Laricobius begins actively 

feeding again in the fall (Oct–Nov).

For soil application, apply the chemical close to the base of the tree (within 0.5 m, or 1–2 ft from 

the trunk) (Cowles et al. 2006). Not only is this standard operating procedure for soil application 

of imidacloprid, it also minimizes the volume of soil in which Laricobius larvae could encounter 

imidacloprid when they drop from the branches to pupate and aestivate. 

22 MONITORING AND MAINTENANCE
INTEGRATING CHEMICAL AND BIOLOGICAL CONTROL OF THE HEMLOCK WOOLLY ADELGID

ASSESSING THE STRATEGY

One of the most challenging parts of implementing an IPM strategy for HWA is assessing its efficacy. In 

concept, efficacy could be assessed by comparing areas where the integrated strategy has been implemented 

with “control” areas where it has not. In practice, however, finding true control stands (i.e., those that have 

received neither predators nor chemical treatments) that also have biotic and abiotic conditions that are 

similar to the treated stands, can be difficult. Aside from chemical and biological control, hemlock health 

and survival are potentially affected by numerous site-specific conditions such as soil quality, aspect, sunlight 

exposure, temperature and moisture regimes, tree age, history of HWA infestation or other disturbances, and 

stand structure and composition. These factors can vary considerably across landscapes, making it difficult 

to confidently attribute differences between stands to IPM treatments. Furthermore, because introduced 

predators like L. nigrinus are widely established and continue to disperse in the eastern U.S., it can be difficult 

to conclude with confidence that certain areas do not have biological control agents present.

Despite these challenges, managers should attempt to assess the efficacy of an IPM strategy, either 

by comparing implementation areas with the best available control areas, and/or by monitoring changes at 

individual sites over time. Efficacy should be evaluated primarily on the basis of hemlock health and survival, 

but could also include long-term trends in HWA population levels, predator population levels, or both.

Hemlock Health 

Visual Crown Ratings

The health of hemlock trees is commonly assessed by making visual ratings of crown condition. 

Specific methods for making these estimates vary and have different advantages and disadvantages. 

Rather than advocating exclusively for a specific method here, we will highlight a few different types 

of health assessments, with cited references that contain more detailed information.

One method for rating hemlock crown health is to utilize the USDA Forest Service Crown 

Condition Classification Guide (Schomaker et al. 2007), which has been designed for use on a wide 

variety of tree species. Schomaker et al. (2007) describes numerous variables that can be used to assess 

tree crowns, but the four following measures can be particularly useful for assessing hemlock crown 

symptoms related to HWA:

• Foliage transparency

An estimate of the amount of skylight visible through the live, normally foliated portion 

of the crown. It is useful for capturing the foliage-thinning effect caused by HWA feeding. 

Values generally decrease as health improves.

• Crown density

An estimate of the proportion of the normal, expected crown area actually present. It is 

ASSESSING THE STRATEGY 23
INTEGRATING CHEMICAL AND BIOLOGICAL CONTROL OF THE HEMLOCK WOOLLY ADELGID

24 ASSESSING THE STRATEGY

useful for accounting for sections of crown that may be missing due to previous branch 

death or breakage. Values generally increase as health improves.

• Crown dieback

An estimate of recent mortality of branches with fine twigs located in the upper and outer 

portions of the crown. Values generally decrease as health improves.

• Uncompacted live crown ratio

An estimate of the length of live crown relative to the total height of the tree. Values generally 

increase as health improves; an exception to this may occur when part of the lower crown 

is lost due to heavy shading and/or adelgid impact, but the upper part of the crown is 

improving due to better sunlight. 

These variables are estimated on a percent basis, ideally by two observers standing at different 

angles from the tree. A rating card illustrating a scale of transparency, density, and live crown ratio 

values can be used to help improve objectivity and consistency between observers (Appendix 1). 

Observers mentally draw crown outlines when making density and transparency measurements 

(Figure 17). After making ratings independently, the two observers briefly discuss their estimates to 

come up with a single, final rating for the tree (Figure 18). These variables can be averaged on a stand 

basis and tracked from year to year.

A variation on the USDA Forest Service crown classification system is that developed by McAvoy 

et al. (2019). In this methodology, five percentage-based variables (live crown ratio, live branches, live 

branch tips, new foliage, and foliage density), all of which increase as crown health improves, are 

averaged to create a single Crown Health Index. Furthermore, the stand averages for each of these 

variables can be combined with an estimate of the percentage of live hemlocks in the stand to create 

a Stand Health Index. These values can also be tracked over time for individual trees and stands to 

determine whether hemlock health is improving, remaining stable, or declining.

Branch Tip Sampling

Another way to estimate and track hemlock health is through branch tip sampling. In this approach, 

hemlock branch tips of about 25–30 cm (10–12 in) in length are evaluated for new shoot production, 

tip dieback, and/or adelgid densities. These branch tip assessments can be made non-destructively 

in the field when branches can be reached by hand from the ground, a ladder, or lift truck. This is 

particularly advantageous when the same trees will be sampled year after year, as it does not contribute 

to canopy loss and can be completed relatively quickly. Alternatively, the branch tips can be clipped 

using hand or pole pruners and evaluated in the field, or later in the laboratory or office. This is 

advantageous when branches cannot be reached from the ground or when detailed measurements on 

branches are desired. 

One rapid method of using branch tips to assess hemlock health is to create indices of new shoot 

growth and dead shoot tips (Mayfield et al. 2015). In this approach, the outer 10 shoot tips on a 25 cm 

(10 in) branch (the terminal tip and the nine most distal side-tips) are identified. Multiply by 10 the 

number of these shoots that represent a) new growth of the most recent growth flush, and b) dead tips, 

to express these variables as percent new growth (positive health measure) and percent dead shoots 

(negative health measure). Several branches (5–10 per tree) can be sampled and averaged to compute 

tree-level means (Mayfield et al. 2015).
INTEGRATING CHEMICAL AND BIOLOGICAL CONTROL OF THE HEMLOCK WOOLLY ADELGID

Figure 17. Crown outlines mentally drawn for rating hemlock crown health. To rate foliage transparency, the observer 

draws a tight outline that clings to the currently-foliated portion of the crown (red dashed line). The relative proportion 

of skylight visible within this outline is used to estimate foliage transparency. In contrast, when rating crown density, 

the observer draws a loose outline that touches the outer branch tips and projects the expected symmetrical crown 

shape for the tree (blue solid line). The relative proportion of skylight blocked by tree structures within this outline is 

used to estimate crown density. When estimating live crown ratio, the height of the live crown (green line) is divided 

by the total height of the tree (yellow line). A rating card (Appendix 1) can be used to aid with the estimation of these 

variables. See the USDA Forest Service Crown Condition Classification Guide (Schomaker et al. 2007) for more 

detailed instructions and guidelines for special scenarios. (Credit: A. Mayfield, USDA Forest Service)

ASSESSING THE STRATEGY 25
INTEGRATING CHEMICAL AND BIOLOGICAL CONTROL OF THE HEMLOCK WOOLLY ADELGID

a b

Figure 18. Visual crown ratings. (a) a rating card is used to make estimates of hemlock health using the USDA Forest 

Service Crown Classification Guide; (b) two observers discuss their individual ratings to arrive at a final assessment 

for each tree. (Credits: A. Mayfield, USDA Forest Service)

26 ASSESSING THE STRATEGY

HWA Population Sampling

HWA per cm and HWA Index

The 25 cm (10 in) branches described above for branch health ratings can also be used to estimate 

HWA density at the same time. This can be done intensively by counting the number of HWA ovisacs 

on the branch and dividing by the cumulative total length of shoot growth to compute adelgids per 

cm. Several branches per tree (5–10) can be used to compute tree-level averages. 

Alternatively, for increased speed, an index can be computed by counting the number of HWA 

ovisacs on each branch up to a pre-defined number (e.g., 100), and stopping when that number is 

reached. Multiple branches per tree can be averaged to create an HWA Index value that ranges between 

zero and the pre-defined maximum count (Cowles et al. 2006, Mayfield et al. 2015). Although these 

kinds of indices will not discriminate differences in adelgid density among very heavily infested trees 

(>100 HWA per 25 cm [10 in] branch), it is an efficient way to distinguish among lightly (index < 

20), moderately (index 20–60), and heavily (index > 60) infested trees without spending an excessive 

amount of time counting adelgids when HWA populations are very high.

Percent Trees and Percent Branches Infested

Another way to assess the level of HWA in a stand is to estimate the percentage of hemlock trees 

that are infested. As noted above, Costa and Onken (2006) present a statistically reliable method 

for estimating the percentage of infested hemlock trees in a stand. Using a semi-random sampling 

strategy, an observer visits between 8 and 100 trees per stand, with the sample size depending on the 

cumulative number of infested trees encountered. A tree is considered infested if at least one HWA 

ovisac is detected after examining 2 branches per tree. 
INTEGRATING CHEMICAL AND BIOLOGICAL CONTROL OF THE HEMLOCK WOOLLY ADELGID

CONCLUSION 27

A modification of this approach is to estimate the percentage of hemlock branches that are 

infested, as presented by McAvoy et al. (2019). In this method, examine the terminal 30 cm (12 in) of 

the underside of a maximum of 5 branches per tree on a minimum of 10 trees per site for a total of 50 

hemlock branches per stand. If one or more HWA is present on the branch, the branch is considered 

infested. The total number of branches infested divided by 50 and multiplied by 100 will yield the 

percent of branches infested. 

Perhaps more important than the exact methods used is that managers choose an assessment 

method they have confidence in replicating, and they use it consistently year after year. Additional 

information for evaluating hemlock health and HWA population is available at the HWA Predator 

Database (Virginia Tech 2020). 

CONCLUSION

As noted above, the strategy and tactics offered in this guide can be considered a starting point for managing 

hemlock stands in a more integrated, sustainable way. It is fully expected that as new information, tools, 

and manager experiences become available, this strategy can and will be modified to achieve better results. 

Managers integrating biological and chemical control of HWA are encouraged to share the specific methods 

and outcomes of their efforts with other managers and researchers. In this way, the HWA research and 

management community can continue to collectively improve our efforts to sustain hemlock and the unique 

ecosystems in which they occur. 
INTEGRATING CHEMICAL AND BIOLOGICAL CONTROL OF THE HEMLOCK WOOLLY ADELGID

GLOSSARY

TERM DEFINITION

abiotic Non-living environmental factors, such as temperature and humidity

aestivation Period of dormancy to survive predictable, unfavorable environmental

conditions, such as temperature extremes, drought, or reduced food

availability

asexual reproduction Type of reproduction by which offspring arise from a single organism and inherit 

the genes of that parent only

biological control The reduction in the abundance of a pest through intentional use of its natural 

enemies (predators, parasitoids, and pathogens)

biotic Living environmental factors, such as plants, animals and micro-organisms

chemical control Using pesticides to control pest insects, weeds, or diseases

crown (tree) Top part of a tree which features branches that grow out from the main trunk and 

support the various leaves/needles used for photosynthesis

emergence (insect) Act of adult insect leaving the pupal exoskeleton, or leaving winter or summer 

dormancy

foundation species Species that plays a large role in structuring an entire community

HWA Hemlock woolly adelgid, Adelges tsugae

invasive Tending to spread prolifically and undesirably or harmfully

IPM (Integrated Pest Management) Series of pest management evaluations, decisions, 

and often a combination of control methods to solve pest problems while 

minimizing risks to people and the environment

larva (pl. larvae) Immature stage of some animals, including insects and mites. In insects with 

complete metamorphosis, it is the stage between the egg and pupa (examples 

include grubs, caterpillars, and maggots)

neonicotinoids Neurotoxic compounds with relatively low mammalian toxicity; the most 

commonly used class of insecticides for control of HWA

non-target effect When control efforts affect a species other than the species they were enacted to 

control (can be positive or negative)

nymph Immature form of invertebrates, including some insects such as the HWA, that 

undergoes gradual metamorphosis; resembles adults

organic duff layer Decomposing dead organic material, such as leaves, bark, needles, and twigs, 

that has fallen to the ground, combined with other organic matter

ovisac Waxy sac into which the females of some insects, including the HWA, place their 

eggs

progrediens generation Generation of the HWA which is present in the spring

pupa (pl. pupae; v. pupate) Non-feeding, inactive stage between larva and adult for an insect with complete 

metamorphosis

sistens generation Overwintering generation of the HWA which is present from summer to early 

spring of the following year

specialist predatory insect Insect that preys on only a limited range of species

28 GLOSSARY
INTEGRATING CHEMICAL AND BIOLOGICAL CONTROL OF THE HEMLOCK WOOLLY ADELGID

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APPENDIX

Crown Density-Foliage Transparency Card for Rating Tree Health

1. Print and fold paper at the line between the top and bottom of the card. Trim the other 3 sides. Laminate the card 

if desired. The card can be re-sized if desired.

2. The crown density-foliage transparency card is a training and field aid. White areas of the card represent skylight 

visible through the crown area, and black areas represent a portion of the tree that is blocking skylight. Use the 

card to calibrate visual estimation. 

• For CROWN DENSITY, hold the card so that “Crown Density” is right-side up (“Foliage Transparency” should 

be upside down). Use the numbers that are right-side up. 

• Conversely, for FOLIAGE TRANSPARENCY, make sure that “Foliage Transparency” is right-side up.

3. The back of the card has two uses: 1) adjustments for CROWN DENSITY when a portion of the crown is missing 

and 2) a general scale for estimating UNCOMPACTED LIVE CROWN RATIO. For more information on use, see 

Schomaker et al. (2007).

34 APPENDIX
The Forest Health Technology Enterprise Team (FHTET) was created in 1995 by the Deputy Chief for State and 

Private Forestry, USDA, Forest Service, to develop and deliver technologies to protect and improve the health of 

American forests. FHTET became Forest Health Assessment and Applied Sciences Team (FHAAST) in 2016. This 

booklet was published by FHAAST as part of the technology transfer series.

http://www.fs.fed.us/foresthealth/technology/

References to pesticides appear in this publication. These statements do not constitute 

endorsement or recommendation of them by the U.S. Department of Agriculture, nor do 

they imply that the uses discussed have been registered. Use of most pesticides is regulated 

by state and federal laws. Applicable regulations must be obtained from the appropriate 

regulatory agency prior to their use.

CAUTION: Pesticides can be injurious to humans, domestic animals, desirable plants, and 

fish or other wildlife if they are not handled or applied properly. Use all pesticides selectively 

and carefully. Follow recommended practices given on the label for the use and disposal of 

pesticides and pesticide containers.

The use of trade, firm, or corporation names in this publication is for the information and convenience of the reader. 

Such use does not constitute an official endorsement or approval by the U.S. Department of Agriculture or the Forest 

Service of any product or service to the exclusion of others that may be suitable.

In accordance with Federal civil rights law and U.S. Department of Agriculture (USDA) civil rights regulations and policies, the 

USDA, its Agencies, offices, and employees, and institutions participating in or administering USDA programs are prohibited 

from discriminating based on race, color, national origin, religion, sex, gender identity (including gender expression), sexual 

orientation, disability, age, marital status, family/parental status, income derived from a public assistance program, political 

beliefs, or reprisal or retaliation for prior civil rights activity, in any program or activity conducted or funded by USDA (not all 

bases apply to all programs). Remedies and complaint filing deadlines vary by program or incident. 

Persons with disabilities who require alternative means of communication for program information (e.g., Braille, large print, 

audiotape, American Sign Language, etc.) should contact the responsible Agency or USDA’s TARGET Center at (202) 720-2600 

(voice and TTY) or contact USDA through the Federal Relay Service at (800) 877-8339. Additionally, program information may 

be made available in languages other than English. 

To file a program discrimination complaint, complete the USDA Program Discrimination Complaint Form, AD-3027, found online 

at http://www.ascr.usda.gov/complaint_filing_cust.html and at any USDA office or write a letter addressed to USDA and provide 

in the letter all of the information requested in the form. To request a copy of the complaint form, call (866) 632-9992. Submit your 

completed form or letter to USDA by: (1) mail: U.S. Department of Agriculture, Office of the Assistant Secretary for Civil Rights, 

1400 Independence Avenue, SW, Washington, D.C. 20250-9410; (2) fax: (202) 690-7442; or (3) email: program.intake@usda.gov. 

USDA is an equal opportunity provider, employer, and lender.