Generalist predators are potential control agents of aphids in cereal fields. Because aphids are low-quality prey for most generalist predators, the availability of alternative high-quality prey may influence the interactions between aphids and their predators by reducing the predation rate due to lowered preference for aphids. We analysed this by testing the ability of six generalist predators (the spiders Erigone atra (Bl.), Clubiona lutescens (Westr.)/reclusa O.P.-C., Pachygnatha degeeri Sundevall, Pardosa prativaga (L.Koch), the carabid beetle Bembidion lampros (Herbst), and the harvestman Oligolophus tridens (C.L. Koch)) to suppress Rhopalosiphum padi (Linné) populations in the presence or absence of alternative prey types (fruit flies Drosophila melanogaster (Meigen) or the collembolan Tomocerus bidentatus (Folsom)). Experiments of 10 days duration were carried out in a microcosm set-up. Without alternative prey all predators except B. lampros were able to reduce aphid population development significantly relative to predator-free controls. The harvest spider O. tridens was the most efficient predator (<90 reduction). Presence of alternative prey (fruit flies) had a significant negative effect on aphid limitation by P. prativaga and a weak positive effect in B. lampros, but did not influence the ability to reduce aphids in E. atra, Clubiona, P. degeeri and O. tridens. In addition, 24-hours' consumption experiments with adult P. degeeri and subadult C. lutescens/reclusa, using R. padi and D. melanogaster as prey types, showed markedly lower consumption rates of aphid than of fruit fly prey. The microcosm arrangement is a simple way to partly simulate the habitat complexity of an agricultural field under laboratory conditions and proved to be a useful tool for investigating complex predator-prey interactions.
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