1917] Parthenogejtesis in the Pear-slug Saw-fly 335 



Observations in the orchard where the first virgin females 

 were obtained showed that the conditions there were similar 

 to those of my experiments. During the first year there were 

 large numbers of the first generation females present, but I did 

 not observe a single male, hence infer that practically all of the 

 eggs laid were unfertilized. These females laid an enormous 

 number of eggs, and apparently almost all of them hatched, for 

 seldom have I seen cherry trees more heavily infested than 

 were these trees during the summer of 1913. The injury was 

 so great that several of the younger trees were killed outright — 

 something unusual in the case of saw-fly infestation. Yet in this 

 same orchard the following spring, scarcely a saw-fly emerged. 

 On June 7, I examined the orchard thoroughly, and did not 

 find an adult or a single egg. It was June 20 before I found 

 eggs, and then only a few of them. The slugs that developed 

 in this orchard in 1913 were very few, and were of no importance 

 from an economic standpoint, in fact most of the trees were 

 absolutely free from them. 



Could it be that climatic conditions killed the saw-flies in 

 this orchard and in my cages? During the spring of 1913 we 

 had a very warm spell in March, and later a cold snap. Could 

 this warm spell have so hurried the development of the saw- 

 flies that they were later killed by the cold? I think not, for 

 in other places in the same vicinity the saw-flies came out in 

 abundance. One orchard, only a few miles away, was badly 

 infested and injured the same spring. Could the saw-flies 

 have been killed by a fungous disease? None whatever was 

 detected. Even the bodies of dead individuals were usually 

 free from fungi. Could it be that the ground was so hard 

 during the emerging period that the adults could not make their 

 way out? I hardly think so. At first I suspected this as being 

 the reason, but after keeping the ground soft in two of my 

 breeding cages for many days, I did not get a single saw-fly to 

 emerge. Besides the thorough examination of the earthen 

 cells showed that the adults did not even get out of them. 



It appears then that we must look elsewhere in order to 

 find the causes for the non-emergence of these second generation 

 parthenogenetic adults. Judging from the facts collected 

 during my investigations of this species, I am inclined to the 

 opinion that parthenogenesis while normal and completely 

 advantageous for the species in the spring parthenogenetic 



