This total includes 90 species and varieties of bacteria, 260 species 
of viruses and rickettsia, 460 species of fungi, 255 species of proto- 
zoa, and 100 species of nematodes (557). Generally speaking, they 
gain entry into the insect either by being ingested with food, 
through wounds or other damaged areas in the integument, or 
through the integument or trachea. Most of them rely on wind, 
rain, streams, healthy or contaminated insects, small mammals, 
or birds for dispersal throughout an insect infestation. Spread is 
occasionally so rapid that all of the insects in an infestation may 
appear to be dying at the same time. 
Many species of forest insects are particularly susceptible to 
viruses of the nuclear-polyhedrosis type. Once inside the host 
these viruses enter the cell nuclei, replicate, and become encapsu- 
lated in many-sided, rodlike crystals, known as polyhedra. In- 
vaded cells are soon destroyed, the polyhedra are then released in 
the body cavity, and the host dies. Diseased larvae usually become 
sluggish, cease feeding, and in some instances move upward in 
the trees. After death they may be seen hanging by their forelegs, 
with the body darkened, decomposed, and liquefied. Eventually 
they completely disintegrate and dry up on the trees. Some of the 
more important eastern forest insects, in addition to the gypsy 
moth and European spruce sawfly, that are subject to polyhedrosis 
virus diseases are eastern tent and forest tent caterpillars, fall 
webworm, linden looper, white-marked tussock moth, winter 
moth, European pine sawfly, red-headed pine sawfly, jack-pine 
sawfly, Virginia pine sawfly, and Neodiprion excitans. 
Forest insects are also affected by several other kinds of 
viruses—cytoplasmic polyhedrosis, granulosis, polymorphic inclu- 
sion, and non-inclusion. Generally speaking, these viruses appear 
to be less effective and less specific in control than those of the 
nuclear-polyhedrosis type. For further information on viruses the 
reader is referred to Bergold (61), Bird (67), Hughes (389), 
Martignoni and Langston (501), and Steinhaus (690). 
Several species of bacteria produce so-called milky diseases in 
the insect host, the best known being the milky disease of Japa-. 
nese bettle larvae (212). 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 Hymen- 
optera, Coleoptera, Diptera, and Orthoptera are susceptible to 
diseases caused by various crystalliferous bacteria, the best 
known of which are several varieties of Bacillus thuringrensis 
(G3), 332). 
Some species of fungi occasionally cause significant reductions 
in insect populations. Aphids are particularly susceptible to at- 
tack by species of Entomophora (=Empusa). The species, E. 
megasperma, was credited with causing a high degree of control 
of the forest tent caterpillar in Ontario from 1949 to 1952. An- 
other species, Beawveria bassiana, reportedly killed over 90 per- 
cent of the larvae of the smaller European elm bark beetle in a 
number of infested elm trees in Connecticut (195). 
Many species of pathogenic Protozoa are also associated with 
forest insects. Generally speaking, they prolong the length of the 
larval stage and reduce the fecundity of surviving adults. 
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