five or more generations per year, whereas in the North, it may produce only one or 
less. 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 powderpost 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 intensified during or following 
periods of drought. 3 
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 a few species of 
trees. Some of the most serious bark beetle pests belong to this group. Many species 
of destructive wood borers also confine their attacks to several closely related 
species. In contrast, numerous other species feed on 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 kinds of places; others deposit them 
on host trees only. When the former condition prevails, it often leads to heavy losses 
among newly hatched larvae, especially where acceptable food is not readily 
available. Infestations of 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 that contain a low ratio of 
preferred to nonpreferred 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 often severe. Occasionally, when a virus epizootic develops, the results may be 
catastrophic for the host insects. Operating singly or in combination, biotic factors 
may limit the duration and magnitude of an outbreak, prolong the intervals between 
outbreaks, or prevent outbreaks entirely. A large number of these control agents 
have been collected and identified in the forests of eastern America, and new ones 
are being added to the list each year. 
Insect control by biotic factors has several advantages not offered by many of the 
other approaches to control now available. Where these factors are well established, 
they usually are self-perpetuating, barring natural catastrophies or interference by 
humans: they adjust to changes in the size of host populations, and they operate 
with practically no adverse side effects. 
The majority of the parasites and predators of insects are other insects. Many 
thousands of species belonging to well over 200 families in 15 orders are known to 
be either parasitic or predacious; many of these attack forest insects. The majority 
of parasitic forms belong to only a few families in the orders Hymenoptera and 
Diptera. Important predators are found in several families of the orders Coleoptera, 
Diptera, Neuroptera, Hemiptera, and Hymenoptera. 
