81 



AN ATTEMPT TO MEASURE THE LOCAL AND 

 SEASONAL ABUNDANCE OF THE SWEDE 

 MIDGE IN PARTS OF YORKSHIRE OVER THE 

 YEARS 1912 TO 1914. 



By F. W. DRY, M.Sc, 

 Fellow of the University of Leeds. 



(With Plate XVIII, 2 Text-figures, and 5 Charts.) 



CONTENTS 



PAGE 



I. Introduction ......... 81 



II. Life-history of the Swede Midge ..... 83 



III. Methods 84 



IV. Statement of results ....... 87 



V. Explanation of results 



(a) Conditions determining the percentage of plants 

 attacked by flies of Brood I of the Swede Midge in a 

 given field ........ 89 



(6) Evidence that flies of the later broods do not always 

 remain in the field in which they have spent the larval 

 and pupal periods ....... 90 



(c) Factors determining the Swede Midge index in different 

 districts 92 



(d) Seasonal variations in the abundance of the Swede 



Midge 95 



(e) Other conclusions drawn from a comparison of the 

 figures collected in the five surveys .... 96 



VI. Summary of chief conclusions . . . . . .99 



I. Introduction. 



The object of this paper is twofold, firstly to describe an attempt to 

 measure the relative density in numbers of one particular insect pest 

 at different times in certain selected places ; secondly, to examine the 

 extent to which the fluctuations in the figures collected can be accounted 

 for. 



Ann. Biol, n 6 



