Inter-relationships of the Parasites of the Frit-fly,Oscinella frit(L.), in eastern North America

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Abstract
Studies are reported on the extent and importance of competition amongst larval and pupal parasites ofOscinella frit.In Ontario, 30–40 per cent. of frit larvae are parasitised byHexacolain July–August and superparasitism is thus likely. Subsequent attack byPolyscelismay occur, but probably unsuccessfully.Competition between the larval parasites and the pupal ecto-parasitesCallitulaandSpalangiais generally slight, because the former species is not abundant and the latter tends to select only healthy hosts.Experiments are described to test the inter-relations of the three pupal parasites by exposing puparia ofDrosophila melanogasterto two species simultaneously and in sequence.WhenCallitulaandSpalangiaare allowed to oviposit simultaneously, the former is more successful when hosts are relatively few, but otherwise the two species are equally successful. WhenCallitulaparasitises puparia recently exposed toSpalangia, equal numbers of the two parasites emerge, but in the reverse experimentCallitulais superior.Callitulais able to parasitise successfully a proportion of puparia already containingSpalangiawhatever the age of the latter; matureSpalangiathus attacked are mostly killed.Similar comparisons ofCallitulaandSpalangiawith the endoparasiticLoxotropashow thatLoxotropais inferior to both the others, but that all three show marked avoidance of multiple parasitism.In general, there is little interference between the various species in the parasite complex associated with the frit-fly.The parasite complex associated with the frit-fly appears to be one that is very well balanced, there being little interference between the various species involved. Moreover, when, for any reason, one species does not attain its accustomed degree of parasitism, this is offset by an increase in the numbers of one or more of the other species.

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