166 MISC. PUBLICATION 273, U. S. DEPT. OF AGRICULTURE 
on the hind wings. The caterpillars are 1 to 114 inches in length, 
black with fine-branched spines on each segment, the middle row 
being bright yellow. The adults are in flight during midsummer. 
The peppergrass beetle (Galeruca externa Say) is a black oval beetle 
about one-fourth inch in length, with a narrow yellow border on the 
elytra, which feeds on lupine in the Great Basin area. In 1934 it 
destroyed the lupines over hundreds of acres in eastern Oregon and 
also fed on the grasses. 
The range crane fly (Zipula simplex Doane) (65) at intervals is 
very destructive to grasslands in California. It is also a serious 
pest of grains, barley, and alfalfa. The adults are grayish brown, 
long-legged flies about one-half to three-fourths of an inch long. 
The females are wingless. The larvae, which are pale brown and 
somewhat roughened, live in small round holes in the ground, from 
which they emerge during the night or on dull days to feed upon any 
green vegetation nearby. During mild or wet seasons vast areas 
of range land may be almost denuded. The spreading of poisoned- 
bran mash, such as is used for grasshoppers, 1s an effective control 
measure. 
NATURAL CONTROL FACTORS 
The question is frequently asked, “Where did these destructive 
forest insects come from?” The answer is that in most cases they 
have been here as long as the forest trees on which they feed. Nearly 
all species of western forest insects, both injurious and beneficial, are 
native to these forests and are distributed throughout the range of 
their favorite host. Occasionally foreign pests are introduced and 
become established where food and climatic conditions are favorable. 
So far, no introduced forest insects of major importance have found 
their way into the forests of the West. The destructive species were 
already here and widely established when the forests were first ex- 
amined. Given favorable conditions for their increase, they can 
suddenly build up their numbers from the few parents which are 
normally present and develop epidemics. 
In the insect world a constant struggle is going on for survival. 
On the one hand, the insects themselves are provided with potential- 
ities for tremendous increase. Some females lay hundreds of eggs 
and some species produce many generations a year, so if all individ- 
uals survived the world would soon be overrun with the progeny. On 
the other hand, the insects must contend with many adverse condi- 
tions that serve to hold their progeny in check. Weather conditions, 
food supply, prevalence of natural enemies, and many other environ- 
mental factors influence their abundance. Some of the more 
important of these factors will be considered. 
CLIMATIC AND ENVIRONMENTAL INFLUENCES 
Climatic factors, such as temperature, moisture, and weather con- 
ditions in general, have an important bearing on the abundance, 
activity, and distribution of insects. 
TEMPERATURE 
Insects as a rule have a very small range of temperatures (50°— 
95° F.) within which they are the most active; and the optimum for 
