INSECT ENEMIES OF EASTERN FORESTS 301 
The most important enemies to forest reproduction—the flower, 
seed, seedling, or young tree—belong to other groups. There are, how- 
ever, some scolytid enemies of reproduction. Several species of the 
genus Conophthorus attack the growing cones of eastern pines and 
prevent the development of the seeds. Of these, C. coniperda (Sz.) 
kills the immature cones of northern white pine and often destroys a 
considerable portion of the seed crop in certain localities, and other 
species of Conophthorus attack the cones of other pines. Other 
scolytids, such as species of Zps, Pityogenes, and Pityophthorus, work 
in the bark of the stems and twigs, and species of Hy/astes attack the 
root collar of young pines that have recently been transplanted or are 
growing under adverse conditions. Thus trees are killed which, al- 
though weakened, would otherwise have survived. When these beetles 
are excessively numerous, healthy young trees may be killed by a 
massed attack, although the beetles suffer heavy mortality in such. 
material. ; 
A number of bark beetles and ambrosia beetles attack, and sometimes 
breed in, healthy trees without necessarily killing them, although the 
tree is always more or less affected. It may be so weakened as to be- 
come easy prey for other insects or disease, its growth may be affected, 
or its timber value may be reduced. 
Dendroctonus valens Lec. and D. terebrans (Oliv.), while preferring 
stumps or sickly trees, often attack healthy pines near the base of the 
trunk and kill areas of bark near the ground level. The young beetles 
of various species of Scolytus and Phloeosinus, after emerging from 
their larval host, feed for some time on the young twigs or leaf petioles 
of healthy trees, killing many twigs and weakening the trees. PAtho- 
rophloeus frontalis Zimm., the native elm bark beetle (//y/urgopinus 
rufipes, and similar species emerge from their larval host in the fall, 
bore into the bark of the trunks of living trees, and feed on the tissues 
and juices of the outer phloem. 
Ambrosia beetles usually breed only in dying, cut, or recently dead 
trees, but a few species, such as the Columbian timber beetle (Corthy- 
lus columbianus Hopk.) attack living trees and breed for generations 
in the living wood. Such a tree often survives and may show but little 
outward evidence of the attack; but, when converted into lumber, the 
heartwood and sapwood may be riddled by old and recent burrows 
and the timber value destroyed or much reduced. 
The greatest amount of damage, however, is done by forms that kill 
forest trees. Such bark beetles vary considerably in their aggressive- 
ness in attacking living trees. Some breed by preference in cut, in- 
jured, or decadent trees, and attack vigorous ones only when they are 
very numerous or when inferior material is lacking. Other species, 
even when present in only moderate numbers, will attack living trees 
in preference to cut or decadent material, provided enough beetles are 
at hand to overcome the tree’s resistance. 
The less aggressive forms find ideal conditions for increase in locali- 
ties where lumbering has been carried on for several years, or where 
timber has been broken by a tornado or injured by a fire or a flood. 
Such conditions often provide suitable breeding material long enough 
for several generations of bark beetles to develop. - The insects may 
thus increase to nearly incredible numbers, and then, when their 
favorite breeding material is lacking, they attack living trees. A few 
