A. Vj. Cameron 09 



prevalent when the first brood of the leaf-miner is on the wing as during 

 the occurrence of the two subsequent generations, showing that the 

 hibernating pupae had not been heavily attacked. 



Continued spells of cool, damp weather are believed to have the effect 

 of reducing the number of ])arasites, but this was certainl)^ not very- 

 marked during the wet summer of 1912. The percentage was only 

 slightly less than that of the following year, when more favourable 

 conditions prevailed. This fact will be seen by consulting Table II, 

 which shows the number of parasites that emerged from infested leaves 

 collected in the field and kept either in breeding jars or cages in the 

 laboratory : 



Table II. Pegomyia hyoscyami and its parasites at FaUowfiehl, 

 Manchester, in 1912 and 1913. 



Month Locality Experiment No. of Pefironiym No. of parasites % of parasites 



leaves were No. emerged emerged to total insects 



collected emerging 



July 1912 Fallowfield 1 86 18 17 



Aug. ,. „ 2 24 6 20 



Sept. „ „ 3 37 21 36 



Total ., 



July 1913 Fallowfield 1 



Aug. „ „ 2 



Sept. „ „ 3 



Total .. 



12. Relation of Pegomyia hyoscyami to its Host Plants. 



Random statements are often made by entomologists of the migra- 

 tion of herbivorous insects from one host plant to another, and, although 

 there are well-authenticated cases of certain species becoming serious 

 pests by deserting common weeds to infest cultivated crops which are 

 closely related, I am not at all convinced that this transition always 

 takes place with that abrupt suddenness which many authors assume. 



To determine whether adults of P. hyoscyami, reared from larvae 

 which had fed on belladonna leaves, would oviposit on the leaves of 

 mangold wurzel, a large number of fertilised females were confined in 

 breeding cages along with fresh mangold plants, but the results were 

 purely negative. In no single instance were eggs deposited. These 

 experiments were repeated with fertilised females reared from mangold 



