PARTHENOGENESIS IN THE PEAR-SLUG SAW-FLY. 



By H. E. EwiNd, Iowa vSlate College. Ames, Iowa. 



Introductory. 



But few of our economic insects have been studied more than 

 the common pear-slug or cherry-slug saw-fly, Caliroa cerasi 

 Linn., yet concerning some points in its biology and life history 

 we are as yet in doubt. Parthenogenesis, known to occur in 

 some of the saw flies, has been suspected in the case of this 

 species, but does not appear to have been sufficiently proved. 

 Mr. R. L. Webster, in his bulletin on this pest* presents a good 

 account of its biology and life history, but in regard to partheno- 

 genesis (complete and successful) he states that it had not been 

 sufficiently proved, yet some important observations were made 

 which are mentioned in this paper. 



Rarity of the Males. 



Males of the pear-slug saw-fly have been described, and 

 were at one time supposed to be fairly common. Mr. Webster, 

 however, noticed early in his work with the species the apparent 

 absence of the males around Ames, Iowa. On page 181 of his 

 bulletin he states: "We have collected and examined large 

 numbers of saw-flies, but have never found a male." He also 

 wrote to Mr. S. A. Rohwer, of the U. S. National Museum in 

 regard to the rarity of the males. Mr. Rohwer examined the 

 collection of the museum, and reported that there were no males 

 there, either from Europe or from America. While working 

 at the Iowa Station in 1911, and while at the Oregon Station, 

 1911-14, I had occasion to examine hundreds of these saw-flies 

 but never found a male. It appears then that if males exist 

 in this country, they must be very rare in certain localities. 



Caliroa cerasi differs from many of the saw-flies in respect 

 to the numerical ratios of the sexes, for in some of the species 

 the males are present in abundance, and mate normally with 

 the females. This was shown to be the case with the chcrrv 



♦Webster. R. L. Tlie Pear-slug. Bui. No. 130. Iowa Agric. Exp. Sta. (1912). 



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