299 - 
Seasonal Populp.tion Trends of &ras shODuer Infestations for 
Different Crons and Habitats 
Systematic observations were made throughout the 1939 season in 
the main grasshopper areas by special survey men, therefore, three of 
these areas xidll be discussed separately. On the basis of weekly esti- 
mates of the number of grasshoppers per sq.uare yard occurring in different 
crops and habitats, population trends have been traced for all of the 
nymphal period and part of the adult period, as shown in table 2, Dor 
most of the habitats the peak of nymphal population was reached by May 
13. The peak of nymphal population for mountain-hay meadows was June 3, 
and for corn June 24 (table 2), 
The trends of grasshopper infestations in the various habitats in 
Montana and ITorth Dakota (northern M. mexicanus area) and in Wyoming, 
South Dakota, and Nebraska (southern M. mexicanus area) represent popu- 
lations containing from 75 to 80 percent M. mexicanus . There was a rapid 
increase in populations from hatching to the peak and then a gradual de- 
crease took place. The range land shown for Montana and North Dakota 
was, for the most part, depleted range of sage brush, Russian- thistle, 
pricklypear cactus, pepper grass ( Lenidium ) . western wheat ( Agrouvron 
smithii ). needlegrass (Stina ). and other grasses (Poa app.). In the northern 
M. mexicanus area there was a second peak in small grain the week of 
July 1, This was probably due to migration of grasshoppers from des- 
troyed fields and distant idle and depleted range lands into grainfields 
that had not been previously infested. 
