F. W. Dry 87 



For each survey in the different districts a figure was calculated 

 which may be taken as a measure of the relative numbers of the Midge 

 per acre of swedes, and may be called the Swede Midge index. This 

 figure was obtained in the following way. For each survey the average 

 percentage of attacked plants in the fields examined in each district was 

 calculated. In speaking of the percentage of attacked plants in a field 

 the abbreviation " % CL " is often used, meaning the percentage of 

 plants showing the crumpled-leaf condition. An average figure is 

 therefore sometimes referred to as the "average % CL." When a 

 calculated average % CL for a district was between two whole numbers, 

 the higher whole number of the two was taken as the index. For 

 example, whether the figure was 7-1, 7-6, or 8-0, the index was taken 

 as 8. The indices are tabulated in Tables 2 and 3 and plotted on Charts 

 1 to 5. 



In my East Riding area there are two chief types of country. In 

 the two Holderness districts the whole area lies below the 100 ft. 

 contour and is quite flat. The boulder clay soil is deep and strong and 

 requires artificial drainage. The Wold part of the area is higher, 

 though nowhere as high as 600 ft., and contains numberless valleys 

 (see fig. 4, Plate XVIII). A thin, comparatively light soil covers the 

 porous chalk, and artificial drainage is unnecessary. 



From figures published by the Board of Agriculture 1 I calculate 

 that the proportion of land under turnips and swedes in the East 

 Riding is about one acre in ten. This proportion is higher than in any 

 other county in Great Britain. Swedes are probably grown on about 

 one acre in twenty. 



IV. Statement of results. 



The more striking features of the appended tables may be briefly 

 summarized. 



1. In May sown fields the attack of the Midge is greater than in 

 June sown fields. 



2. The first brood of 1912 was a very large one. That of 1913 was 

 a very small one. The progress of the Midge in the East Riding area 

 from the first brood of 1913 to the third brood of 1914 is shown in a 

 simple manner in the following table : 



1 Abstract of Agricultural Returns for Great Britain, collected 4th June, 1910. 



