FOREST INSECTS 
Insects are by far the most numerous of all the forms of animal 
life inhabiting the forest. They are remarkably well adapted to 
their surroundings and occupy an incredibly wide variety of eco- 
logical niches. They serve many functions in the economy of the 
forest and are as essential a part of the complex association of 
organisms comprising it as are the trees themselves (412). While 
the majority of species are either beneficial or innocuous, many 
species are exceedingly injurious. 
Beneficial forest insects are represented by a great many 
species, some of which are enormously abundant. Some feed on 
forest debris and aid in its deterioration; others feed on organic 
matter in the duff and soil and contribute to improvements in soil 
fertility. Some contribute to stand improvement by attacking and 
killing decadent, diseased, or overmature trees and making way 
for younger growing stock. Many others are parasites or preda- 
tors of destructive species of insects. 
Harmful forest insects are those that are responsible for eco- 
nomic loss. They include (1) species that damage or destroy the 
flowers and seeds of trees and which are particularly important 
pests in seed orchards and seed production areas; (2) species that 
stunt, deform, or kill young trees by damaging or destroying the 
terminals, laterals, or roots of reproduction and in plantations; 
(3) species that cause loss of vitality, growth reduction, and often 
the death of trees by eating the foliage; and (4) species that feed 
under the bark or in the wood of living trees and girdle and kill 
them or riddle them with tunnels are important pests in the forest. 
These are not all: Numerous others that bore into and damage or 
destroy green logs, storm-felled timber, green-sawn and seasoned 
lumber, rustic construction, poles, posts, crossties, mine props, 
and all manner of finished products from flooring to furniture are 
highly destructive. 
Most of our forest insects are native to the continent and are 
usually distributed throughout the ranges of their hosts. Some are 
destructive at normal population levels, but the majority normally 
occur in such low numbers as to be of little or no consequence. A 
few of the latter, however, are capable of great and rapid in- 
creases in numbers when favorable environmental conditions 
prevail. 
Eastern forests are also inhabited by many species of intro- 
duced insects, a few of which are widely distributed and extremely 
destructive. These include the gypsy moth, European pine shoot 
moth, balsam woolly aphid, European pine sawfly, and the smaller 
European elm bark beetle. Many species of natural enemies of 
several introduced pests have also been imported and established 
in eastern forests. 
Conditions conducive to forest insect outbreaks are only partly 
understood. It appears though that outbreaks are most likely to 
occur (1) in pure stands rather than in stands of mixed composi- 
tion, (2) in overmature rather than immature stands, and (3) in 
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