F. W. Dry 99 



by parasites. As I have already remarked, it may be that the relation 

 of the Midge to the weather is not so simple as it may at present appear 

 to be. 



VI. Summary of chief conclusions. 



1. The Swede Midge was found to be present in 1912, 1913, and 

 1914 both at Garforth in the West Riding of Yorkshire and in all parts 

 of the area in which I worked in the East Riding. At the same time it 

 was possible to measure differences, sometimes very marked, in the 

 relative numbers of the Midge at different places and at different times. 



2. Conditions determining the percentage of plants attacked by 

 Brood I of the Midge in a given field are: 



(1) Date of sowing (p. 89), 



(2) Distance from a swede field of the previous year, and 



(3) Size of field (p. 89). 



3. In an early sown swede field adjacent to an early sown swede 

 field of the previous year a drop, often a very marked one, in the per- 

 centage of attacked plants could usually be plotted as the distance 

 in that field from the swede field of the previous year increased 

 (pp. 89, 90). 



4. An appreciable number of flies of broods later than the first 

 sometimes leave the field in which they have spent the larval and pupal 

 periods, and lay their eggs in another field (p. 91). 



5. Factors determining the Swede Midge index in a given district 

 seem to be: 



(1) Average date of sowing (p. 92). 



(2) Distance from the sea (p. 94). 



6. The prosperity of the Swede Midge appears to depend very 

 largely on summer weather (p. 95). In this connection, however, the 

 relation between the Midge and any parasites requires investigation. 

 I have reared Proctotrypid flies from larvae of the Midge (p. 98). 



7. Very likely the Midge would cease to exist in the Holderness 

 Coast district if it were not for the immigration of flies into that district. 



8. There is a great mortality at some stage or stages of the life- 

 history. 



9. In 1912 in the East Riding the Turnip Flea Beetle, Phylloireta 

 nemorum, was a competitor of some importance with the Midge. 



7—2 



