90 



Swede Midge in parts of Yorkshire 



could be drawn for other similarly situated 1913 fields and for 1914 

 fields near to 1913 fields. The graph shows a quick drop in % CL as 

 the distance from the 1912 field increases, the first few rows being 

 badly attacked, while the middle and far side of the field are not much, 

 though a little, worse attacked than any part of a field not near to a 

 1912 field. Thus in a small field near to a swede field of the previous 

 year, if both fields were early sown, the percentage of attacked plants 



Fig. 5. Graph showing distribution of plants infected by the Swede Midge (Brood I) in a 1913 swede 

 field adjacent to a 1912 swede field. 



The rows are numbered from the side of the field next to the 1912 field. 



is usually high. But in larger fields so situated the attack is to a large 

 extent concentrated on one side and the field as a whole is not so badly 

 attacked. 



(b) Evidence that flies of the later broods do not always remain in the 

 field in which the// have spent the larval and pupal periods. 



Before discussing various points which follow it is important to 

 know whether any appreciable number of flies of the later broods leave 

 the field in which they have spent the earlier stages of their life- 

 history. 



