Parasitic and predacious insects differ widely in habits and behavior. A parasite 
usually requires only one host in which to complete its development; a predator 
usually requires several to many hosts for its development. A parasite usually does 
not kill its host until it has completed its own development; a predator usually kills 
its host as soon as it is encountered. Host selection by parasites is generally a 
function of the egg-laying female adult only; whereas each individual predator must 
search out hosts in all of the feeding stages. 
Forest insects are also fed upon by many other forms of animal life, principally 
spiders, mites, birds, and many mammals. Birds, especially woodpeckers, are 
sometimes effective in suppressing outbreaks. Many other species of birds are also 
thought to play important roles in control (1/90, 470, 564). Small mammals such as 
shrews are often effective in controlling species that spend part of their lives on the 
ground (/70). 
The literature contains many references to the role of parasites and predators in 
insect control. A good introduction to the subject is in several publications (26, 48, 
IFO S205 218-2403, 284, 621, 1183, 1203, 1213). 
Many species of forest insects are also subject to a considerable degree of control 
by pathogenic micro-organisms. Some of the latter, especially the viruses in the 
family Baculoviridae, frequently occur in the form of epizootics and decimate or 
eliminate outbreaks over large areas. The baculo viruses are particularly useful 
because they infect only invertebrates. Notable examples are (1) a polyhedrosis 
virus disease that ultimately plays a leading role in the control of gypsy moth 
outbreaks in the Northeast, and (2) a disease of the same type that caused the 
collapse of an outbreak of the European spruce sawfly over a region of several 
thousand square kilometers in eastern Canada and the Northeastern States during 
the late 1930's. 
There are more than 1,300 different kinds of micro-organisms, most of which are 
pathogenic, that have been found associated with insects. This includes 22 types of 
viral diseases in 826 hosts, and a total of 1,271 host-virus records (8/5), 100 
species of bacteria (907), 750 species of fungi (907), 300 species of protozoa (907), 
several rickettsiellas, and about 1,000 species of nematodes (99/). Generally 
speaking, they gain entry into the insect by being ingested with food, during 
hatching from contaminated eggs, through wounds or other damaged areas in the 
integument, or through the tracheae. Most of them rely on wind, rain, streams, 
healthy or contaminated insects, small mammals, or birds for dispersal throughout 
an insect infestation or outbreak. Spread is occasionally so rapid that all of the 
insects in an infestation may appear to be dying at the same time. 
Many species of forest insects are particularly susceptible to viruses of the 
nuclear-polyhedrosis type (873). Once inside the host these viruses enter the cell 
nuclei, replicate, and some virions become encapsulated in many-sided, rodlike 
crystals, known as polyhedra. Invaded cells are soon destroyed, the virions and 
polyhedra are then released in the body cavity, and the host dies. Diseased larvae 
usually become siuggish, cease feeding, and in some instances move upward in the 
trees. After death they may be seen hanging by their forelegs, with the body 
darkened, decomposed, and liquified. Eventually they completely disintegrate and 
dry up on the trees. 
Forest insects are also affected by several other kinds of viruses—cytoplasmic 
polyhedrosis, granulosis, polymorphic inclusion, and noninclusion. Generally 
speaking, these viruses appear to be less effective and less specific in control than 
