Hyperparasitoid aggregation in response to variation in Aphidius ervi host density at three spatial scales |
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Authors: | SHON SCHOOLER,ANTHONY IVES,& JASON HARMON |
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Affiliation: | Department of Zoology, University of Wisconsin-Madison, U.S.A. |
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Abstract: | 1. This article investigates the pattern of hyperparasitism of the host Aphidius ervi Haliday (Hymenoptera, Aphidiidae), a primary parasitoid of the pea aphid, Acyrthosiphon pisum (Harris) (Homoptera: Aphididae) at three spatial scales. 2. In the laboratory, the hyperparasitoid Asaphes lucens (Provancher) (Hymenoptera: Pteromalidae) was introduced into cages containing sixteen alfalfa plants with varying numbers of A. ervi mummies (the stage susceptible to hyperparasitism). The pattern of hyperparasitism at the end of the 48-h trials showed no density-dependent hyperparasitoid aggregation, although there was strong density-independent hyperparasitoid aggregation. 3. In the field, the density of A. ervi mummies was manipulated in twelve 2 × 2-m plots containing 1309–1654 alfalfa stems. Variation in hyperparasitism among plots showed no density-dependent aggregation, although there was strong density-independent aggregation. 4. Finally, at the largest scale of the study, the distribution of hyperparasitism was sampled among twelve alfalfa fields within a 5 × 3-km area. At this scale there was both density-dependent and density-independent hyperparasitoid aggregation. 5. The natural variation in A. ervi mummy density is greatest at the larger scales of study. Therefore, density-dependent hyperparasitism occurs only when there is high natural variation in mummy density. |
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Keywords: | Acyrthosiphon pisum Aphildius ervi Asaphes lucens Hyperparasitoid aggregation |
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