60 MISC. PUBLICATION 273, U. S. DEPT. OF AGRICULTURE 
Some leaf-feeding insects are skeletonizers in their early stages and 
then devour all of the leaf as they become more mature. Some 
mine the interior of the leaf when very young and then, later on, 
eat all of it. 
Outbreaks of leaf chewers do not always result in the death of the 
defolated trees. For instance, large forest areas in Washington, 
Canada, and Alaska have been badly defoliated by the hemlock bud- 
worm for 2 or more years in succession and yet most of the trees 
have recovered. On the other hand, outbreaks of the hemlock looper, 
the pine butterfly, and the Douglas fir tussock moth have resulted 
in the death of billions of feet of standing timber, with a high per- 
centage of the stand killed over hundreds of thousands of acres. 
While the work of leaf feeders is easily detected, considerable 
injury frequently occurs before their activities are noticed. Since 
voung caterpillars are more easily killed by poison than older ones, 
early detection and control are important. 
As these leat chewers actually swallow and digest their leafy food, 
the method of artificial control is to spray or dust the foliage with 
a stomach poison, such as an insecticide containing arsenic. Where 
small trees that can be reached with dusting or spraying machinery 
are involved, the application of such a poison is a simple operation. 
Treating large forest areas is quite a different matter, and this usually 
can be done only by means of airplanes. Application of insecticides 
by this method is discussed in the section on forest insect control. 
Insects comprising the group of leaf eaters are mostly either cater- 
pillars (Lepidoptera) or sawflies (Hymenoptera), but a few beetles 
do similar work. No attempt will be made to list all the insects 
which under certain conditions may prove destructive, and in the 
following pages are mentioned only those that have proved particu- 
larly injurious and with which the forester should, if possible, become 
familar. 
KEY TO DIAGNOSIS OF INJURY FROM IMPORTANT DEFOLIATING INSECTS 
A. Foliage appearing thin or sparse. Leaves chewed, mined, skeletonized, 
or stripped from the trees. 
1. Leaves chewed, and defoliated part of tree covered with silken 
web. Work of caterpillars with three pairs of true legs 
and less than six pairs of prolegs. 
a. Leaves and buds at tips of branches webbed together 
and fed upon by nearly hairless caterpillars, that 
wriggle violently backwards or fall to the ground 
when disturbed_____~_ bud moths or budworms, page 78. 
b. Large, dense, conspicuous, silken tents formed at end 
of branches or in crotches; made by very hairy 
caterpillars with blue, red, or yellow markings. 
tent caterpillars, page 72. 
c. Loosely woven tents formed at ends of branches of 
broadleaved trees; made by yellowish-brown or 
gray caterpillars clothed with long white hairs, 
arising from black and orange tubercles. 
fall webworm, page 68. 
d. Defoliated portion or entire tree covered with a very 
light fine cobweb of silken threads. 
(1) Caterpillars nearly naked or with only fine 
hairs. 
aa. Dark-green caterpillars with fine, 
closely set hairs and two lateral 
white stripes on each side. Feed- 
ing on pine_-_-- pine butterfly, page 62. 
