BIRDS AND INSECTS 167 



To illustrate the methods by which the control of 

 insect pests may to some extent be effected two 

 examples may be quoted, in which valuable economic 

 measures have been the result of careful scientific 

 work. The maggot of the fly Chortophila brassicce 

 is, in many parts of this country, very destructive of 

 cabbages, cauliflowers, and brussels sprouts. There 

 is evidence that in some cases the whole of the first 

 setting of these plants has been destroyed, and in 

 very many others that over 50 per cent of the plants 

 have been either destroyed or seriously checked in 

 their early growth. 



It was found, on studying the natural history of the 

 insect, that the adult female, which makes its appear- 

 ance early in the month of May, approaches the 

 young plants and lays her eggs on the ground close 

 to the stem. Soon after the eggs are laid the young 

 maggots emerge and burrow down into the earth to 

 begin their attack upon the roots. If the plant is 

 not already well established, and there are sufficient 

 maggots present, the whole of the roots may be 

 destroyed and the plant perishes ; but if the plant 

 is well established, or the maggots not too numerous, 

 it may recover and suffer no more than a temporary 

 check in growth. Now it occurred to Professor 

 Tracey, of Detroit, U.S.A., that if a paper disk were 

 placed round the base of the stem at the time of 

 setting, the maggots would be prevented from burrow- 

 ing down into the soil to attack the roots. As a result 

 of several experiments paper was found to be unsuit- 

 able, and it has been replaced by squares of a coarse 

 roof- felting. 



The use of these disks proving to be extraordinarily 

 successful in America, it was decided to try some 

 experiments on their use in this country. Under the 

 direction of Dr. Imms and Mr. Wadsworth a field 



