Aggregation by Foraging Insect Parasitoids in Response to Local Variations in Host Density: Determining the Dimensions of a Host Patch

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Abstract
(1) We examined techniques for analysing the spatial scale at which parasitoids aggregate in response to host patchiness. (2) We attempted to demonstrate with simulations that current analyses based upon B, the slope of the linear regression of percentage parasitism on host density, are flawed. B is inappropriate due to its dependence upon the variance of host density measurements, and, therefore, upon the nature of the underlying host distribution. (3) An alternative analysis is proposed, based upon the correlation coefficient of the same linear regression, calculated over a series of increasing quadrat sizes. (4) The proposed analysis was applied to field data of the foraging behaviour of Argochrysis armilla (HymenopteraL Chrysididae), a parasitoid of the solitary, groundnesting wasp, Ammophila dysmica (Hymenoptera: Sphecidae). A. armilla aggregated in areas of high host nest density both during and after nest excavation. Parsitoids responded maximally to host patchiness over areas of 3-50 m2. (5) Attempts to analyse parasitoid foraging behaviour indirectly through the resultant patterns of parasitism are criticized, due to (i) the tendency of such indirect analyses to ignore temporal variation in host distribution, (ii) the tenuous link between patterns of foraging and patterns of parasitism, and (iii) the inability of studies of parasitism to distinguish active, behaviourally mediated aggregation from passive, demographically mediated aggregation. (6) The current use of B as a measure of the strength of an aggregative response may also be inappropriate, but may be rectified by measuring host density on a logarithmic scale.