1S MISC. PUBLICATION 657, U. S. DEPT. OF AGRICULTURE 
Diseases 
Insects are subject to many fatal diseases, which sometimes are 
potent factors in suppressing an outbreak of a harmful pest. These 
diseases are caused by many different micro-organisms, including filter- 
able viruses, bacteria, and fungi. Few of these have been adequately 
studied. One of the most common examples is a wilt disease which 
spreads rapidly during outbreaks of various caterpillars. The cater- 
pilars suddenly sicken and die, and are seen hanging from leaves and 
twigs. At first, they are filled with liquid but later present a black- 
ened shriveled appearance. 
RESUME OF NATURAL CONTROL FACTORS 
Under normal conditions, the operation of these physical, nutri- 
tional, and biological forces counteracts the enormous reproductive 
capacity of insects and tends to keep the destructive and beneficial 
ones more or less in balance. The few survivors of harmful species, 
which at such times escape their enemies, continue to live and feed 
on their hosts without doing conspicuous injury. Thus, defohating 
insects usually feed on a few leaves or needles, but the damage is so 
small as to escape notice. The bark beetles kill an occasional tree or 
breed in down logs and broken tops. The aggregate damage is neg- 
ligible, and the annual growth in the forest exceeds this small drain, 
so that there is a net accretion in the volume of wood in the stand. 
Infestations which exist under these conditions are termed normal 
or endemic, and it is a hopeless and unwise undertaking to try to 
exterminate the insects by control measures. 
Under certain conditions, however, the natural balance may be 
broken by any one of a number of factors. The beneficial insects or 
other enemies of harmful species become reduced in numbers; the 
resistance of the trees is lowered through defoliation, drought, fire, 
overmaturity or stagnation; large quantities of slash or other breeding 
material become available ; or climatic factors become especially favor- 
able; and the injurious species breed rapidly and in excessive numbers, 
and a destructive outbreak soon develops. 
Within a few seasons a high percentage of a timber stand may be 
killed by bark beetles, and the destruction may continue for years and 
spread over large areas. Defolators may suddenly increase within 
an area, and after destr oying the foliage of valuable timber over large 
acreages, disappear with equal swiftness. There are many factors 
which come into play in bringing about these sudden changes, and it 
is often difficult to isolate the responsible causes. Outbreaks of such 
a character are called epidemic infestations and require immediate 
attention and drastic control measures. 
SILVICULTURAL CONTROL OF FOREST INSECTS 
There are many types of second-growth stands in the East where 
it is quite possible, through silvicultural practices applied to the grow- 
ing stands, to bring about conditions unfavorable for the development 
of outbreaks of certain insects or to maintain the stand in sufficient 
vigor to avoid serious losses. Some examples of the general principles 
involved in the application of forest practices to insect control are 
here discussed under several different conditions of growth, and fur- 
