INSECT ENEMIES OF WESTERN FORESTS 77 



trees. In the protection of ornamental, park, and shade trees, 

 rather intensive spray programs are justified in order to prevent 

 damage. In protecting large timber stands, airplane spraying is 

 now available as an effective weapon in defoliator control. The 

 most difficult problem involved is one of deciding when an outbreak 

 may become sufficiently damaging to justify the expense of air- 

 plane spraying, for most outbreaks subside of their own accord 

 without reaching the stage of inflicting severe forest damage. 

 Control methods are discussed on page 221. 



The leaf -eating insects include all those leaf -feeding forms that 

 have biting mouth parts and actually bite into and swallow their 

 leafy food. They may be divided into three groups: (1) The leaf 

 chewers, which feed externally upon and devour any part of the 

 leaf; (2) the leaf skeletonizers, which eat out the green chlorophyll 

 and leave only the network of veins and midribs; and (3) the leaf 

 miners, which burrow through and feed between the surfaces of 

 the leaves or needles. Some leaf -feeding insects are skeletonizers 

 in their early stages and then devour all of the leaf as they become 

 more mature. Some, when very young, mine the interior of the leaf 

 and later eat all of it. 



Outbreaks of leaf chewers do not always result in the death of 

 the defoliated trees. For instance, large areas of hemlock forest 

 in Washington, Canada, and Alaska have been badly defoliated by 

 the black-headed budworm for two or more years in succession, 

 and yet most of the trees have recovered. On the other hand, out- 

 breaks of the hemlock looper, the pine butterfly, the Douglas-fir 

 tussock moth, and even one recent outbreak of the black-headed 

 budworm on Vancouver Island, British Columbia, have caused 

 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 in heavy de- 

 foliations, considerable injury often takes place before their 

 activities are noticed. Since young caterpillars are more easily 

 killed by poison than older ones, provided they can be reached, 

 early detection and control are highly desirable. 



As these leaf chewers actually swallow and digest their leafy 

 food, the method of artificial control most frequently used in the 

 past was to spray or dust the foliage with a stomach poison, such 

 as one of the arsenicals. DDT and other new contact and stomach 

 insecticides developed during World War II have now largely re- 

 placed the older chemicals, since these new materials are outstand- 

 ingly lethal to most leaf chewers. Where small trees can be reached 

 with ground sprayers, the application of these insecticides is a 

 simple operation. Large forest areas can be treated satisfactorily 

 only by airplane or helicopter. Application of insecticides from 

 the air is discussed on page 223. 



Insects comprising the group of leaf eaters are mostly either 

 caterpillars (Lepidoptera) or sawflies (Hymenoptera), but a few 

 beetles do similar work. Only those that have proved particularly 

 injurious and with which the forester should become familiar are 

 discussed here. 



