94 Swede Midge in parts of Yorkshire 



(District XI), where the average sowing date is the first week of June, 

 has very low indices. 



I am also aware that the presence or absence of the cresses Nasturtium 

 palustre and N. silvestre may prove to be a factor of importance. 

 J. F. Robinson 1 records these cresses from the Holderness part of my 

 area, but not from the Wolds. This point requires investigation. 



(2) Distance from the sea. 



The Swede Midge is less numerous in seaside districts than in inland 

 districts. I have shown this to be the case on p. 88 and the charts show 

 it quite clearly. The frequent failure of the Midge to increase in number 

 from one brood to another in these seaside districts may be noted. 



An explanation of these low indices was suggested to me by an 

 accidental observation at Garforth early in August 1912. A bowl of 

 water happened to be standing about a foot and a half from the ground 

 near to a swede field. Within five minutes of each other two flies got 

 on to the surface of the water in this bowl. As soon as a Swede Midge 

 fly drops on to a surface of water it is helpless. This suggested to me 

 that in districts near the sea the Swede Midge indices are kept low by 

 the drowning of flies in the sea. Flies of the first brood must move 

 greater or less distances if they are to find swedes. I have shown that 

 some flies of the later broods leave the fields in which they have spent 

 the earlier stages of their life-history, so that the later broods could 

 suffer as well as the first. The midges are by no means strong fliers 

 for if one is liberated in quite a gentle breeze it appears to be 

 blown helpless by the wind. Two fields in the Flamborough district 

 (District III) in 1913 do, I think, throw light on the problem. They 

 were comparatively small plots next to ground which had grown swedes 

 in 1912. The sowing dates and percentages of plants attacked by 

 Brood I were: 



M. May, 3rd week. 10%. 

 N. May, 3rd week. 6 %. 

 In the other fields in the district the highest percentage of attacked 

 plants was 2-5 %, and the average 0-7 %. These figures suggest that 

 when the flies have only a short journey to make near the sea many of 

 them accomplish it safely, but that when they have to move further in 

 search of swedes many of them are lost in the sen. 



I have examined wind records kept at Bridlington for the period 

 June 1st to Sept. 30th of 1913 and 1914. In 1913 the frequency and 



1 The Flora of the East Biding of Yorkshire L902. 



