Forest Insect Control 
The impact of naturally occurring adverse environmental factors on forest insects 
is so great that the majority of species never occur in sufficient numbers to 
constitute an economic hazard. Even those insects that become sufficiently abun- 
dant at times to be considered “pests” are also subject to control by these factors, 
though usually to a lesser degree and frequency (62/, 630). Depending on the 
insect and the value of the forest resources affected, a given degree of natural 
control may reduce damage to an acceptable level. 
Natural Control 
Natural contro! results when naturally occurring adverse environmental factors 
prevent insect populations from reaching or exceeding harmful proportions. It may 
be effected by a single factor, such as abnormally low winter temperatures, or by 
several factors working in combination; it never ceases to function entirely, but its 
impact may fluctuate greatly from place to place and time to time (2/, 492, 62/, 
674). 
Physical factors.—Temperature is one of the most important physical factors 
affecting forest insects. It not only sets limits to their distribution but also often 
profoundly affects their abundance. The optimum temperature for many eastern 
species appears to lie between 24° and 27° C; outside this range, insect activity 
gradually declines and eventually ceases. When temperatures climb to 50° C or 
drop to around — 18° C or lower, death results. Rapid changes in temperatures, such 
as occur in the fall and spring, also may cause mortality even though absolute lethal 
temperatures are not reached. 
Insects react in many ways to escape the rigors of adverse temperatures. During 
the summmer, leaf-feeding larvae may move from the upper to lower surfaces of 
leaves, or from the periphery of the crowns to the shaded interior to escape 
excessive heat. Solar-sensitive eggs may be deposited in the shade (340). Adults 
find relief from heat by moving into the interior of stands, to the shaded portions of 
trunks, or to the interior of crowns. During cooler-than-normal days, they may seek 
out individual, exposed trees, the sunny side of trunks, or trees along the edges of 
stands. To escape the effects of winter cold, a species may hibernate in the egg, 
larval, pupal, or adult stage in areas where the likelihood of lethal temperatures 
occurring is greatly lessened. Because of physiological changes that occur in the 
dormant state, many northern species are able to survive temperatures well below 
—18° C. Southern species are much less likely to survive exposure to such 
temperatures. 
Outbreaks of leaf-eating insects may be suppressed by late-spring frosts that kill 
the young, tender foliage which the young larvae depend on for food. Late springs 
may result in greatly delayed emergence from hibernation, and cool summers may 
result in a slowdown in the rate of larval development. Prolonged developmental 
periods add up to an extended period of exposure to parasites, predators, and 
disease pathogens, which may lead to unusually heavy losses. The number of 
generations produced per year may be related to photoperiod and the length of the 
season of biologically effective temperatures. In the South, a species may produce 
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