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Density-dependent foraging behaviors in a parasitoid lead to density-dependent parasitism of its host

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Empirical studies of spatial heterogeneity in parasitism by insect parasitoids have focused largely on patterns, while the many possible underlying mechanisms have been little studied in the field. We conducted experimental and observational studies on Tachinomyia similis (Diptera: Tachinidae) attacking western tussock moths (Orgyia vetusta; Lepidoptera: Lymantriidae) on lupine bushes at Bodega Bay, Calif., USA. We examined several foraging behaviors that have been hypothesized to create density-dependent variation in parasitism rates, including spatial aggregation of parasitoids to high host density, mutual interference among searching parasitoids and decelerating functional responses of the parasitoid. At the spatial scale of individual bushes, we detected both aggregation to a high density and a decelerating functional response. The resulting spatial pattern of parasitism was best fit by two models; one included an effect of parasitoid aggregation and the other included an effect of aggregation and a decelerating functional response. Most of the variation in parasitism was not correlated with density of O. vetusta.

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We would like to thank P. Chesson, P. de Valpine, A. Hastings, L. Hurd, B. Inouye, J. Rosenheim, and two anonymous reviewers for discussions or comments on the text, M. Greaves, J. Combs, J. Robertson, and the Spatial Group for much needed assistance in the field. This work was sponsored by an NSF grant to S.P. Harrison and an NSF predoctoral fellowship to J. Umbanhowar.

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Correspondence to James Umbanhowar.

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Umbanhowar, J., Maron, J. & Harrison, S. Density-dependent foraging behaviors in a parasitoid lead to density-dependent parasitism of its host. Oecologia 137, 123–130 (2003).

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  • Parasitoid aggregation
  • Functional response
  • Interference
  • Spatial heterogeneity