BIOLOGICAL CONTROL 
Biological control is possible because of the existence in nature 
of a continuing interplay between the abundance of insects and 
their natural enemies, and because many of the latter are subject 
to manipulation one way or another. Traditionally, it has con- 
sisted of the use of parasites, predators, and disease pathogens to 
hold populations in check, but within recent years the concept has 
been broadened to include various other biotic methods for reduc- 
ing and/or maintaining insect populations at tolerable levels. So 
far, almost all biological control efforts against forest insects 
have been confined to the use of parasites, predators, and 
pathogens. 
Generally speaking, biological control efforts against forest in- 
sects have been limited to (1) the importation and establishment 
of foreign parasites and predators of introduced pests; (2) the 
transfer of parasites, predators, and disease pathogens from one 
region to another within the country; (8) the augmentation of 
established parasite and predator populations with fie!d-collected 
or laboratory-reared individuals; and (4) the use of microbial 
sprays to control outbreaks. 
The majority of importations of parasites and predators were 
made prior to World War II, with emphasis on enemies of the 
gypsy moth, brown-tail moth, satin moth (387, 124), European 
pine shoot moth, European pine sawfly, European spruce sawfly, 
balsam woolly aphid, and larch casebearer. Since World War II, 
importations have been limited mostly to enemies of the balsam 
woolly aphid and the smaller European elm bark beetle. Alto- 
gether, a total of 156 species of parasites and predators had been 
imported and liberated against 59 species of introduced and 
native pests by 1960. Of these, a total of 44 were successfully 
established (205). Since the early 1930’s, the Canadian govern- 
ment has also imported large numbers of many species of para- 
sites and predators against a number of pests, many of which also 
occur in the United States (484). Some of these have spread into 
adjoining areas of the United States. Large numbers of others 
have been shipped to this country and liberated in infested stands. 
A polyhedrosis virus disease of the gypsy moth accidentally 
introduced into the United States in the early 1900’s has since 
played an important role in the control of outbreaks of its host. 
Like others of its kind, this virus is most effective in dense popu- 
lations and is almost unnoticeable in light infestations. Limited 
field trials have indicated that direct spraying of the virus in 
water formulations has considerable promise for direct control. 
Applications of polyhedrosis virus sprays have been used to 
suppress populations of the European pine sawfly (66), the Vir- 
ginia pine sawfly (485), and the Swaine jack pine sawfly (654). 
Virus epizootics have been initiated in populations of the forest 
tent caterpillar by disseminating the virus during one generation 
(685). 
Viruses are usually applied as sprays, but they may also be 
applied as dusts if first incorporated with powder. Sprays may be 
applied by hand-operated sprayers, mist blowers, or aircraft. 
Progress has been made in the control of a number of lepidop- 
terous defoliators with Bacillus thuringiensis. Aerial applications 
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