F. W. Dry 93 



June sown fields, so that the larger number of June sown fields in the 

 late sowing districts helps to keep the indices low in those districts, but 

 a glance at Tables '2 and 3 will show that the average numbers of attacked 

 plants for May sown fields in the early sowing districts are higher than 

 for May sown fields in late sowing districts. 



There is no evidence of any brood of the Midge in Yorkshire earlier 

 than the first brood which attacks swedes. The question at once 

 presents itself, therefore, what is it which brings to a close the long 

 winter's rest ? It cannot be mere lapse of time, for if it were we should 

 never find an interval between two broods of one summer during which 

 flies are relatively scarce or not to be found at all. During the Christmas 

 and Easter terms of 1913 — 14 and the Christmas term of 1914 I have 

 kept maggots in sand or soil indoors at average temperatures of from 

 55° F. to 70° F., and have reared a number of flies from them ; but from 

 maggots kept out of doors under the same moisture conditions, I have 

 not reared a single fly. This suggests that temperature is an important 

 factor, if not the only one, in bringing about the emergence of the first 

 brood of midges. Xow from observations in the field in 1911 on the 

 length of time during which the flies of a brood could be found, it seems 

 unlikely that the average lifetime of the fly is more than a week. We 

 are thus able to understand the importance to the Midge of the average 

 sowing date of swedes. 



Swedes grow faster in the later sowing parts of my East Riding area 

 than in the earlier sowing parts, doubtless owing to slightly higher 

 temperature. Moreover, throughout my East Riding area, earlier sowing 

 than is actually the practice is prevented by the fear of Leaf Mildew which 

 attacks early sown swedes more than late sown ones. This mildew may 

 therefore be looked upon as, indirectly, a limiting factor of the Swede 

 Midge. 



It is interesting to find that in the United States the chief way of 

 combating that dangerous Cecidomyiid enemy of wheat, the Hessian 

 Fly, is deliberate late sowing. 



There mav. of course, be other factors contributing to the greater 

 success of the Midge in the early sowing districts. One factor may be 

 the proportion of land under swedes. This is somewhat higher on the 

 Wolds than in Holderness, because a four course rotation is the rule on 

 the Wolds, while a five course one is the practice in Holderness. In 

 Holderness, too, more mangels are grown as a root crop and the pro- 

 portion of grass land is larger. Soil differences may be another factor. 

 But it is especially notable that the Little Weighton wold district 



