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



The Monterey pine resin midge {RetinodipJosis resinicoJoides 

 Wms.) is another small pitch midge which inhabits the resin exuda- 

 tions of Monterey pine but apparently is not injurious to the trees. 



FEEDERS ON THE INNER BARK OF YOUNG TREES 



The most frequent damage to young trees by insects that feed on 

 the inner bark is that suffered by intermediate or suppressed trees 

 growing under crowded conditions or those weakened by drought, 

 fire, or mechanical or other injury. Vigorous, young, dominant trees 

 in the stand have a good chance to escape damage from these insects, 

 except under conditions where they become epidemic. Usually the 

 normal damage of this character in the virgin forests is of more 

 benefit than otherwise, since it represents a natural thinning process 

 and the release of the more dominant trees from competition. At 

 times, however, such damage may become serious when outbreaks of 

 bark beetles or other cambium- or root-feeding insects sweep through 

 the young stands and kill a high percentage of thrifty as well as 

 weakened individuals. 



The insects which feed on the inner bark of trunk or roots of 

 young trees are usually those which also feed on thin bark of older 

 trees. These include certain groups of bark beetles, bark borers, and 

 bark weevils. Since most of these insects do their greatest damage 

 to older, mature trees they will be discussed later under another 

 heading (p. 95). 



Many species of bark beetles (Scolytidae) inflict their greatest 

 damage on small or thin-barked trees. Many of these are rarel}?-, if 

 ever, primary and aggressive in their attacks upon large trees, but 

 may breed in windfalls, slash, or large trees that are dying or have 

 been attacked first by other bark beetles. Breeding in such trees or 

 felled material, they may emerge in large numbers and become very 

 destructive to the small trees in the stand. 



In pines, the pine engraver beetles of the genus Ips are the ones 

 most frequently responsible for this type of damage. Less frequently 

 species of Pityogenes or Pityophthorus are involved. 



In young stands of Douglas fir Pseudohylesinus nehidosus Lee. 

 and Scolytus unispmosus Lee. frequently kill groups of small trees, 

 particularly in the vicinity of slashings. 



Small balsam firs are similarly affected by species of Scolytus, 

 Pseudohylesinus, and Pifyokteines. Young spruce and hemlock also 

 may be killed by species of these and other genera. 



Young redwoods, cedars, cypresses, junipers, and related cupressine 

 trees are frequently killed by species of Phloeosinus, which breed 

 in the trunks and limbs of dying or dead larger trees. 



DEFOLIATORS OF YOUNG TREES 



The insects that feed upon the leaves of young trees are in nearly 

 every instance the same species as those that feed on the leaves of 

 older trees. Defoliating insects usually show no particular choice as 

 to the age or size of tree that they attack, and young trees in the 

 forest may be fed upon by almost any leaf-feeding form. In some 

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

 insects. This was particularly noticeable in the case of the hemlock 



