82 Swede Midge in parts of Yorkshire 



It is, of course, notorious that the abundance of insect pests varies 

 enormously in different years and in different localities, but so far as 

 I know little has been done in the way of actually measuring such 

 fluctuations in abundance. Unless such measures can be obtained 

 there is, of course, no definite basis on which to correlate differences 

 of abundance with differences of conditions. Various factors may be 

 suggested as of importance in determining the degree of prosperity of 

 an insect, but without a statistical basis it must necessarily be difficult 

 or impossible to discuss such factors with precision. 



The only way in which greater precision can be ensured appears to 

 be to undertake a census over a period of years and preferably over an 

 area which includes districts which differ amongst themselves in the 

 conditions to which the insect is exposed in them. The conclusions 

 which are drawn from such a census should then so far as possible be 

 subjected to the test of experiment. 



The present paper is an account of work carried out on those lines 

 on the Swede Midge, Contarinia (Diplosis) nasturtii, Kieff . The country 

 worked in was the area in the East Riding of Yorkshire shown on the 

 accompanying charts, and also a small area round Garforth in the 

 West Riding. 



My object was to obtain a figure for each of the districts into which 

 I divided my area, which should be roughly proportional to the number 

 of Swede Midge individuals per acre of swedes. With many crops and 

 many insect pests methods like those shortly to be described could not 

 be used. In the case of the Swede Midge, however, a combination of 

 fortunate circumstances makes it quite practicable to carry out a 

 fairly accurate census, and to do so, if desired, two or three times in 

 one summer. These particular circumstances are briefly as follows: 



1. The number of swede plants in a row can easily be counted. 



2. It is easy to recognise at a glance the characteristic dis- 

 torted appearance of the swede plant produced by Swede Midge 

 attack. 



3. Signs of Swede Midge attack on a plant last, as a rule, about six 

 weeks after the eggs have been laid. 



4. Swede Midge maggots remain on the plant upon which they have 

 been born until they have completed the feeding stage. They do not 

 move from one plant to another. 



5. So far as I am aware the Midge never kills a plant. However 

 badly a plant may be attacked, it remains alive and shows, by its 

 distorted appearance, that it has been attacked. 



