THE ALFALFA CATERPILLAR. 



23 



preceding winter may have killed the hibernating Pteromalus larvse 

 (fig. 11). In the same year Mr. E. N. Wilson found the species 

 quite numerous in the Imperial Valley of California. As many as 20 

 per cent of the Eurymus pupse were parasitized by it. 



This parasite seems to be distributed over a considerable area, for, 

 besides being present in Arizona and California and, as stated, at 

 Wellington, Kans., it has been reared during the season of 1913 and 

 found to be quite abundant at Salt Lake City, Utah, by Mr. Eock- 

 wood, and at Nashville, Tenn., specimens were raised by Mr. Larri- 

 mer from a single pupa of Eurymus. 



It seems almost certain that this parasite winters as a larva with- 

 in the pupal shell of the host. The first lot collected in a pupa of 

 the alfalfa caterpillar in December were discovered 

 as larvse in January and soon thereafter turned to 

 pupse (fig. 12), issuing as adults in March. The 



Fig. 11. — Pteromalus 

 eurymi: Larva. 

 Greatly enlarged. 

 (Original.) 



Fig. 12. — Pteromalus 

 eurymi: Pupa. 

 Greatly enlarged. 

 (Original.) 



Fig. 13. — Pteromalus 

 eurymi: Adults is- 

 suing from chrys- 

 alis of alfalfa cat- 

 erpillar. Enlarged 

 nearly three diam- 

 eters. (Original.) 



eggs are laid in pupse of Eurymus, from 40 to 114 parasites develop- 

 ing in one pupa. About 80 to 90 per cent of these are females and 

 the rest males, and the adults issue from one or more tiny holes in 

 the pupa of their host. (See fig. 13.) 



The combined length of the egg, larval, and pupal stages in the 

 warmer weather of August is from 12 to 15 days, while the pupal 

 stage was found to cover 4 days in the month of August and 12 

 to 15 days in February, the variation being due to differences of 

 temperature. Thus several generations are possible each season, and 

 thus, with abundant egg production and high percentage of females, 

 gives rise to a rapid increase in the number of parasites, so that by 

 late August the multiplication of the host species is checked. 



