Insect enemies of western fokests 59 



by the annual rings, while for the year of defoliation the ring at the 

 base of the tree is enlarged over that of the previous year. Unable 

 to throw off the excessive moisture brought up by the roots, the water 

 and sap accumulate under the bark and ferment. Often this sets up 

 an attractive influence that draws the bark beetles into the area, and 

 trees are then killed which otherwise might have recovered. 



The extent to which a tree may be injured by defoliation will de- 

 pend upon the tree species, whether evergreen or deciduous, the posi- 

 tion of the tree within the stand, its general health, the insect species 

 involved, and the time of year when the defoliation occurs. Since 

 evergreens cannot replace their leaves as readily as deciduous trees, 

 they are much more seriously injured by defoliation than those that 

 normally shed their leaves each year. One year of severe defoliation 

 may be enough to kill such trees as Douglas fir, hemlock, and pon- 

 derosa pine. Alders and oaks, on the other hand, can sometimes 

 withstand several seasons of defoliation without fatal injury. Domi- 

 nant trees are more resistant than their suppressed neighbors, and 

 vigorous trees have a better chance of resisting attacks than those 

 weakened from one cause or another. Defoliators usually show little 

 preference for weakened trees as do many of the bark beetles and are 

 more apt to feed indiscriminately on whatever foliage of their 

 favorite hosts happens to be at hand, 



Outbreaks of defoliators are characteristically sporadic. For 

 many years the forester may not observe a single specimen of some 

 important leaf -feeding insect, and then without warning a sudden 

 outbreak may oc6ur and the forest is swarming with millions of 

 caterpillars or slugs tliat devour everything in their path. Such 

 outbreaks usually go through a 3- to 5-year cycle. First, there is a 

 preepidemic stage in which the insect becomes unusually numerous. 



Then there is the epidemic stage, which usually lasts for 3 years, 

 the first year showing evident damage, the second year a peak of 

 damage, and the third year one of declining numbers but still with 

 evident injury. Third, there is the post-epidemic period in which 

 the insect returns to a normal or quiescent status. This decline in 

 the epidemic is brought about by natural control factors such as an 

 increase in parasitic enemies and disease or through some climatic 

 condition which is unfavorable to a continued activity of the de- 

 foliators. 



The aim in control is not to eradicate the insects but to protect 

 the forest from heavy and concentrated feeding during the height of 

 occasional epidemics. Although more expensive methods can always 

 be used on individual trees of high value, such as ornamental, park, 

 and shade trees, spraying and airplane dusting are the only methods 

 now used to protect large timber stands during outbreaks of defoliat- 

 ing insects. These methods are discussed under the heading "Con- 

 trol of defoliating insects" (p. 175). 



The leaf -eating insects include all of 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, 

 that feed externally upon and devour any part of the leaf, (2) the 

 leaf skeletonizers, that eat out the green chlorophyll and leave only 

 the network of veins and midribs, and (3) the leaf miners, that bur- 

 row through and feed between the surfaces of the leaves or needles. 



