408 



Yearbook of Agriculture 1949 



until a high percentage of the forest 

 stands has been killed. Then, even more 

 suddenly than it appeared, the epi- 

 demic subsides. 



This sporadic behavior of forest-in- 

 sect populations indicates that complex 

 factors govern the abundance of cer- 

 tain species in the forest. Parasites, 

 predators, unfavorable weather, resist- 

 ance of the trees due to growth vigor, 

 all tend to hold populations in check. 

 On the other hand, conditions that 

 will tend to weaken the trees, such as 

 drought, preponderance of a favored 

 food tree, failure of parasites and pred- 

 ators, overmaturity, and windfalls and 

 slash, all provide favorable conditions 

 for the destructive species to breed up 

 in numbers. Man, himself, has at times 

 aggravated serious insect outbreaks by 

 his method of using the forest. 



Although science has not yet been 

 able to uncover and appraise all the 

 factors that influence the abundance of 

 forest-insect populations, it has shown 

 that there are dominant conditions that 

 must be taken into account in main- 

 taining productive forests free from ex- 

 cessive losses due to insect pests. The 

 most successful control methods that 

 have been developed up to the present 

 time (and no doubt those that will be 

 used in the future) are based upon the 

 strategy of using nature's methods as 

 far as possible in holding down destruc- 

 tive insect populations. 



Furthermore, the kinds of insects 

 that attack forest trees include many 

 species that vary widely in their habits 

 and in the character and amount of 

 damage they do. Some insects attack 

 only the flowers; others the cones and 

 seeds. The activity of these insects does 

 not damage the tree itself, but at times 

 so much of the seed crop is destroyed 

 that reproduction of the forest is re- 

 tarded. Sucking insects, such as scales 

 and aphids, attack foliage and stems; 

 they rarely kill the tree outright but 

 gradually weaken it and slow down the 

 growth rate. The most effective tree 

 killers, however, are the defoliators and 

 bark beetles, whose activities destroy 

 vital plant organs and bring about an 



immediate and often fatal effect upon 

 the growth functions of the tree. Other 

 insects that cause great damage are ter- 

 mites and some wood borers, which 

 feed only on the wood after the tree is 

 dying or dead and destroy material that 

 otherwise could be put to use. 



Trees are defoliated mostly by the 

 larvae of certain moths and sawflies 

 and to a lesser extent by both the adult 

 and larval forms of some beetles. De- 

 foliators can kill trees by depriving 

 them of foliage, thus stopping the man- 

 ufacture of the plant food so that the 

 trees slowly starve. Some of the historic 

 defoliations of the past have been re- 

 corded not only in the chronicles of the 

 time but also in the annual rings of sur- 

 viving trees. Outbreaks of the spruce 

 budworm in the New England States 

 and of the fir tussock moth in the West 

 are recent examples of widespread de- 

 foliating epidemics. 



Insects that feed between the bark 

 and wood find their nutrition in the 

 sugars and starches that are in solution 

 in the cells of the inner bark and cam- 

 bium. To reach these they mine 

 through the corky bark into the inner 

 bark layer, where they introduce fun- 

 gi that develop in the sapwood and 

 stop the flow of the sap. The leaves, 

 deprived of water, quickly wilt and the 

 tree dies. Bark beetles make up the 

 bulk of the destructive cambium feed- 

 ers. Certain species of bark beetles are 

 particularly adapted to mature stands 

 of pine and in a number of Western 

 States take a heavy toll from virgin 

 forests that are the main reserve of 

 timber supplying the Nation's need for 

 high-quality soft pine. In some regions 

 during the past two decades these 

 insects have destroyed more merchant- 

 able timber than was cut by the saw- 

 mills and destroyed by fires, combined. 

 Characteristic of the bark beetle infes- 

 tations is their capacity to flare up into 

 epidemics of spectacular proportions. 



Termites and wood borers do not 

 kill or damage living trees and, in na- 

 ture's economy in the forest, may be 

 of benefit in that they accelerate the 

 deterioration and decay of dead trees 



