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



or suck the juices. Trees can withstand a great deal of such feeding 

 without being seriously affected, and some such insect work is 

 going on more or less constantly. If the feeding is heavy, the 

 growth of the tree is retarded. If a high percentage of the leaf 

 surface is destroyed, the tree may die. The damage done to the 

 forest by defoliators is difficult to estimate, since a large part of it 

 involves only a loss of increment and not the death of trees. On 

 the other hand, when epidemics of defoliators occur, their ability 

 to destroy timber, especially coniferous timber, over large areas 

 in a short time places them at the top of the list of destructive 

 forest insects. 



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

 depend upon the tree species, whether the tree is evergreen or de- 

 ciduous, its position 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 decidu- 

 ous 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 ponderosa pine. Alders and oaks, on the other hand, 

 can sometimes withstand several seasons of defoliation without 

 fatal injury. Dominant trees are more resistant than their sup- 

 pressed 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, but are more likely to feed indis- 

 criminately on whatever foliage of their favorite host 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 occur and the forest may be swarmed with 

 millions of caterpillars or slugs that devour everything in their 

 path. Some defoliators, like the tent caterpillars, appear nearly 

 every year at widely separated points in the forest. Others appear 

 as major outbreaks at long intervals of time. Frequently, out- 

 breaks go through a 3- to 5-year cycle. First, there is a preepi- 

 demic 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 de- 

 cline in the epidemic may be brought about by natural control 

 factors, such as an increase in parasitic enemies and disease, or 

 through some climatic condition unfavorable to a continued activ- 

 ity of the defoliators. 



The aim in control of native forest defoliators is not to attempt 

 eradication but to protect forests from severe damage, either by 

 preventing the build-up of epidemics by "catching them while they 

 are small" or by reducing populations at the peak of epidemics to 

 prevent heavy, concentrated feeding, which would be fatal to the 



