96 Swede Midge in parts of Yorkshire 



investigated the relation between the Midge and a Proctotrypid parasite 

 which I reared from some maggots in the summer of 1914, and have 

 searched for further parasites. For it is quite possible that the slaughter 

 of 1912 was largely performed by parasites, and would have taken place 

 even if that summer had been as fine as the preceding one. 



It is recorded for the Hessian Fly that a summer in which the fly 

 multiplies greatly is often followed by one in which it is extremely 

 reduced, and this reduction is attributed to parasites. P. Marchal 

 records 1 that in Vendee in 1894 the Hessian Fly increased greatly 

 during the summer, but that it was very reduced in numbers in 1895. 

 His explanation is that this was largely due to parasites which had also 

 increased in 1894, though they commenced to hibernate earlier than 

 the Hessian Fly. The flies of the last brood of 1894 were thus able to 

 lay their eggs without the larvae hatching from those eggs being attacked 

 by the parasite. But in 1895 the parasites hatched out in such numbers 

 that they were able to kill a very large proportion of the maggots of 

 the first brood of the Hessian Fly. A similar thing may have happened 

 to the Swede Midge in 1912. I am therefore aware that the relation 

 between the Midge and the weather is very likely not so simple or 

 direct as it may appear to be from a glance at Table 1. 



(e) Other conclusions drawn from a comparison of the figures collected 

 in the five surveys. 



In 1912 although I did not carry out a systematic Brood-I survey 

 I was able to see that there were large differences between the percentages 

 of infected plants which I found in different districts. But in 1913 the 

 first brood was reduced almost to the same dead level over all the country 

 in which I worked, at Garforth and in the East Riding. This general 

 levelling may have been due to two causes: 



(1) The killing of a larger proportion of the Midge individuals 

 in the more densely populated districts. 



This quite possibly happened, though I have no evidence on 

 the point. 



(2) The migration of flies from more densely to less densely 

 populated districts. 



Such movements of flies probably did take place, for I have shown 

 that while flies of the first brood must move greater or less distances if 

 they are to find swedes, Hies of the later broods also sometimes move 



1 "Les Cecidomyics dee Cm-ales et lours parasites," Ann. <S'oc. Enlom. France, 1897, 

 Vol. LXVT, p. 12. 



