Apanteles glomeratus (L.) als Parasit von Aporia crataegi (L.) (Hymenoptera: Braconidae).

Abstract

The date of oviposition in Apanteles glomeratus (L.) (parasitizing Aporia crataegi L.) is less determined by the stage of development of the caterpillar than by the flight period of the Braconid wasp. In opposition to Pieris brassicae even still the second and third larval instar are parasitized, but the first one is obviously preferred. Parasitized caterpillars develop more slowly during the first stages than healthy ones, and so they begin later to envelop themselves by spinning their hibernation nests. The imagines of Ap. glomeratus emerge from cocoon during the second half of May. But they find caterpillars of A. crataegi suitable for parasitization not before the beginning of July. The meantime is obviously passed as a further generation in Pieris brassicae. The impossibility for Ap. glomeratus to complete a full year cycle in A. crataegi alone is caused at first by the length of embryonic development (3 weeks) of this host, on the other hand by the uniform growth of the specimens of an A. crataegi population which is brought about by the diapause during the third larval instar of each generation. Ap. glomeratus itself is parasitized by a number of different Ichneumonidae and Chalcididae with the result that all together nearly 50 per cent, of the Braconid wasps are eliminated. In the centres of the outbreak of A. crataegi in Western Germany (Oberrheinische Tiefebene) the parasitization by Ap. glomeratus was found up to 1955 (incl.) only 20-30%. Towards the margin of the outbreak areal the percentage was higher, as like as in the centres during 1956. According to theoretical considerations a parasite alternately attacking two different hosts will be hardly able to accomodate itself to circumstances like the increasing population density of these hosts by means of own increase. In case that the increasing two hosts are obtainable at the same time, one of them will be displaced by parasitization and compensated by the other. Under both conditions variable factors of biotop can be more or less effective. So during an outbreak of A. crataegi Ap. glomeratus is generally not able to increase its own population density in a similar way as known from outbreaks of Pieris brassicae. But the Braconid wasp may perhaps contribute towards the fact that A. crataegi is nearly always relatively rare during normal years.