often severe. Occasionally, as when a virus epizootic develops, the 
results may be catastrophic. 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 col- 
lected 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 unwise interference 
by man; 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 pre- 
daceous; many of these attack forest insects. The majority of 
parasitic forms belong to only a few families in the orders Hy- 
menoptera and Diptera. Important predators are found in several 
families of the orders Coleoptera, Diptera, Neuroptera, Hemip- 
tera, and Hymenoptera. 
Parasitic and predaceous 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 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 (284, 535). Small mammals such as 
shrews are often effective in controlling species that spend parts 
of their lives on the ground. 
The literature contains many references to the role of parasites 
and predators in insect control. A good introduction to the subject 
is found in publications by DeBach (187), Sweetman (701), 
Clausen (144), Balch (25), Turnbull and Chant (722), Thompson 
(714), and Burkner (118). 
Many species of forest insects are also subject to a consider- 
able degree of control by pathogenic micro-organisms. Some of 
the latter, especially the viruses, frequently occur in the form of 
epizootics and decimate or eliminate outbreaks over large areas. 
Notable examples are (1) a polyhedrosis virus disease which has 
played a leading role in the control of gypsy moth outbreaks in 
the Northeast for many years, and (2) a disease of the same type 
which caused the collapse of an outbreak of the European spruce 
sawfly over a region of several thousand square miles in eastern 
Canada and the Northeastern States during the late 1930’s. 
Well over 1100 different kinds of micro-organisms, most of 
which are pathogenic, have been found associated with insects. 
18 
