duction, or to kill it; or they may be applied to the environment 
to render it less favorable for the insect either through alteration 
in food supply, in microclimatic factors, or in the abundance and 
effectiveness of natural enemies. 
The choice of materials or methods in artificial control de- 
pends on several things: whether the insect is a native or intro- 
duced species; the factors known or thought to be responsible for 
its abundance; the need for quick and effective reductions in its 
numbers; and cost. If a native species is involved, the aim usually 
is not to eradicate it, but rather to reduce and maintain its num- 
bers to a tolerable level; if it is an introduced pest, the aim may 
be to eradicate it or to slow down or prevent its spread. 
Artificial control methods designed to suppress existing out- 
breaks, usually have short-term effects only; those that are used 
for the prevention of outbreaks may be long lasting. Control costs 
vary considerably, depending on the insect and its habits, the 
methods and materials used, and the size and accessibility of in- 
fected stands. It may be limited to only a few dollars per acre 
where infested stands are treated with aerial applications or 
insecticides or it may cost several dollars per tree when treat- 
ments are made to individual trees. Occasionally, control costs 
may be retrieved through the salvage and sale of infested timber. 
Some types of control may be used with complete safety; others, 
in which hazardous materials are involved, must be used with 
great care. 
NATURAL CONTROL 
Natural control results when naturally occurring adverse en- 
vironmental 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 en- 
tirely, but its impact may fluctuate greatly from place to place 
and time to time (9, 302, 309). 
PHYSICAL FACTORS.—Temperature is one of the most im- 
portant 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 75° and 80° F. As temperatures depart 
from this range, in either direction, activity gradually declines and 
eventually ceases. When they climb to 120° or drop to around 0° 
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 summer, 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. 
Adults find relief from heat by moving into the interior of stands, 
to the shaded portions of trunks, or to the interior of crowns. Dur- 
ing 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 utilize areas in 
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