Forest & Disease Leaflet 86

U.S. Department of Agriculture Forest Service Douglas- Tussock Boyd E. Wickman, Richard R. Mason and Galen C. Trestle1

The Douglas-fir tussock moth curred in British Columbia, , ( pseudotsugata McDun- , , , Cali- nough) is an important defoliator fornia, , and , of true and Douglas-fir in but the area subject to attack is more Wesern North America. Severe extensive (fig. 1). tussock moth outbreaks have oc- Outbreaks of the Douglas-fir tus- sock moth appear to develop almost 1Boyd E. Wickman and Richard R. explosively, and then usually subside Mason are principal insect ecologists, abruptly after a year or two. Some Pacific Northwest Forest and Range Ex- outbreaks in New Mexico, however, periment Station, Forest Service, U.S. have persisted in the same general Department of Agriculture. Galen C. Trostle is retired; he was entomologist, area for as long as 7 years. During a Pacific Northwest Region, Forest Ser- severe outbreak, caterpillars are found vice, U.S. Department of Agriculture, crawling on the ground or on at the time of this study. Outbreaks of the tussock moth have caused similar serious damage in Brit- ish Columbia, Idaho, , and New Mexico (fig. 2).

Hosts The tussock moth has three pre- ferred 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 (south- ern Washington, Oregon, and Idaho), Douglas-fir, white fir, and grand fir are all equally acceptable. In the south (California, Nevada, Arizona, and

Figure 2.-- A stand of white fir in north- eastern California heavily damaged by defoliation from the Douglas-fir tussock moth. Figure 1. -- Distrubution of host type where F-701857 Douglas-fir tussock moth may be found and location of outbreaks. trees, brush, and buildings, but once an outbreak subsides, finding caterpil- lars 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 Washing- ton 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 de- foliated areas amounted to 10 percent of the grand fir and 33 percent of the Douglas-fir.

2 New Mexico), white fir is the preferred and more easily visible. At that time, host. they may feed on both current and old In any locality, after the caterpillars foliage, although current needles are have eaten the preferred foliage, they preferred. Defoliation occurs first in feed on many other trees and under- the tops of trees and the outermost por- story shrubs. Larvae have been found tions of the branches, and then in the feeding on subalpine fir, red fir, pon- lower crown and farther back on the derosa pine, Jeffrey pine, sugar pine, branches (fig. 4). lodgepole pine, western hemlock, En- gelmann spruce, and western larch af- ter 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,

F-701859 Figure 4.--Defoliation occurs first in the tops of trees and outermost branches where new foliage is located.

By August, as the larvae reach pu- pation, the upper crowns of most of the firs may be completely bare (fig. 5). At this time, large numbers of col- orful caterpillars drop from defoliated F-701858 trees and crawl almost everywhere. The brown tips, bare twigs, and dam- Figure 3.--Young tussock moth larvae feed aged needles give the stands a brown, first on the undersides of new foliage, causing it to shrivel and die. dead appearance (fig. 6). 3 . Description

The adult male, as shown on the cover of this publication, is an ordi- nary looking gray-brown to black-brown moth with feathery anten- nae 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 whit- ish spots. The hindwings are a contrast- ing brown. The female (fig. 7) is very different in appearance, having tiny rudimen- tary 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 con- spicuously darker at the tip because of F-701860 a dense coat of exceptionally long, dark Figure 5.--By late summer, dense larval hairs. Her eggs are deposited in a mass populations can completely strip the upper on top of the cocoon from which she crown of foliage. emerged. The 1-inch-long (2.5-cm) masses have a gray mottled appearance (fig 8).

F-701861 F-701863 Figure 6.--A stand in the Blue Mountains Figure 7.--The adult female is unable to fly severely defoliated by the tussock moth. and remains on top of her cocoon.

4 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 es- pecially 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 air- F-701864 Figure 8.--Eggs are deposited in a mass on borne caterpillar hairs. 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 simi- lar to horns are located right behind the head; a similar but longer pencil is on the posterior. Four dense, buff- F-701866 Figure 10.--Mature larva feeding on fir foliage.

Life Cycle

The tussock moth produces one gen- eration each year. Adults appear from late July into November, depending on the season and location. Male flight activity is apparently stimulated by F-701863 F-701865 light and temperature. It begins about Figure 9.--Young larvae are very small and midday, peaks in late afternoon, and have long, fine body hairs. rapidly diminishes at dusk. 5 The female emits a sex pheromone Since the female moth does not fly, that attracts males during their flight major dispersal is by windborne young period. Females mate soon after they larvae. When the larvae drop off foli- emerge from their pupal cocoon. They age, they produce long silk threads cling tenaciously to the outside of the that, combined with their hair and light cocoon and lay eggs on its surface im- weight, allow them to be carried by mediately after mating. In light infes- air currents. The distance most cater- tations, cocoons with their attached pillars travel, however, rarely exceeds egg masses usually are scattered on the 500 m (0.25 mi). Any longer range dis- foliated twigs throughout the tree persal by a few individuals does not crowns. In heavy infestations, they are produce new outbreaks. concentrated on the lower parts of Larvae growth is slow at first, but trees. Where defoliation is severe, egg becomes progressively faster as larvae masses on cocoons may be bunched pass through four to six moults. They on the trunk, on the lower side of larger also eat proportionately more. They limbs, or on objects some distance require current year’s foliage when from the tree. young, but once they are about half Each female lays her eggs in a single grown, they can complete their devel- mass (about l/2 inch (1.3 cm) in diam- opment on a diet of old foliage. eter) of a dry, tough, frothy, substance Pupation occurs any time from late containing many hairs from her body. July to the end of August inside a thin She may lay only a few, or as many as cocoon of silken webbing mixed with 350 nearly spherical, white eggs, in larval hairs. The pupal stage lasts from one to three layers in this mass. The 10 to 18 days, depending on tempera- number of eggs per mass varies with ture. Then the moth emerges to begin geographical location, but on the av- the life cycle again. erage, egg masses from the northern part of the moth’s range contain fewer Natural Control eggs than those from the southern part. Many natural controls exist that After egg laying is complete, the fe- keep the number of tussock low male dies, leaving the eggs to over- most of the time. In populations that winter in the gray, woolly mass at- persist at low densities, over 90 per- tached to the cocoon. cent of the larvae and at least 75 per- Egg hatch in late May or early June cent of the pupae and eggs are killed coincides with bud burst and shoot each generation by natural causes. If elongation of the host trees. One to such mortality does not take place, the seven days after the eggs hatch, the population will increase rapidly. tiny, hairy caterpillars crawl to new Overwintering eggs are usually needle growth and begin feeding. 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.

F-701867 F-701868 Figure 11.--A small wasp, Teleneomus Figure 12.--A parasite fly, Carcelia yalensis, californicus (Ashmead), ovipositing in tus- lays eggs on the back of a mature larva. sock moth eggs in the laboratory.

After the surviving eggs hatch, some At outbreak numbers, tussock moth young larvae are lost while dispersing populations have escaped their usual from the egg mass to feeding sites on natural controls and have been affected new foliage. Others are eaten by birds, by additional mortality factors related spiders, and predaceous . Insect to their high numbers. A nuclear poly- parasites attack all ages of larvae, but hedrosis virus capable of wiping out a parasitic fly, Carcelia yalensis Sell- large numbers of larvae and pupae usu- ers, is an especially effective enemy ally appears only in relatively dense (fig. 12). It lays its eggs on the backs populations. Its presence, in combina- of mature tussock moth larvae. The tion with other mortality factors, fre- maggot enters the caterpillar’s body quently causes collapse of outbreak and eventually kills the host in the populations. When diseased larvae die, pupal stage. Pupae are also killed by a their internal organs liquefy (fig. 13). variety of wasplike parasites that at- The virus spreads through the popula- tack the cocoon directly. Birds also tion when these bodies fall to the prey on cocoons and can destroy a sig- ground and rupture or lie smeared over nificant proportion of the pupal popu- the foliage. Virus particles may per- lation at low tussock moth densities. sist in the environment for many years thereafter.

7 vention. These activities must be well integrated to insure adequate protec- tion 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 popu- lations that are increasing in density need to be detected early and con- trolled 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 num- ber of larvae in the population can be estimated by sampling foliage in the lower crown of host trees. Therefore, F-701869 if an outbreak is developing, it can be Figure 13.--Larvae killed by natural virus hang detected 1 or 2 years early by an in- head downward from the foliage. crease in the number of insects, so that suppression, if needed, can be initiated In the absence of viral disease, out- before significant damage to trees oc- break populations of tussock moth are curs. ultimately limited by the quantity and The need to suppress a tussock moth quality of available food. Early defo- population depends on the impact an liation of the current year’s needles uncontrolled outbreak is expected to eliminates the preferred food and have on the forest. Decisions for con- forces larvae to feed on less nutritious trol must be based on a thorough evalu- older needles. This causes starvation ation of the potential of the insect and lowered production and survival population and the resource values at of eggs, and helps reduce population stake. Models of insect population numbers to a level where they are behavior, stand dynamics, and eco- again regulated by the usual natural nomic variables are available for use enemies. in integrated pest management sys- tems. Applied Control The prevention of outbreaks is the ultimate aim of pest management. Management recommendations to Much additional knowledge of prevent tree damage from tussock insect-host relations and stand dynam- moth outbreaks involve four major ics, however, is needed before silvi- activities: early detection, evaluation, cultural systems can be suppression, and pre 8 prescribed to reduce susceptibility of not endanger humans, livestock, crops, host stands. There is some indication beneficial insects, fish, and wildlife. that fir growing in pine sites and fir Do not apply pesticides when there is stands growing on warm, dry sites are danger of drift, when honey bees or most susceptible to Douglas-fir tus- other pollinating insects are visiting sock moth damage. Where offsite fir plants, or in ways that may contami- is well established and conversion to nate water or leave illegal residues. the proper tree species would be un- Avoid prolonged inhalation of pes- economical, some form of annual ticide sprays or dusts; wear protective population monitoring should be con- clothing and equipment if specified on sidered to detect increases in popula- the container. tion numbers. The preferred way to If your hands become contaminated keep losses low is to work toward with a pesticide, do not eat or drink healthy, thrifty stands, growing on a until you have washed. In case a pes- proper site. ticide is swallowed or gets in the eyes, Two materials are registered and follow the first-aid treatment given on available that might be used to reduce the label, and get prompt medical at- outbreak population numbers. Car- tention. If a pesticide is spilled on your baryl is the only chemical presently skin or clothing, remove clothing im- registered for aerial application against mediately and wash skin thoroughly. the tussock moth. The microbial pes- Do not clean spray equipment or ticide Bacillus thuringiensis (B.t.) is a dump excess spray material near commercially available material that ponds, streams, or wells. Because it is is not hazardous to most beneficial difficult to remove all traces of herbi- insects, birds, small mammals, and cides from equipment, do not use the aquatic systems. However, results with same equipment for insecticides or B.t. against the tussock moth have not fungicides that you use for herbicides. been consistent. If control is necessary, Dispose of empty pesticide contain- a State or Federal pest control special- ers promptly. Have them buried at a ist should be consulted for current rec- sanitary land-fill dump, or crush and ommendations. bury them in a level, isolated place. Caution: Pesticides used improperly can be injurious to man, , and NOTE: Some States have restric- plants. Follow the directions and heed tions on the use of certain pesticides. all precautions on the labels. Check your State and local regula- Store pesticides in original contain- tions. Also, because registrations of ers under lock and key--out of the reach pesticides are under constant review of children and animals--and away by the Federal Environmental Pro- from food and feed. tection Agency, consult your county Apply pesticides so that they do agricultural agent or State exten

9 sion specialist to be sure the intended Mason, R. R. use is still registered. 1978. Tussock moth life tables. In McGraw-Hill Yearbook of Science and The United States Department of Technology, p. 181-183. McGraw-Hill Agriculture has issued a series of Book Co., New York. Douglas-fir tussock moth handbooks and additional issues are in press. Cop- Mason, Richard R. ies can be obtained from the U.S. Gov- 1978. Detecting suboutbreak populations of the Douglas-fir tussock moth by sequential ernment Printing Office. sampling of early larvae in the lower tree crown. U.S. Department of Agriculture For References est Service Research Paper PNW-238, 9 p. Pacific Northwest Forest and Range Experi ment Station, Portland, Oreg. Beckwith, R. C. 1976. Influence of host foliage on the Mitchell, R. G. Douglas-fir tussock moth. Environmental 1979. Dispersal of early instars of the Entomology 5(1):73-77. Douglas-fir tussock moth. Annals of the En- tomological Society of America Brookes, M. H., R. W. Stark, and R. W. 72(2):291-297. Campbell (editors). 1978. The Douglas-fir tussock moth: A Torgersen, Torolf R. synthesis. U.S. Department of Agriculture 1977. Identification of parasites of the Technical Bulletin 1585, 331 p. Douglas-fir tussock moth based on adults, cocoons, and puparia. U.S. Department of Campbell, Robert W., and Lorna C. Agriculture Forest Service Research Paper Youngs. PNW-215, 28 p. Pacific Northwest Forest 1978. Douglas-fir tussock moth: An anno- and Range Experiment Station, Portland, tated bibliography. U.S. Department of Ag Oreg. riculture Forest Service General Technical Report PNW-68, 168 p. Pacific Northwest Wickman, B. E., R. R. Mason, and H. G. Forest and Range Experiment Station, Port Paul. land, Oreg. 1975. Flight, attraction, and mating behav- ior of the Douglas-fir tussock moth in Daterman, G. E., Linda J. Peterson, Oregon. Environmental Entomology Richard G. Robbins, Lonne L Sower, 4(3):405-408. G. Doyle Daves, Jr., and Ronald G. Smith. 1976. Laboratory and field bioassay of the Wickman, Boyd E. Douglas-fir tussock moth pheromone, 1978. Phenology of tussock moth. In (Z)-6-heneicosen-11-one Environmental McGraw-Hill Yearbook of Science and Entomology 5(6):1187-1190. Technology, p. 183-185. McGraw-Hill Book Co., New York. Mason, R. R. 1976. Life tables for a declining population Wickman, Boyd E. of the Douglas-fir tussock moth in north 1978. Tree mortality and top-kill related to eastern Oregon. Annals of the Entomologi defoliation by the Douglas-fir tussock moth cal Society of America 69(5): 948-958. 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, Revised January 1981 Portland, Oreg.

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