INSECT ENEMIES OF EASTERN FORESTS 11 
iar relations exist between Scolytus ventralis in western firs and the 
fungus Trichosporium symbioticum. It is likely that similar rela- 
tionships exist between many other bark beetles and fungi. 
The Dutch elm disease, caused by Ceratostomella ulin, is dependent 
on certain insects for its spread from tree to tree. At present the two 
species of insects known to be of greatest importance are Scolytus 
multistriatus and Hylurgopinus rufipes, both bark beetles. 
Future research will bring to light many more cases and possibly 
demonstrate that fungi are necessary for many other types of insect 
feeding. Further information on insects and fungi may be found 
in the following: Craighead (774); Leach, Orr, “and Christensen 
(271); Caird (79) : ; Rumbold (362, 363, 364); Bramble and Holst 
(54); Craighead and St. George (720) ; and Leach (270). 
THE CONTROL OF FOREST INSECTS ° 
In the insect world a constant struggle for survival is going on. 
On the one hand, the insects themselves are provided with potentialities 
for tremendous increase. ‘The females lay hundreds of eggs, and some 
species produce many generations a year. If all individuals sur- 
vived, the world would soon be overrun with the progeny. On the 
other hand, insects must contend with many adverse conditions, which 
serve to hold their numbers in check. Weather conditions, abundance 
or lack of food, prevalence of natural enemies, and many other factors 
have an influence in determining their abundance. Some of the more 
important of these factors are considered in the following paragraphs. 
NATURAL CONTROL FACTORS AND INFLUENCES 
Climatic factors such as temperature, moisture, and unusual weather 
conditions have an important bearing on the activity, periodic abund- 
ance, and distribution of insects. 
TEMPERATURE 
Asarule there is a rather short range of temperatures (50° to 95° F.) 
within which insects are most active, and the optimum for many of the 
Temperate Zone species appears to be between 75° and 80°. Tem- 
peratures either above or below this optimum range limit their ac- 
tivity. Few insects can withstand temperatures above 120°, and this 
makes possible the control of many species of bark- and wood-boring 
insects merely by exposing the infested logs to direct sunhght. 
Low winter temperatures often act as effective checks on insects that 
hibernate as immature stages in exposed locations above the snow line. 
° Prepared from material supplied by all authors and partly adapted from U. S. 
Dept. Agr. Misc. Pub. 273 (Keen, 262). This manuscript was prepared in 1941, 
but its publication was delayed by World War II. With the research energies of 
the Division turned to immediate war problems, no opportunity was offered 
to fully revise the manuscript to include the uses of the new insecticides and of 
aerial spraying. Some revision has been attempted in the section on chemical 
control to introduce these newer developments and this section should be referred 
to in considering chemical control of the various species discussed. 
