513 



Experimental Institute at Wadenswil. Examinations of the hiber- 

 nating pupae showed that those found in exposed situations were 

 of a dark colour and never yielded adults ; these emerged only from 

 the bright green pupae taken from sheltered positions. The cater- 

 pillars of the summer generation usually pupate on the food-plant, 

 but cold weather causes those of the autumn generation to seek more 

 elevated situations that protect them against damp and cold. Mild 

 weather in autumn may sometimes lead to this migration not taking 

 place and in these cases the cold weather later on destroys the 

 unprotected pupae. Parasitised larvae pupate earlier than others and 

 are unable to reach suitable shelters ; the collection of the winter 

 pupae of P. brassicae therefore has little value and may even be 

 harmful. 



As the pupal stage of the parasites lasts longer than that of the host, 

 the host-larvae are already present when the parasites begin to 

 oviposit. Ichneumonid and Braconid parasites are more numerous 

 than Tachinids. The Hymenoptera chiefly concerned are Apanteles 

 (Microgaster) glomeratus, Pimpla instigator, and Pteromalus puparum, 

 the last-named being the most numerous. In autumn Pteromalus 

 appear to parasitise female pupae by preference. 



Experiments were made to ascertain where the eggs of the first 

 generation of P. brassicae are laid. There does not seem to be any 

 preferred food-plant. The eggs were found, singly, on Brassica 

 oleracea (cabbage), Knautia arvensis, Plantago minor, Taraxacum 

 officinale (dandelion), Heracleum sphondylium, Trifolium pratense and 

 other plants. No eggs were laid on meadow grasses or on Caltha 

 palustris. The egg-stage lasts 10-12 days, the larval 34-38 and the 

 the pupal 14-18 in summer and up to 5 months in winter. The entire 

 life-cycle occupies 58-68 days in summer. In the present case the 

 first generation appeared in April and May and the second from the 

 end of Jmie to the end of July. The second did comparatively little 

 damage as a result of the extensive parasitisation of the first. This 

 occurred chiefly on the first day of the larval stage, so that it appears 

 that the Hymenoptera concerned, Pimpla and Pteromalus, mainly 

 attack the younger stages. In these experiments Tachinids, which 

 are frequently found in winter pupae, were not observed, but the 

 larvae examined were not more than 8 days old, and Tachinids appear 

 to oviposit in older ones. Other experiments are described in which 

 the parasites were placed near eggs about to hatch. The short 

 (3-4 day) egg-stage of Apanteles glomeratus appears to be due to the 

 fact that this Braconid oviposits on the skin of the larva and a long 

 egg-stage might entail the destruction of the egg when the host-larva 

 moults. Loos has calculated this loss at 40 per cent, in the case of 

 Tachinid parasites [see this Review, Ser. A, vii, p. 499]. The eggs 

 of Pimpla investigator are laid within the body of the larva and ovi- 

 position is not conflned to young individuals, being carried on for a 

 fortnight. From 14 to over 100 eggs are laid in one host. As 

 Pteromalus puparum deposits eggs from the beginning to the end of 

 the larval stage of the host it necessarily happens that some of the 

 adult parasites emerge at a time when no larvae of P. brassicae are 

 available. This is no disadvantage as P. puparum attacks other 

 larvae, such as those of Hyponomeuta malinellus. More than 100 eggs 

 may be laid in one host. 



