We investigated whether two biocontrol agents, the parasitoid wasp Aphelinus asychis Walker and the predatory ladybird beetle Harmonia axyridis Pallas, would act in concert to reduce densities of an aphid (Macrosiphum euphorbiae Thomas) that attacks greenhouse-grown roses (Rosa hybrida L.). We conducted three types of experiments: feeding trials in microcosms that examine predator preference for mummies versus aphids; a longer-term pairing of the two natural enemies and aphids on whole plants in large cages; and a trial release of ladybird beetles into a rose greenhouse infested with the aphid and parasitoid. In the microcosm feeding trials both larvae and adults of H. axyridis fed on aphids, but also on parasitoid mummies, raising the possibility that intraguild predation of parasitoids by H. axyridis could disrupt aphid control. In cages, ladybird beetles dampened peak aphid densities during an outbreak without altering densities of parasitoid pupae or the ratio of parasitoids to aphids. In our whole-greenhouse release, we saw no evidence that H. axyridis disrupted aphid control by resident A. asychis. Together, these results suggest that H. axyridis can complement aphid biocontrol by the parasitoid A. asychis, rather than disrupting control through intraguild predation. © 2004 Elsevier Inc. All rights reserved.
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