The Douglas-fir tussock moth

(Orgyia pseudotsugata McDunnough) is an important defoliator

of true firs and Douglas-fir in

Wesern North America. Severe

tussock moth outbreaks have ocForest Insect

& Disease

Leaflet 86

U.S. Department

of Agriculture

Forest Service

Douglas-Fir Tussock Moth

Boyd E. Wickman, Richard R. Mason and Galen C. Trestle1

curred in British Columbia, Idaho,

Washington, Oregon, Nevada, California, Arizona, and New Mexico,

but the area subject to attack is more

extensive (fig. 1).

 Outbreaks of the Douglas-fir tussock moth appear to develop almost

explosively, and then usually subside

abruptly after a year or two. Some

outbreaks in New Mexico, however,

have persisted in the same general

area for as long as 7 years. During a

severe outbreak, caterpillars are found

crawling on the ground or on

1

Boyd E. Wickman and Richard R.

Mason are principal insect ecologists,

Pacific Northwest Forest and Range Experiment Station, Forest Service, U.S.

Department of Agriculture. Galen C.

Trostle is retired; he was entomologist,

Pacific Northwest Region, Forest Service, U.S. Department of Agriculture,

at the time of this study.
Outbreaks of the tussock moth have

caused similar serious damage in British Columbia, Idaho, California, and

New Mexico (fig. 2).

Hosts

The tussock moth has three preferred hosts, and its preference appears

to depend on locality. In the northern

part of its range (British Columbia and

northern Washington) Douglas-fir is

preferred; in the central area (southern Washington, Oregon, and Idaho),

Douglas-fir, white fir, and grand fir are

all equally acceptable. In the south

(California, Nevada, Arizona, and

Figure 1. -- Distrubution of host type where

Douglas-fir tussock moth may be found

and location of outbreaks.

trees, brush, and buildings, but once

an outbreak subsides, finding caterpillars is difficult.

Defoliation by the tussock moth

kills or top-kills many trees, weakens

additional trees that are eventually

killed by bark beetles, and retards tree

growth for several years. For example,

a very large outbreak in the Blue

Mountains of Oregon and Washington killed 39 percent of all trees in the

heavily defoliated areas. Within these

areas were patches where nearly all the

trees died. Top-kill in the heavily defoliated areas amounted to 10 percent

of the grand fir and 33 percent of the

Douglas-fir.

2

Figure 2.-- A stand of white fir in northeastern California heavily damaged by

defoliation from the Douglas-fir tussock

moth.

 F-701857
New Mexico), white fir is the preferred

host.

In any locality, after the caterpillars

have eaten the preferred foliage, they

feed on many other trees and understory shrubs. Larvae have been found

feeding on subalpine fir, red fir, ponderosa pine, Jeffrey pine, sugar pine,

lodgepole pine, western hemlock, Engelmann spruce, and western larch after the preferred hosts have been

stripped.

The insect also infests ornamental

spruce and fir trees in many montane

cities and towns. Some infestations

occur on ornamental spruce many

miles from the nearest host forests

known to be infested with tussock

moth populations.

Damage

Usually the first indication of attack

appears in late spring. Larvae from

newly hatched eggs feed on current

year’s foliage, causing it to shrivel and

turn brown (fig. 3). These larvae are

inconspicuous, but by mid-July they

are larger, more colorful,

Figure 4.--Defoliation occurs first in the tops

of trees and outermost branches where new

foliage is located.

Figure 3.--Young tussock moth larvae feed

first on the undersides of new foliage,

causing it to shrivel and die.

and more easily visible. At that time,

they may feed on both current and old

foliage, although current needles are

preferred. Defoliation occurs first in

the tops of trees and the outermost portions of the branches, and then in the

lower crown and farther back on the

branches (fig. 4).

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F-701859

By August, as the larvae reach pupation, the upper crowns of most of

the firs may be completely bare (fig.

5). At this time, large numbers of colorful caterpillars drop from defoliated

trees and crawl almost everywhere.

The brown tips, bare twigs, and damaged needles give the stands a brown,

dead appearance (fig. 6).

.

3
Description

The adult male, as shown on the

cover of this publication, is an ordinary looking gray-brown to

black-brown moth with feathery antennae and a wingspread of 1 to 1 1/4

inches (2.5-3.1 cm). The forewings are

gray brown and have two indistinct,

irregular dark bars and two vague whitish spots. The hindwings are a contrasting brown.

The female (fig. 7) is very different

in appearance, having tiny rudimentary wings, small threadlike antennae,

and a large abdomen. She is usually

about 3/4 inch (1.9 cm) long and is

grayish in color. Her abdomen is conspicuously darker at the tip because of

a dense coat of exceptionally long, dark

hairs. Her eggs are deposited in a mass

on top of the cocoon from which she

emerged. The 1-inch-long (2.5-cm)

masses have a gray mottled appearance

(fig 8).

Figure 5.--By late summer, dense larval

populations can completely strip the upper

crown of foliage.

Figure 7.--The adult female is unable to fly

and remains on top of her cocoon.

Figure 6.--A stand in the Blue Mountains

severely defoliated by the tussock moth.

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4

F-701861 F-701863
F-701863

colored tussocks are located forward

along the middle of the back (fig. 10).

The rest of the body, except for the

legs and head, is covered with short

hairs radiating from red, buttonlike

centers.

Larvae may vary from light to dark,

but two main color variations are especially noticeable in mature

larvae--light cream or “blonde” and

dark brown or “brunette.” Larval body

hairs irritate the skin of some people.

Those working in the woods during an

outbreak may develop an itching rash

from contact with the larvae or airborne caterpillar hairs. Figure 8.--Eggs are deposited in a mass on

top of female cocoons.

Young larvae are 1/8 to 1/4 inch (4-7

mm) long and have long, fine body

hairs which later develop into tufts (fig.

9).

Mature larvae are up to 1-1/4 inches

(3.1 cm) long and very colorful. Two

long, dark tufts or pencils of hair similar to horns are located right behind

the head; a similar but longer pencil is

on the posterior. Four dense, buffF-701864

Figure 10.--Mature larva feeding on fir

foliage.

Figure 9.--Young larvae are very small and

have long, fine body hairs.

Life Cycle

The tussock moth produces one generation each year. Adults appear from

late July into November, depending on

the season and location. Male flight

activity is apparently stimulated by

light and temperature. It begins about

midday, peaks in late afternoon, and

rapidly diminishes at dusk.

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F-701866

5
The female emits a sex pheromone

that attracts males during their flight

period. Females mate soon after they

emerge from their pupal cocoon. They

cling tenaciously to the outside of the

cocoon and lay eggs on its surface immediately after mating. In light infestations, cocoons with their attached

egg masses usually are scattered on the

foliated twigs throughout the tree

crowns. In heavy infestations, they are

concentrated on the lower parts of

trees. Where defoliation is severe, egg

masses on cocoons may be bunched

on the trunk, on the lower side of larger

limbs, or on objects some distance

from the tree.

Each female lays her eggs in a single

mass (about l/2 inch (1.3 cm) in diameter) of a dry, tough, frothy, substance

containing many hairs from her body.

She may lay only a few, or as many as

350 nearly spherical, white eggs, in

one to three layers in this mass. The

number of eggs per mass varies with

geographical location, but on the average, egg masses from the northern

part of the moth’s range contain fewer

eggs than those from the southern part.

After egg laying is complete, the female dies, leaving the eggs to overwinter in the gray, woolly mass attached to the cocoon.

Egg hatch in late May or early June

coincides with bud burst and shoot

elongation of the host trees. One to

seven days after the eggs hatch, the

tiny, hairy caterpillars crawl to new

needle growth and begin feeding.

Since the female moth does not fly,

major dispersal is by windborne young

larvae. When the larvae drop off foliage, they produce long silk threads

that, combined with their hair and light

weight, allow them to be carried by

air currents. The distance most caterpillars travel, however, rarely exceeds

500 m (0.25 mi). Any longer range dispersal by a few individuals does not

produce new outbreaks.

Larvae growth is slow at first, but

becomes progressively faster as larvae

pass through four to six moults. They

also eat proportionately more. They

require current year’s foliage when

young, but once they are about half

grown, they can complete their development on a diet of old foliage.

Pupation occurs any time from late

July to the end of August inside a thin

cocoon of silken webbing mixed with

larval hairs. The pupal stage lasts from

10 to 18 days, depending on temperature. Then the moth emerges to begin

the life cycle again.

Natural Control

Many natural controls exist that

keep the number of tussock moths low

most of the time. In populations that

persist at low densities, over 90 percent of the larvae and at least 75 percent of the pupae and eggs are killed

each generation by natural causes. If

such mortality does not take place, the

population will increase rapidly.

Overwintering eggs are usually

heavily parasitized by small

6
wasps (fig. 11). Birds, such as the

mountain chickadee and redbreasted

nuthatch, also feed on tussock moth

eggs and may destroy a large portion

of the overwintering masses.

Figure 11.--A small wasp, Teleneomus

californicus (Ashmead), ovipositing in tussock moth eggs in the laboratory.

Figure 12.--A parasite fly, Carcelia yalensis,

lays eggs on the back of a mature larva.

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After the surviving eggs hatch, some

young larvae are lost while dispersing

from the egg mass to feeding sites on

new foliage. Others are eaten by birds,

spiders, and predaceous insects. Insect

parasites attack all ages of larvae, but

a parasitic fly, Carcelia yalensis Sellers, is an especially effective enemy

(fig. 12). It lays its eggs on the backs

of mature tussock moth larvae. The

maggot enters the caterpillar’s body

and eventually kills the host in the

pupal stage. Pupae are also killed by a

variety of wasplike parasites that attack the cocoon directly. Birds also

prey on cocoons and can destroy a significant proportion of the pupal population at low tussock moth densities.

F-701868

At outbreak numbers, tussock moth

populations have escaped their usual

natural controls and have been affected

by additional mortality factors related

to their high numbers. A nuclear polyhedrosis virus capable of wiping out

large numbers of larvae and pupae usually appears only in relatively dense

populations. Its presence, in combination with other mortality factors, frequently causes collapse of outbreak

populations. When diseased larvae die,

their internal organs liquefy (fig. 13).

The virus spreads through the population when these bodies fall to the

ground and rupture or lie smeared over

the foliage. Virus particles may persist in the environment for many years

thereafter.

7
vention. These activities must be well

integrated to insure adequate protection from the pest.

Defoliation of trees is easily detected

from the ground or air when outbreaks

occur. To prevent damage caused by

such defoliation, tussock moth populations that are increasing in density

need to be detected early and controlled before tree defoliation occurs.

Population trends can be determined

by annual monitoring of adult males

or larvae. Adult numbers are evaluated

by attracting flying males to sticky

traps with a sex pheromone. The number of larvae in the population can be

estimated by sampling foliage in the

lower crown of host trees. Therefore,

if an outbreak is developing, it can be

detected 1 or 2 years early by an increase in the number of insects, so that

suppression, if needed, can be initiated

before significant damage to trees occurs.

The need to suppress a tussock moth

population depends on the impact an

uncontrolled outbreak is expected to

have on the forest. Decisions for control must be based on a thorough evaluation of the potential of the insect

population and the resource values at

stake. Models of insect population

behavior, stand dynamics, and economic variables are available for use

in integrated pest management systems.

The prevention of outbreaks is the

ultimate aim of pest management.

Much additional knowledge of

insect-host relations and stand dynamics, however, is needed before silvicultural systems can be

Figure 13.--Larvae killed by natural virus hang

head downward from the foliage.

In the absence of viral disease, outbreak populations of tussock moth are

ultimately limited by the quantity and

quality of available food. Early defoliation of the current year’s needles

eliminates the preferred food and

forces larvae to feed on less nutritious

older needles. This causes starvation

and lowered production and survival

of eggs, and helps reduce population

numbers to a level where they are

again regulated by the usual natural

enemies.

Applied Control

Management recommendations to

prevent tree damage from tussock

moth outbreaks involve four major

activities: early detection, evaluation,

suppression, and pre

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8
prescribed to reduce susceptibility of

host stands. There is some indication

that fir growing in pine sites and fir

stands growing on warm, dry sites are

most susceptible to Douglas-fir tussock moth damage. Where offsite fir

is well established and conversion to

the proper tree species would be uneconomical, some form of annual

population monitoring should be considered to detect increases in population numbers. The preferred way to

keep losses low is to work toward

healthy, thrifty stands, growing on a

proper site.

Two materials are registered and

available that might be used to reduce

outbreak population numbers. Carbaryl is the only chemical presently

registered for aerial application against

the tussock moth. The microbial pesticide Bacillus thuringiensis (B.t.) is a

commercially available material that

is not hazardous to most beneficial

insects, birds, small mammals, and

aquatic systems. However, results with

B.t. against the tussock moth have not

been consistent. If control is necessary,

a State or Federal pest control specialist should be consulted for current recommendations.

Caution: Pesticides used improperly

can be injurious to man, animals, and

plants. Follow the directions and heed

all precautions on the 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 when 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 container.

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.

Do not clean spray equipment or

dump excess spray material near

ponds, streams, or wells. Because it is

difficult to remove all traces of herbicides from equipment, do not use the

same equipment for insecticides or

fungicides that you use for herbicides.

Dispose of empty pesticide containers promptly. Have them buried at a

sanitary land-fill dump, or crush and

bury them in a level, isolated place.

NOTE: Some States have restrictions on the use of certain pesticides.

Check your State and local regulations. Also, because registrations of

pesticides are under constant review

by the Federal Environmental Protection Agency, consult your county

agricultural agent or State exten

9
sion specialist to be sure the intended

use is still registered.

The United States Department of

Agriculture has issued a series of

Douglas-fir tussock moth handbooks

and additional issues are in press. Copies can be obtained from the U.S. Government Printing Office.

References

Beckwith, R. C.

 1976. Influence of host foliage on the

 Douglas-fir tussock moth. Environmental

 Entomology 5(1):73-77.

Brookes, M. H., R. W. Stark, and R. W.

Campbell (editors).

 1978. The Douglas-fir tussock moth: A

 synthesis. U.S. Department of Agriculture

 Technical Bulletin 1585, 331 p.

Campbell, Robert W., and Lorna C.

Youngs.

 1978. Douglas-fir tussock moth: An anno-

 tated bibliography. U.S. Department of Ag

 riculture Forest Service General Technical

 Report PNW-68, 168 p. Pacific Northwest

 Forest and Range Experiment Station, Port

 land, Oreg.

Daterman, G. E., Linda J. Peterson,

Richard G. Robbins, Lonne L Sower,

G. Doyle Daves, Jr., and Ronald G. Smith.

 1976. Laboratory and field bioassay of the

 Douglas-fir tussock moth pheromone,

 (Z)-6-heneicosen-11-one Environmental

 Entomology 5(6):1187-1190.

Mason, R. R.

 1976. Life tables for a declining population

 of the Douglas-fir tussock moth in north

 eastern Oregon. Annals of the Entomologi

 cal Society of America 69(5): 948-958.

Mason, R. R.

 1978. Tussock moth life tables. In

 McGraw-Hill Yearbook of Science and

 Technology, p. 181-183. McGraw-Hill

 Book Co., New York.

Mason, Richard R.

 1978. Detecting suboutbreak populations of

 the Douglas-fir tussock moth by sequential

 sampling of early larvae in the lower tree

 crown. U.S. Department of Agriculture For

 est Service Research Paper PNW-238, 9 p.

 Pacific Northwest Forest and Range Experi

 ment Station, Portland, Oreg.

Mitchell, R. G.

 1979. Dispersal of early instars of the

 Douglas-fir tussock moth. Annals of the En-

 tomological Society of America

 72(2):291-297.

Torgersen, Torolf R.

 1977. Identification of parasites of the

 Douglas-fir tussock moth based on adults,

 cocoons, and puparia. U.S. Department of

 Agriculture Forest Service Research Paper

 PNW-215, 28 p. Pacific Northwest Forest

 and Range Experiment Station, Portland,

 Oreg.

Wickman, B. E., R. R. Mason, and H. G.

Paul.

 1975. Flight, attraction, and mating behav-

 ior of the Douglas-fir tussock moth in

 Oregon. Environmental Entomology

 4(3):405-408.

Wickman, Boyd E.

 1978. Phenology of tussock moth. In

 McGraw-Hill Yearbook of Science and

 Technology, p. 183-185. McGraw-Hill

 Book Co., New York.

Wickman, Boyd E.

 1978. Tree mortality and top-kill related to

 defoliation by the Douglas-fir tussock moth

 in the Blue Mountains outbreak. U.S. De-

 partment of Agriculture Forest Service Re-

 search Paper PNW233, 47 p. Pacific North-

 west Forest and Range Experiment Station,

 Portland, Oreg.

10

Revised January 1981

*U.S. GOVERNMENT PRINTING OFFICE:1980O-327-869