which to hibernate either in the egg, larval, pupal, or adult stage 
where the likelihood of lethal temperatures occurring are greatly 
lessened. Because of physiological changes which occur in the dor- 
mant state, many northern species are able to survive tempera- 
tures well below 0°. Southern species are much less likely to 
Survive exposure to such temperatures. 
Outbreaks of leaf eating insects may be suppressed by late 
spring frosts which 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 slow-down in the rate of larval development. Prolonged de- 
velopmental 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 is related to photoperiod and the length of the season of 
biologically effective temperatures. In the South, a species may 
produce five or more generations per year; whereas in the North, 
it may produce only one. Outbreaks of such a species often appear 
to develop with explosive suddenness in the South. In the North, 
it may require several years for an equivalent outbreak to occur. 
Insect populations are also affected by atmospheric moisture 
and by moisture conditions in host trees and in the soil. For 
example, various species of wood borers, such as powder-post 
beetles, breed successfully only in very dry wood; whereas others, 
such as the ambrosia beetles, require wood with a much higher 
moisture content. Damage to black locust by the locust borer and 
to pines by various species of bark beetles may be greatly intensi- 
fied during or following periods of drought. 
A considerable degree of control also may be effected by other 
adverse weather conditions. For example, heavy, beating rains may 
dislodge and destroy large numbers of larvae that feed on the 
foliage of trees; moist weather may result in the development of 
disease epizootics and the decimation of insect populations. 
HOST FACTORS.—Many species of forest insects feed on only 
one species of tree. Some of the most serious bark beetle pests. 
belong to this group. Many species of destructive wood borers also 
confine their attacks to a single species or to closely related 
species. In contrast, numerous other species feed on several to a 
great many tree species, often in different families. Many leaf 
eating species belong to this group. The females of some insects 
deposit their eggs in all manner of places; others deposit them on 
host trees only. This often leads to heavy losses among newly- 
hatched larvae, especially where acceptable food is not readily 
available. Infestations of a single-host species that develop in 
weak or decadent trees may disappear or be greatly reduced when 
the ratio of these trees to healthy vigorous trees drops below a 
critical level in a stand. Populations of multiple-host species, 
many of which attack trees in all stages of health or vigor, may 
be held to low levels in mixed stands which contain a low ratio 
of preferred to non-preferred host species. 
BIOTIC FACTORS—Parasites, predators, and pathogenic 
micro-organisms play important roles in the natural control of 
insects. Their effects may not always be evident, but they always 
bring some degree of pressure to bear on their hosts, and it is 
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