92 MISC. PUBLICATION 273, U.S. DEPT. OF AGRICULTURE 



ern Oregon, and eastern Washington, which might have destroyed 

 nearly 2 billion board feet of timber had it not been brought under 

 control with aerial spraying on a vast scale. Such extensive out- 

 breaks have often been followed by disastrous forest fires. 



The Douglas-fir tussock moth shows a decided preference for 

 Douglas-fir and species of true fir and can increase to epidemic 

 numbers only when feeding on these preferred host trees. It will, 

 however, feed on other species of conifers when they are mixed 

 with firs, and even on the underbrush. Defoliation first occurs at 

 the tops of trees, and as this foliage is destroyed the caterpillars 

 work down on the lower foliage. 



The caterpillars of this tussock moth are striking-looking crea- 

 tures, with brightly colored tufts of hairs. When full grown they 

 are from 3 4 to 1 inch long, with gray or light-brown bodies and 

 black, shiny heads. Two long brushes, or pencils, of black hairs 

 fully y 4 inch long, suggest horns, directly behind the head, and a 

 similar but longer tuft is at the posterior end of the body. On the 

 upper side of the first four and the last abdominal segments are 

 dense, light-brown, or cream-colored tufts of hairs about 1 16 inch 

 long, and numerous red spots. Along the sides of the body are 

 somewhat broken, narrow, orange stripes, while the lower side of 

 the body is nearly naked, with the prolegs only sparsely covered 

 with hairs. 



The dull, brownish-gray moths are far more ordinary looking 

 than their handsome larvae. The males are about 1 2 inch long and 

 have a wing expanse of nearly 1 inch. The wingless females are 

 about one-half as long as the males. 



Normally there is one generation a year. The small, hairy cater- 

 pillars hatch from the eggs early in the spring and do a great deal 

 of migrating before settling down to feed on the new foliage. At 

 this stage they are easily carried long distances by the wind. Early 

 in June evidence of their work becomes conspicuous just after the 

 small larvae have killed the new foliage by girdling the base of 

 expanding buds, and by the first of August defoliation may be so 

 severe as to be noticeable over wide areas. The hairy larvae do 

 considerable traveling during their search for food and drop from 

 the trees by long silken strands, causing a conspicuous webbing 

 of the forest. They reach full growth early in August and then 

 pupate inside grayish-brown, spindle-shaped cocoons, which are 

 covered with hairs and attached to various parts of the trees and 

 neighboring shrubs. Moths appear the latter part of August and 

 mate soon after emergence. The winter is passed in the egg stage. 

 Eggs are laid by the wingless female moth in masses on top of 

 the old cocoon from which she has emerged and are covered with 

 a frothy white substance liberally mixed with hairs from her abdo- 

 men. These cocoons and egg masses are among the most conspicu- 

 ous evidences of tussock moth prevalence. 



One of the most important parasites of the Douglas-fir tussock 

 moth is a tachinid fly, somewhat larger and more hairy than the 

 ordinary housefly. The larvae of this fly live in the caterpillars 

 and emerge after the caterpillars have formed their cocoons. A 

 very small wasplike insect, Tricliogramma minutum Riley, attacks 



