BATE OF MULTIPLICATION OF HESSIAN FLY. 3 



economical of time, therefore, to dissect the females and count the 

 number of ova they contained, and this procedure has been followed 

 out beginning with the fall brood of 1918. 



Before the dissection of female flies and the counting of their ova 

 had been carried very far, other difficulties appeared. These hinged 

 chiefly on the question of what females should be counted. At first 

 it seemed simple enough to count the number of ova contained in a 

 given number of females and average up the results. This, however, 

 proved to be insufficient because of the great variation in the results 

 obtained from different lots of females. The females vary greatly in. 

 size, and, correspondingly, in the number of ova they contain; and 

 the average number of ova per female varies in different fields. 



In the study to determine some of the reasons for this condition of 

 affairs, it became evident that the number of ova contained in a 

 female is related to the number of puparia in a tiller. On the aver- 

 age, the greater the number of puparia in a tiller the smaller the 

 number of eggs the resulting females can lay. Since the average 

 number of puparia in infested tillers varies in different fields and in 

 different years, the difficulty of making a fair average can be readily 

 seen. There also appears to be a relation between the date when 

 wheat is sown and the average number of eggs for females of the 

 fall brood. Puparia obtained from late-sown wheat developed fe- 

 males with a reduced capacity for egg production. When work was 

 begun on the spring brood in 1920 it was found that the average 

 number of eggs varied markedly for the two principal generations. 

 All of these conditions and undoubtedly others have to be dealt with 

 in some manner in arriving at a fair estimate. 



In view of these facts, separate counts have been made for the 

 principal generations. In the case of the fall brood all of the puparia 

 found in a number of plants taken from various fields were caged 

 and every female which emerged was dissected. Most puparia were 

 taken from fields known to be sown early and which, consequently, 

 contained flies of all ages ; or else they were taken from fields where 

 the age of the plants was unknown. This avoided the giving of un- 

 due weight to fields sown late. In this manner many of the data 

 accumulated have been eliminated from the general average for the 

 fall brood. 



In the case of the spring brood, all plants were obtained from a 

 number of fields just before harvest. All their puparia were re- 

 moved and caged. In two cages extremely long stubbles, which 

 probably contained all the puparia of the original plants, were used. 

 In this generation allowance must be made for an enormous mor- 

 tality due to parasitism and weather. Usually it is not expected 

 that more than from 3 to 10 per cent of the flies will remain alive 

 at emergence time in the fall, and, of course, allowance must be made 



