155 MISC. PUBLICATION 273, U. S. DEPT. OF AGRICULTURE 



on the hind wings. The caterpillars are 1 to I14 inches in length, 

 black Tvith 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 [TijmJa simplex Doane) {60) 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, is 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 liave a very small range of temperatures (50°- 

 95° F.) within which they are the most active; and the optimum for 



