KATE OF MULTIPLICATION OF HESSIAN" FLY. 7 



Going on the assumption that under normal conditions practically 

 every egg is laid, let it be assumed that none of the fly stages are 

 destroyed by enemies. For the sake of round numbers let 285 be 

 taken as the average number of eggs laid by the fall generation and 

 230 as the average by the spring generation. It may also be taken 

 for granted that one male will fertilize several females, since Enock 7 

 proved by careful methods that one male successfully fertilized as 

 many as six females. This will take care of the preponderance of 

 females in the spring brood. 



Starting now with a single female of the spring brood emerging 

 in the fall, it is found that she can lay 230 eggs. One-half of these 

 eggs in the spring will produce females, each of which may lay 285 

 eggs. This will give 32,775 as the product of one female working 

 through two generations in one wheat crop, with no allowance made 

 for multiplication in volunteer wheat. The flies which develop in 

 volunteer wheat in the East are usually comparatively small in num- 

 ber and are heavily parasitized. If it be assumed that all these flies 

 remain alive in the stubbles until the next fall, there will be a total 

 of 60 per cent of the flies emerging as females, or 19,665. If these 

 all lay 230 eggs, there will be the enormous total of 4,522,950 flies 

 developing in the fall wheat of the second year. The resulting fe- 

 males would number 2,261,475 and would give rise in the second 

 spring to 644,520,375 fly stages. This result has been obtained in 

 two crop years, not allowing for any multiplication in volunteer 

 wheat. 



The question may be looked at from another point of view. Prob- 

 ably infestation of stubbles is never as low as one puparium per 

 acre, although the foregoing computation began with one fly. In 

 one field studied quite carefully during the summer and fall of 1919 

 the stubbles were found to be infested at the rate of 4,900,000 per 

 acre. A careful examination of the puparia in these stubbles just 

 before the emergence season showed that only 4.4 per cent contained 

 living fly larvae, parasites having destroyed practically all of the 

 remainder. This 4.4 per cent amounts to 215,600 flies per acre. If 

 60 per cent of these were females there would be 129,360 females per 

 acre ovipositing in the next crop of wheat and producing in it the 

 next fall 29,752,800 ova. With similar calculations from a few 

 other fields as a basis, a big outbreak in wheat sown in the fall of 

 1919 was looked for. As a matter of fact, however, it was found 

 after the emergence season was over that many of the expected flies 

 had failed to emerge from the stubbles. A careful count made in 

 the field mentioned above showed that only about one-third of the 

 healthy fly larvae had transformed. This amounted to about 1.4 

 per cent of the total number of flies. 



7 Op. cit. 



