INSECT ENEMIES OF EASTERN FORESTS i 
when there are no convection currents of air. Except on cloudy days. 
the most suitable period for aerial spraying is just after dawn. — 
When properly applied, the concentrated spray mixtures have a dis- 
tinct advantage over dusts. The loss from drift is less, the deposit 
is heavier, and adherence is better. This method of using the con- 
centrated sprays has been more fully treated by Potts (356, 357). 
USE OF BENEFICIAL INSECTS IN THE CONTROL OF 
FOREST-TREE PESTS 
: By R. C. Brown 
It has already been pointed out that practically all forest insects 
are attacked by numbers of parasitic and predaceous insects that nor- 
mally exert a very definite influence in keeping the forest insects in 
check. The forest entomologist is therefore interested in the use of 
these beneficial insects, whenever possible, as a control measure. Ob- 
viously those types of control in which nature itself will lend a helping 
hand should be utilized to the fullest extent. It may seem presump- 
tuous on the part of man to attempt natural control. Man is, however, 
continually upsetting the balance of nature, and the purpose of bio- 
logical control is to restore to equilibrium, as far as possible, the 
normal balance between a host and its natural enemies. 
CONTROL OF INTRODUCED FOREIGN INSECTS 
When a forest insect is accidentally introduced into a new favorable 
environment without its native parasites and predators, and finds an 
abundance of favored food, it is likely to develop into a serious pest. 
Many important forest insects have been accidentally introduced into 
North America from Europe. Most of these imported insects have 
gained entrance into the Northeastern States or Canada and present 
some of the most serious problems confronting forest entomologists 
in these regions. The gypsy moth, the brown-tail moth, the satin moth, 
the European pine shoot moth, the larch casebearer, the larch sawfly, 
the European spruce sawfly, several species of pine sawflies, the smaller 
European elm bark beetle, the elm leaf beetle, the beech scale, the 
fir bark louse, and two birch leaf-mining sawflies are all native Euro- 
pean species, which have been introduced and are now firmly estab- 
lished in the Northeast. Several of these have become serious pests 
in natural woodlands and forest plantations. 
The gypsy moth alone has already cost State and Federal Govern- 
ments many millions of dollars in attempts to control it and prevent 
its westward spread Other introduced forest insects have already 
spread over rather large areas in the United States and Canada, and 
in the aggregate thousands of acres of forests and plantations are 
affected annually. The presence of these insects was not detected until 
they had spread over such large areas that it was too late to attempt 
eradication. It appears that the absence of their natural enemies was 
one of the main reasons for their tremendous increase. 
Generally speaking, insect parasites are fairly specific in their host 
relationships; therefore it is a rare occurrence to find a native parasite 
that has become an effective enemy of an introduced pest. 
