20 CIRCULAR 849. U. S. DEPT. OF AGRICULTURE 



August 19, 1936, R. H. Painter, while collecting at about 12,500 feet 

 in the Medicine Bow Mountains, Wyo., noted adults of this species 

 darting about among the rocks. W. L. Schipull, Forest Supervisor, 

 Montezuma National Forest, Colo., reported that on July 13, 1938, 

 he observed thousands of moths in a dense stand of Engelmann spruce 

 and Alpine fir at an elevation of 10,500 feet. The writer observed 

 thousands of moths late in June 1941, in remote canyons of the moun- 

 tains of northern Nevada at 6,000 to 9,000 feet. The} 7 were actively 

 feeding on flowers of various shrubs in bloom at that time. It is not 

 known whether these adults originated locally or were migrants from 

 the Plains. 



The moths reappear in September in greatly reduced numbers and 

 begin depositing their eggs. Dissection of females in June showed the 

 eggs to be barely discernible in the ovaries, whereas in September, when 

 the moths emerged from estivation, the eggs were almost fully devel- 

 oped. 



Life cycle. — Eggs deposited on October 14, 1925, at Wichita, Kans., 

 hatched in from 8 to 18 days, and the larvae molted seven times before 

 pupating the following April. The periods required for the various 

 stages and the width of the head capsule in the different larval instars 

 were as follows: 



Stage: Period Width of head 



Instar: (days) capsule (mm.) 



First 16 to 48 0.30 



Second 13 to 73 0.40 to .50 



Third 4 to 70 .50 to .80 



Fourth 3 to 42 .75 to 1.10 



Fifth 6 to 11 1.10 to 1.65 



Sixth 4 to 18 1.50 to 2.50 



Seventh.... 9 to 25 1.90 to 3.00 



Total 167 to 194 



Pupa 25 to 32 



Egg to adult 216 to 231 



The great variation in the duration of the first to fourth instars was 

 due to differences in the instar in which the larvae hibernated. In 

 spite of these variations the total number of days from egg to adult 

 varied but little. 



The pupae were 18 to 22 mm. long and 6 to 7 mm. in diameter. 



Reproductive capacity. — The number of eggs dissected from 21 adults 

 collected at lights in Garden City and Manhattan, Kans., September 

 27 and 28, 1934, ranged from 318 to 2,241, and averaged 1,150. One 

 month later 16 adult females from the same sources were dissected. 

 At that time the smallest number of eggs was 208, the largest 1,325, 

 and the average 661. Undoubtedly a part of the full complement of 

 eggs had been deposited by most of these females before capture. 

 The enormous reproductive capacity of this species, as revealed by 

 dissections, partly explains its ability to produce seemingly sudden 

 outbreaks. After a favorable estivation period large numbers of eggs 

 are deposited, and, if followed by favorable hibernation conditions, 

 innumerable larvae appear the following spring. When growth in 

 native pasture lands affords enough food the larvae do not migrate to 

 cropland. Frequently, however, the most intense infestations are in 

 pastures that have been grossly overgrazed, so that the larvae are 

 forced to migrate to adjoining alfalfa fields or wheatland. 



