86 Swede Midge in parts of Yorkshire 



of two rows widely separated from each other in each field visited. 

 If the figures from the two rows agreed fairly closely, I took the per- 

 centage of attacked plants calculated from those two rows as the 

 percentage for the field. If there was a considerable difference between 

 the two figures, I would examine one or more further rows. Sometimes 

 in the Brood II or Brood III surveys, when it was necessary to hurry 

 through the survey as quickly as possible, I would examine only a 

 single row. This I took care was about half way between the two rows 

 examined in the first survey. I also found out whether each field was 

 sown in May or June by asking people I met in the fields, or failing them 

 I would estimate by eye, a thing which I was able to do with very fair 

 accuracy. In this way a figure for the percentage of attacked plants 

 and the approximate sowing date for each field visited were obtained. 

 Each year I examined about 200 fields, examining the same fields in 

 all the surveys of any one year. 



I should point out that as the leaves grow from the heart of the 

 plant to the outside and finally fall off, signs of attack do not last for 

 more than about five or six weeks. 



In 1913 in the East Riding the three broods of that summer did 

 not, so far as I was able to tell, overlap. In 1914 they did overlap. 

 I call the second survey the Brood II survey, and the third the Brood III 

 survey, because I am sure that flies of those particular broods were 

 respectively responsible for the greater part of the attack in each case. 

 But it is not impossible that in the second survey some plants were 

 counted which had actually been infected by flies of the third brood, 

 and that in the third survey some plants were counted which had been 

 infected by flies of a fourth generation. The two later surveys which 

 were hurried through quickly are therefore surveys of the numbers of 

 plants which could be seen to have been attacked at the stated times 

 at which the surveys were carried out. The dates of the surveys and 

 the tests applied to their accuracy are given in the Appendix. These 

 tests show, I consider, that the figures obtained in my five surveys were 

 sufficiently accurate for the purposes to which I put them. After the 

 conclusion of the last survey charts were constructed, one for each year, 

 on which all the fields examined were marked. The East Riding area 

 was then divided into eleven districts and Oarforth made a twelfth. 

 The ground occupied by the town of Bridlington is not included in 

 any district. All small plots near to swede fields of the previous 

 year were omitted from calculations for a reason which will appear 

 shortly. 



