Forest Insects 
Insects, the most abundant form of life on earth, also are numerically dominant 
in the forest. For over a period of some 350 million years they have evolved to 
occupy an incredibly wide variety of ecological niches. They serve many functions 
in the economy of the forest and are as essential a part of the complex associations 
as are the trees themselves (674). Competition between people and insects for the 
forest and its products increased with the rise of human population. We demand 
much more of the shrinking forest environment now than we did in previous 
centuries. So, this competition is an ever-increasing problem that must be under- 
stood before it can be resolved. 
By no means are all forest insects economically destructive. A multitude of 
species are beneficial. Along with other organisms they are important decomposers 
of forest debris—dead trees, fallen leaves, and such (542, 543). Thus, they help 
return nutrients to the soil, increasing its fertility (306). Others hasten the death of 
decadent, diseased, or overmature trees, making way for vigorous younger growing 
stock (S3/). Many species of insects are parasites or predators of the destructive 
ones (62/). Some innocuous species are hosts for parasites of damaging insects, 
enabling the parasite population to remain at times when the population of destruc- 
tive insects has declined or is in an unsusceptible stage. 
Harmful forest insects and related forms are those that are responsible for 
economic loss. They include (1) species that damage or destroy the flowers and 
seeds of trees, and are particularly important pests in seed orchards and seed 
production areas; (2) species that stunt, deform, or kill nursery stock or young trees 
by damaging or destroying the buds, shoots, or roots; (3) species that cause loss of 
vitality, growth reduction, and often the death of trees by eating the foliage; (4) 
species that feed under the bark or in the wood of living trees and girdle and kill 
them or riddle them with tunnels; and (5) other destructive species that bore into 
and damage or destroy green logs, storm-felled timber, green-sawed and seasoned 
lumber, rustic construction, poles, posts, crossties, mine props, and all manner of 
finished products from flooring to furniture. 
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 commonly 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 increases 
in numbers when favorable environmental conditions prevail. 
Eastern forests are also inhabited by many species of introduced insects, a few of 
which are widely distributed and extremely destructive. These include the gypsy 
moth, European pine shoot moth, balsam woolly adelgid, 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 composition, (2) in overmature rather than in immature 
stands, and (3) in plantations rather than in natural stands. They may also develop 
