50 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 

 defoliated trees. For instance, large forest areas in Washington, 

 Canada, and Alaska have been badly defoliated by the hemlock bud- 

 Avorm 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 worb of leaf feeders is easily detected, considerable 

 injury frequently occurs before their activities are noticed. Since 

 young caterpillars are more easily killed by poison than older ones, 

 early detection and control are important. 



As these leaf 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. Wliere 

 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 

 familiar. 



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 

 gi-ay caterpillars clothed with long wliite 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. 



