EXPERIMEXTS OX THE BREEDiyPr OF THE MAXOOLD FLY. 



89 



It was suspected that these predaeeous insects attacked and killed a few of the 

 mangold and dock flies. It is pretty certain, however, that a sufficient supply of 

 material was maintained to render this factor negligible. 



Inside the cage sugar-beets were sown in three of the compartments and mangolds 

 in the remaining two. The drills ran lengthwise and were placed 27 inches apart. 

 Throughout the summer the plants received careful attention, being singled, 

 cultivated and kept free from weeds. Up to about the time that the seedlings were 

 inches above the ground there was a period of severe drought, which terminated 

 on 25th June. It was therefore thought expedient to" water the plants occasionally 

 to prevent their flagging. With the extra attention paid to them they developed 

 into strong healthy plants with large roots, rather better than those outside the cage, 

 which were seriously affected by the attacks of black flv (ApJii.'^ rmnicis, L.). 



Fig. 2. 



The scheme of the experiments was as follows. In compartment I., in which 

 sugar-beets were grown, specimens of P. bicoJor reared from docks (Rumex obtusifoUus) 

 were liberated. In compartments II. and III. sugar-beets were also grown, and the 

 flies liberated were respectively P. hijoscyami reared on belladonna and P. hyoscyami 

 reared on mangolds. In compartments IV. and V. mangolds were sown, and into the 

 first of these specimens of P. bicolor reared from docks were introduced : in the other 

 were specimens of P. hyoscyami. The method of procedure adopted was to collect 

 as many nuned leaves of dock, belladonna and mangolds as possible containing the 

 infesting maggots. These were then isolated in different breeding boxes andTbred 

 through to the pupal condition in the insectarv (fig. 2) attached to the field experiment 



