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



gether without scoring the wood. Seedlings and saplings an inch 

 or more in diameter are sometimes killed by the attacks. 



DEFOLIATORS OF YOUNG TREES 



Usually defoliating insects show no particular choice as to the 

 age or size of tree they attack, and young trees in the forest may 

 be fed on by almost any leaf -feeding form. However, some of the 

 young trees in the stand are actually avoided by defoliating in- 

 sects. This was particularly noticeable in hemlock looper out- 

 breaks, where heavy defoliations ceased when stands of young 

 growth were reached. On the other hand sometimes young trees 

 are fed on in preference to older trees. This has been noted in 

 some examples of spruce budworm attack on young Douglas-fir. 

 Also, small trees may be fed on because of their closeness to the 

 ground or their more tender succulent growth. For instance, 

 young, low-growing pines have been seriously damaged by inva- 

 sions of Mormon crickets and grasshoppers which, when in epi- 

 demic numbers, chew all green foliage within reach (p. 209) . 



Since the damage done by defoliators to mature forest trees is 

 usually of greater importance than that done to young trees, this 

 group of insects will be discussed in the following section. 



INSECTS INJURIOUS TO MATURE FOREST TREES 



The insects that prey on young forest trees and that may, during 

 the formative years, cause serious injury through stunting, de- 

 forming, or halting growth are of little importance after the trees 

 have reached maturity. The mature trees may still be fed upon by 

 root-feeding or terminal-feeding insects, but such damage within 

 reasonable limits can be borne without fatal consequences, and the 

 small loss in growth on mature trees is of minor importance. 



There are three principal groups of insects that cause the death 

 of mature trees — (1) those that bore into the inner bark of the 

 main trunk and cut off the supply of moisture and food; (2) those 

 that feed on the leaves and cause severe defoliation; and (3) those 

 that suck the juices from twigs or the main stem. 



In many ways the defoliators are the most dangerous forest 

 pests. They are primary and aggressive in their activities; that is, 

 they attack healthy, vigorous trees as readily as undernourished, 

 weakened ones (fig. 31). The injury does not always kill the tree, 

 but often so weakens it that it becomes readily susceptible to bark- 

 beetle attack. On the other hand, the miners of the inner bark 

 usually direct their attack against trees previously weakened 

 through drought, defoliation, fire, or some other cause. A few 

 species of bark beetles are aggressive and primary in their attacks 

 and tremendously destructive, but most species are decidedly sec- 

 ondary, attacking only trees that are already weakened or dying. 

 A few sap-sucking insects, notably the spruce aphid and the 

 balsam woolly aphid, are primary killers of mature trees. 



It is true that the destruction of older, mature trees in the 

 forest through attack by insects is more or less of a natural proc- 



