INSECT ENEMIES OF WESTERN FORESTS 73 



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 vir- 

 gin 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 be- 

 come 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 

 Miners in the Inner Bark and Phloem (p. 126). 



Many species of bark beetles (Scolytidae) inflict their greatest 

 damage on small or thin-barked trees. Many of these are rarely, if 

 ever, primary and aggressive in their attacks on 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 fre- 

 quently species of Pityogenes or Pityophthorus are involved. 



In young stands of Douglas-fir Pseudohylesinus nebulosus Lee. 

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

 particularly in the vicinity of slashings. 



Small true firs are similarly affected by species of Scolytus, 

 Pseudohylesinus, and Pityokteines. Young spruce and hemlock also 

 may be killed by species of these and other genera. 



Young redwoods, cedars, cypresses, junipers, and related cu- 

 pressine trees are frequently killed by species of Phloeosinus, 

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



Although bark beetles (Scolytidae) are primarily enemies of 

 large forest trees, a few species appear to be of importance in kill- 

 ing large seedlings and saplings through attack on the roots. Spe- 

 cies of the genera Hylastes, Hylurgops, and Pseudohylesinus have 

 been found doing this type of damage. Normally these are sec- 

 ondary bark beetles which breed in stumps and under the bark of 

 trees killed by fire or insects, but they appear to be primary in 

 attacking the roots of suppressed or weakened seedlings. The 

 attacking beetles make entrance burrows at the ground line and 

 construct winding galleries, which extend downward into the 

 larger roots and are partly filled with frass. The larvae work 

 through the cambium, away from the egg tunnels, and feed to- 



