casebearer, and the smaller European elm bark beetle. By 1982 a total of 264 
species of parasites and predators had been imported and liberated against 60 
species of introduced and native pests. Of these, a total of 50 are known to be 
established. Beginning in the early 1930’s, the Canadian government also imported 
large numbers of many species of parasites and predators against a number of pests, 
many of which also occur in the United States (227, 788). Some of these biological 
control agents 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 
populations and is almost unnoticeable in light infestations. A microbial spray 
formulation of this virus has been developed and registered under the name 
Gypchek (732). 
Applications of polyhedrosis virus sprays have been used to suppress populations 
of the European pine sawfly (//2), the Virginia pine sawfly (790), the Swaine jack 
pine sawfly (/095), and the redheaded pine sawfly (292, 1/45). Virus epizootics 
have been initiated in populations of the forest tent caterpillar by disseminating the 
virus during one generation (//44). Other important eastern forest insects that are 
subject to polyhedrosis virus diseases are European spruce sawfly, eastern tent 
caterpillar, fall webworm, linden looper, whitemarked tussock moth, winter moth, 
jack pine sawfly, and the blackheaded pine sawfly. 
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. Experimental vectoring with contaminated parasites or 
males may simplify distribution of viruses in the future. 
Considerable progress has been made in the control of a number of lepidopterous 
defoliators with Bacillus thuringiensis. The HD-1 isolate of B. thuringiensis is now 
used commercially against many forest insect defoliators, including gypsy moth, 
spruce budworm, Douglas-fir tussock moth, tent caterpillars, webworms, and 
cankerworms. 
Chemical Control 
The use of chemicals to suppress forest insect populations is usually a method of 
last resort. It should be the practice to use them only when other forms of control, 
either natural or artificial, fail or threaten to fail in the prevention or control of 
destructive populations. Chemical insecticides sometimes may complicate the reso- 
lution of an insect problem (642,755). Depending on the situation, chemicals may 
be applied to a single tree or to forested areas covering thousands of hectares or 
square kilometers. The aim, therefore, is usually limited to the suppression of 
injurious populations to tolerable levels. 
Many different types of equipment and techniques are available for applying 
insecticides (993). Aerial applications are made to large areas by fixed-wing 
aircraft (fig. 1) or helicopters (5/, 59, 90, 354, 627, 641). Individual trees, small 
groups of trees, and seed orchard trees (fig. 2) may be treated by ground equipment 
such as mist blowers (290, 994) or by knapsack sprayers. Large individual trees 
may be treated by mist blowers and by hydraulic sprayers (S38). Logs are treated by 
knapsack and power sprayers (/040). Fogging machines are sometimes used around 
14 
