92 Swede Midge in parts of Yorkshire 



(Brood II) is plotted for various rows in Field K at Driffield, sown June, 

 last week, which was next to Field L, sown June, first week, which was 

 attacked by the first brood of the Midge. The two lots of swedes were 

 separated by a hedge and about thirty yards of barley. The graph 

 shows a very marked drop in the percentage of plants attacked in 

 Field K as the distance from Field L increased. 



In 1914 the evidence of such movement was not so clear. 



(c) Factors determining the Swede Midge index in different districts. 



The indices for each district in each survey are given in Tables 2 

 and 3, and plotted on Charts 1 to 5. On the charts for the Brood II 

 and Brood III surveys I have also shown in which districts the index 

 exceeded the index of the previous survey and in which districts the 

 index was not greater. In Chart 5 the indices are the highest and the 

 differences between the various districts are greatest. 



The two factors which seem to be most important are: 



(1) Average date of sowing, 



(2) Distance from the sea. 

 (1) Average date of sowing. 



In the northern part of my East Riding area, as I have shown 

 (p. 88), the Midge is more numerous than in the southern part, and a 

 glance at the charts will emphasize this fact. Mr Henry Ullyott, C.C., 

 farmer and agricultural valuer, has kindly provided me with what in 

 his experience are the average sowing dates for these two parts of my 

 area. For each of the districts in the northern part the estimate which 

 he gives is the last week in May ; for all those in the southern part, the 

 first week in June. In my survey work I examined fields just as I came 

 to them, and the numbers of May sown and June sown fields which I 

 visited in the northern and southern parts are as follows: 



1913 1914 



Number of May Number of June Number of May Number of June 

 sown fields sown fields sown fields sown fields 



Northern part 52 23 G9 11 



Southern part 27 49 55 54 



In 1914, Mr Ullyott tells me, sowing was earlier than usual through- 

 out the East Riding area. At Gaiforth, where the Midge is more 

 numerous than in the East Riding, sowing is earlier. The average date 

 there is the third week of May. Neither in 1913 nor 1914 did I come 

 across any fields sown later than the last week in May. 



I have already shown that May sown fields are worse attacked than 



