of trees is common in dense, overstocked

stands of pure, even-aged, young sawtimber (fig. 1), but also occurs among dense

clumps of pine in stagnating mixed-conifer

stands. One million or more trees containing more than 1 billion board feet of

timber may be killed each year during an

outbreak. Such extensive tree killing may

deplete timber supplies, adversely affect

levels and distributions of stocking, disrupt management planning and operations, and increase forest fire danger by

adding to available fuels.

Western Pine Beetle

Clarence J. DeMars, Jr.,1 and Bruce H. Roettgering2

Usually, the beetles breed in and kill

scattered, overmature, slowgrowing,

decadent, or diseased trees and trees

weakened by stand stagnation, lighting, fire, or mechanical inU.S. Department

of Agriculture

Forest Service

Forest Insect

& Disease

Leaflet 1

F--703000

Figure 1.—Group tree killing by western

pine beetle in a dense, young-growth

ponderosa pine stand.

The western pine beetle,

Dendroctonus brevicomis LeConte,

can aggressively attack and kill ponderosa and Coulter pine trees of all ages

and vigor classes that are 6 inches (15

cm) or larger in diameter, including apparently healthy trees. Group killing

1

Research Entomologist, U.S. Department of Agriculture, Forest Service, Pacific Southwest Forest and

Range Experiment Station, Berkeley, Calif.

2

Supervisory Entomologist, U.S. Department of Agriculture, Forest Service, Pacific Southwest Region,

San Francisco, Calif.

F--703001
jury. This tree mortality need not be a loss,

but may be considered part of the normal

ecological process of succession through

which a forest matures and replaces itself. Some of the trees killed under these

conditions, however, may be on residential or recreation sites. These highly prized

trees are often impossible to replace and

expensive to remove.

The western pine beetle is most damaging in California, but its range extends

northward into Oregon, Washington,

Idaho, and southern British Columbia;

eastward into Montana, Nevada, Utah,

Colorado, Arizona, New Mexico, and

western Texas; and southward into northwestern Mexico (fig. 2). It is commonly

found in forests with a midelevation level

between 2,000 and 6,000 feet (600 to

1,800 m). In Canada, beetle populations

can be found below 1,000 feet (300 m)

and in Mexico above 8,000 feet (2,400

m).

Hosts, Damage, and Evidence of

Attack

Normal attack and development occur

only in ponderosa and Coulter pine. Pitch

tubes, 1/4 to1/2 inch (6 to 13 mm) in diameter, formed on the tree trunk around

entry holes made by attacking female

beetles (fig. 3), are usually the first evidence of infestation. The pitch tubes are

white to red-brown masses of resin and

boring dust found in the crevices between

the bark plates. Relatively few, widely

scattered, white pitch tubes, 1 inch (25

mm) or larger in diameter, usually indicate that the attacks were not successful

and that the tree will survive. Close examination of successfully attacked trees,

however, reveals small, reddish-brown

Figure 2.—Range of the western

pine beetle.

pitchtubes and dry, reddish-brown boring

dust in the bark crevices and around the

base of the tree.

The western pine beetle initially attacks

midway up the tree, but the first sign of attack may often be predaceous checkered

beetles seen at eye level scurrying over the

bark in search of western pine beetle prey.

Attacking adult beetles carry spores of a

blue-staining fungus, Ceratocystis minor

(Hedg.) Hunt, in special pouch-like structures in their heads called mycangia. As the

beetles chew their way through the bark,

the spores of this wilt-causing fungus dislodge and begin to germinate. In trees

2
Figure 4.—To feed on a western pine beetle

brood, woodpeckers have stripped off the outer

bark of the tree, exposing the orange inner bark.

attacked in early or midsummer, it takes

only a few weeks for the fungus to invade and block the conductive vessels

of the inner bark and sapwood. Once

the vessels are blocked, the foliage begins to fade, first to a pale green and

then to yellow, straw, and sorrel. Finally, perhaps after a year, the foliage

may turn red brown. This fading is the

first evidence of damage to the tree that

is visible at a distance.

In trees attacked in late summer or

fall, the fungus develops more slowly,

and many infested trees do not fade until

the next spring. These green trees are

often first seen to be infested when

woodpeckers flake off the outer bark

as they search for and feed on developing beetle larvae. This flaking exposes

the bright-orange inner bark, making

these trees visible from as far away as

300 feet (100 m) (fig. 4.).

Figure 3.—Pitch tubes made by the western

pine beetle: (A) unsuccessfu/ attack, a

pitched-out beetle (ejected by resin flow) stuck

to underside of pitch tube; and (B) successful

attack, a small, red-brown tube with c/ear

opening.

F--703004

3

F--703003 B

A F--703002
Western pine beetle infestation of a

dying pine can be confirmed by removing the bark to expose the winding,

crisscrossing egg galleries in the inner

bark and on the surface of the sapwood

(fig. 5). The egg galleries are slightly

wider than the adult beetles that construct them and are usually tightly

packed with boring dust. The western

pine beetle is the only bark beetle that

makes this particular type of egg gallery in ponderosa and Coulter pines.

Figure 5.—Sinuous egg galleries characteristics of the western pine beetle in ponderosa

and Coulter pine.

Life Stages and Development

Western pine beetles pass through the

egg, larval, pupal, and adult stages

during a life cycle that varies in length

from about 2 months in warm weather

to 10 months in cool weather. All stages

are completed beneath or in the bark of

infested

trees, except for a brief period when the

adults fly to find new trees to attack.

In the northern part of their range and

at higher elevations, the beetles produce

two generations each year, with attacks in

early June and late August. In the South

and at lower elevations, the beetles

produce three and sometimes four

generations each year, with attacks as

early as March and as late asNovember.

During an attack period, which may

last 3 weeks, each female lays about 60

tiny pearl-white eggs individually in

niches cut into the sides of the egg

gallery (fig. 6A). Some of these parent

females may emerge and reattack to

establish additional galleries elsewhere

in the same tree or in other host trees.

After incubating from 1 to 2 weeks, the

eggs hatch.

The larvae are small white grubs that

feed first in the phloem, where they construct a short gallery. They then mine into

the middle bark where most of their development takes place (fig. 6B). After

completing four larval stages, they transform into pupae and then into adults. As

these brood adults feed on the middle and

outer bark, fungal spores collect in their

mycangia. Once the adult insects emerge,

they are ready to renew the

attack-infestation cycle in living trees.

When the female beetles successfully

attack a tree, they release minute amounts

of behavioral chemicals into the air. These

odors (pheromones) attract males and

other females to the tree, causing a mass

attack that tends to overcome the tree’s

natural resistance. If numerous beetles

are flying and fair weather persists, adjacent trees may be attacked, resulting in

a group of infested trees. Usually, an

F--703005

4
5

Besides attracting western pine

beetles themselves, the pheromones also

attract their natural enemies, such as

predaceous checkered and ostomid

beetles. The ability of these beetles to

sense the pheromones makes them effective predators during the critical attack phase.

Conditions Affecting Outbreaks

Several conditions often work together to influence the number of

beetles and the beetle-caused tree mortality in a given area. The significant

conditions follow.

Food supply.—The availability of

suitable host material—phloem and inner bark—is a key condition influencing

western pine beetle outbreaks. Most trees

are either too healthy or too weak to provide material in which beetle numbers can

increase. Healthy trees can withstand

many attacks before the beetles are successful, the brood is established, and new

adult beetles are produced. Weak trees,

such as those that have been smog damaged, diseased, or suppressed by competition, although easily killed, also produce

relatively few beetles.

The thick, nutritious phloem and inner bark of healthy trees become host

material for attacking western pine

beetles when these trees undergo sudden and severe moisture stress. Healthy

trees ordinarily produce abundant

amounts of resin, which pitch out or

eject attacking beetles. But, when suddenly deprived of moisture, stressed

trees cannot produce sufficient resin

flow to resist attack, and their nutritious food supply becomes available to

beetles. In these trees, almost all attacking beetles can succeed and reproduce

Figure 6.—Life stages of the western pine beetle:

(A) adult and eggs five times the natural size and

(B) midstage larvae, natural size.

insufficient number of beetles are

flying, or bad weather delays the

arrival of more beetles to the vicinity

of the newly attacked tree. Either of

these conditions will usually result in

the typically isolated dead tree.

B F--703007

A F--703006
many times their number of offspring,

increasing the beetle population to outbreak levels.

Moisture stress results when the

water balance between the foliage and

the roots changes dramatically. An imbalance may result from increased water loss from the needles (transpiration),

decreased water uptake by the roots

(physiological drought), or from a combination of the two.

Any condition that results in excessive

demand for moisture, such as tree crowding, competing vegetation, or sudden

exposure to severe sunlight; or any condition that reduces the ability of the roots

to supply water to the tree, such as mechanical root damage, root disease, soil

compaction, or drought, can cause moisture stress and increase susceptibility to

attack by the western pine beetle.

Loss of attacking beetles.—Flying adult

beetles may fail to locate a suitable host tree

or may fail to initiate attack once they arrive. Bark beetle losses during this period,

which include losses from predation by

checkered beetles, are nearly impossible to

measure accurately, but they appear to be

considerable.

Tree resistance.—Healthy pines produce

sufficient resin flow at the attack site to (1)

pitch out (eject) the beetles or (2) soak the

phloem tissue surrounding the egg galleries

with resin. The trees thereby inhibit larval

and fungal development.

Natural enemies.—More than 80 species of predaceous and parasitic insects have

been found in bark infested by the western

pine beetle, but the nature of the relationships among them is largely unknown. Two

important species of checkered beetles

Figure 7.—Adult, black-bellied clerics

(Enoclerus lecontei) feed on western pine

beetles attacking the tree. Two times the

natural size.

6

Common parasites are Roptrocerus

xylophagorum Ratzeburg, Dinotiscus

burkei Crawford, and Coeloides sp. nr.

brunneri Vierick.

Woodpeckers remove the outer bark

from infested trees to feed on the larvae. Larvae that are not eaten are left

with only a thin layer of protective bark,

increasing their susceptibility to desiccation and parasitism.

Woodpeckers, predators, and parasites play a significant role in reducing

the number of the developing brood

within a tree. Although they help stabilize conditions at low beetle population levels, their action alone cannot

control outbreaks.

F--703008

known to prey on the western pine

beetle are Enoclerus lecontei (Wolcott)

(fig. 7) and Enoclerus sphegus Fabricius. The blue-green ostomid

Temnochila chlorodia Mannerheim

and the fly Medetera aldrichii Wheeler

are also important predators of the

western pine beetle.
Cold temperatures.—Winter temperatures below - 20 ° F ( - 27 °C) and

persisting for several days can cause

heavy brood mortality in those portions

of the tree above the snowline. Effects

are only temporary, however, and after

a few generations, the population usually recovers.

Control

Landowners have two basic alternatives when choosing the control strategy most appropriate for their needs:

beetle population suppression and damage prevention.

Suppression.—Over the years, several

suppression methods have been tried to

help reduce beetle populations enough to

lower tree mortality significantly. These

methods have included the removal of

infested trees by logging, felling infested

trees and peeling off and burning the bark,

and felling infested trees and applying

toxic residual sprays to kill emerging

beetles. Because adult beetles can fly

many miles and produce many offspring,

effective suppression methods require the

location (spotting) and treatment of all,

or nearly all, infested trees over extensive areas in a short period of time.

Timely spotting and treatment are difficult and expensive tasks that require cooperation among many landowners. Consequently, the results have often been unsatisfactory. Also, these projects have failed because the basic underlying cause for the

population outbreak—an abundance of

stressed trees—has not changed. Typically,

if a habitat favorable to high-level western

pine beetle populations persists, suppression—by whatever means—will probably

fail to reduce tree mortality significantly.

Prevention.—Where ponderosa and

Coulter pine contribute appreciably to land

value, such as in commercial forests, developed recreation sites, and urban forests, preventing tree killing by the western pine beetle

is often more appropriate than attempting

to suppress beetle populations. Landowners can prevent unacceptable damage on

their land by maintaining thrifty, vigorous

trees or stands that do not afford a suitable

food supply for the beetle.

In mature pine forests east of the

Cascade-Sierra Nevada crest, removal of

trees with a great chance of damage by

beetles in a program known as sanitation

cutting can salvage valuable timber that

might otherwise be lost if the trees were to

die gradually. Trees with a high risk of damage by beetles characteristically have poor

vigor and can be recognized by crown symptoms such as dead tops, branches, and twigs

and short, sparse, poorly colored foliage.

Also, they may be older, slow-growing trees

that are heavily infected with dwarf

mistletoe, that are root diseased, or that

have been struck by lightning.

The thinning of dense, 70- to

80year-old sawtimber stands is an effective silvicultural method for managers of both small and large holdings.

Reducing stand stocking to 55 to 70

percent of the basal area needed for full

site utilization will relieve the competitive stress among the remaining trees,

improve their vigor, and make them less

prone to successful bark beetle attack.

Prevention can also take the form

of minimizing injury or disturbance to

individual trees or sites. Careful logging practices and care in develop7
ing urban forest land are simple, yet

effective, ways to prevent damage by

western pine beetles.

Individual tress of particularly high

value that are predisposed to beetle attack by temporary injury or disturbance

may be given a protective residual bark

spray to prevent successful attack. If

protection for 1 or 2 years would allow the tree to regain its normal vigor,

such spraying may be viable. The

sprays are costly, however, and should

not be considered if trees have a severe

root disease, are badly injured by construction, or are seriously damaged in

other ways.

Insecticide use is governed by the

provisions of the Federal Insecticide,

Fungicide, and Rodenticide Act, as

amended. This act is administered by

the Environmental Protection Agency.

Persons contemplating insecticide use

should obtain the names of materials

currently registered. Private landowners can get information from a Cooperative Extension agent at their

land-grant college, State agricultural

experiment station, county Extension

office, or their local State forestry office. Federal resource managers contemplating action against the western

pine beetle should contact the Forest

Pest Management Staff, Forest Service,

U.S. Department of Agriculture, for assistance and advice.

References

Koehler, C. S.; Wood, D. L.; Scarlett, A. L. Bark

beetles in California trees. Coop. Ext. Leafl.

21034. Berkeley, CA: University of California,

Cooperative Extension Service; 1978. 8 p.

Koerber, T. W., comp. Lindane in forestry . . . a continuing controversy. Gen. Tech. Rep. PSW-14.

Berkeley, CA: U.S. Department of Agriculture,

Forest Service, Pacific Southwest Forest and

Range Experiment Station; 1976. 30 p.

Miller, J. M.; Keen, F. P. Biology and control of the

western pine beetle. Misc. Publ. 800. Washington, DC: U.S. Department of Agriculture; 1960.

381 p.

Smith, R. H.; Trostle, G. C.; McCambridge, W. F.

Protective spray tests on three species of bark

beetles in the Western United States. J. Econ.

Entomol. 70(1): 119-125; 1977.

Stark, R. W.; Dahlsten, D. L., eds. Studies on the

population dynamics of the western pine beetle,

Dendroctonus brevicomis LeConte (Coleoptera:

Scolytidae). Berkeley, CA: University of California, Division of Agricultural Science; 1970. 174

p.

Pesticides used improperly can be injurious to human beings,

animals, and plants. Follow the directions and heed all precautions on

labels. Store pesticides in original containers under lock and key--out of

the reach of children and animals--and away from food and feed.

Apply pesticides so that they do not endanger humans, livestock,

crops, beneficial insects, fish, and wildlife. Do not apply pesticides where

there is danger of drift when honey bees or other pollinating insects are

visiting plants, or in ways that may contaminate water or leave illegal

residues.

Avoid prolonged inhalation of pesticide sprays or dusts; wear

protective clothing and equipment, if specified on the label.

If your hands become contaminated with a pesticide, do not eat or

drink until you have washed. In case a pesticide is swallowed or gets in

the eyes, follow the first aid treatment given on the label, and get prompt

medical attention. If a pesticide is spilled on your skin or clothing, remove

clothing immediately and wash skin thoroughly.

NOTE: Some States have restrictions on the

use of certain pesticides. Check your Store and

local regulations. Also, because registrations

of pesticides are under constant review by the

U.S Environmental Protection Agency, consult your local forest pathologist, county agriculture agent, or State extension specialist

to be sure the intended use is still registered. Revised

* GPO: 1983 0 - 416-953 November 1 982