those of the nuclear-polyhedrosis type. Further information on viruses is available 
(96,215,022, S16) 11455 J:195). 
Several species of bacteria produce so-called milky diseases in the insect host, 
the best known being the milky disease of Japanese beetle larvae (350). The larvae 
of many closely related species of beetles are also affected by this disease. Scores of 
species of Lepidoptera and apparently several species of Hymenoptera, Coleoptera, 
Diptera, and Orthoptera are susceptible to diseases caused by various 
crystalliferous bacteria, the best known of which are several varieties of Bacillus 
thuringiensis Berliner (343, 344, 347, 549, 550, 709). 
Some species of fungi occasionally cause significant reductions in insect popula- 
tions. Aphids are particularly susceptible to attack by species of Entomophthora. 
The species E. megasperma Cohn was credited with causing a high degree of 
control of the forest tent caterpillar in Ontario. Another species, Beauveria bas- 
siana (Balsamo) Vuillemin, reportedly killed more than 90 percent of the larvae of 
the smaller European elm bark beetle in a number of infested elm trees in Connecti- 
cut (3//). Pathogenic fungi, such as Paecilomyces sp., Beauvaria bassiana, Meta- 
rhizium anisopliae, etc., killed nearly one-third of the southern pine beetles in the 
field (S67). Laboratory testing has confirmed the activity of these fungi against the 
southern pine beetle (954). 
Many species of pathogenic Protozoa are also associated with forest insects (652, 
1094, 1125). Generally speaking, these pathogens prolong the length of the larval 
stage and reduce the fecundity of surviving adults. 
Numerous species of nemotodes may be found in the gut, the hemocoel, or in 
particular organs such as the Malpighian tubules of insects. Some of these kill the 
host when they emerge, some cause death by initiating the action of lethal microbial 
disease agents (/265). Some cause injury but not death (9/). Certain species 
infesting bark beetles riddle the intestines and gonads of their hosts, block the 
ducts, and cause the ovaries to shrivel. This may lead to a marked reduction in egg 
production by infested female beetles and a sharp decline in beetle populations 
(S26). The literature on the subject has been reviewed (/270). 
Applied Control 
When natural control factors fail to hold populations to economically tolerable 
levels, it may be necessary to apply silvicultural, chemical, or other controls to 
enhance, supplement, or serve as substitutes for natural control. Depending on the 
insect and its biology and ecological relationships, a wide variety of materials and 
methods are available for suppressing populations. These may be applied directly 
against the insect to interfere with its growth or reproduction, or to kill it; or they 
may be applied to the environment to render it less favorable for the insect through 
alteration in food supply, in microclimatic factors, or in the abundance and effec- 
tiveness of natural enemies. 
The choice of materials or methods in artificial control depends on several things: 
whether the insect is a native or introduced species, the factors known or thought to 
be responsible for its abundance, the need for quick and effective reduction in its 
numbers, and cost. If a native species is involved, usually the aim is not to eradicate 
it, but rather to reduce its numbers to a tolerable level; if it is a recently introduced 
pest, the aim may be to eradicate it or to slow down or prevent its spread. 
Artificial controls designed to suppress existing outbreaks usually have short- 
term effects only; those that are used for the prevention of outbreaks may be long 
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