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

Abstract

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 (Hymenoptera: 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. Parasitoids 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.

Escape from parasitism: spatial and temporal strategies of a sphecid wasp against a specialised cuckoo wasp

“…Rosenheim et al (1989) have shown that correlations between nest density and parasitoid activity are low if the observation patches are too small or too large. Therefore, we determined host nest density on patches of different sizes and evaluated the validity of the patch size used (see also Rosenheim et al 1989).…”

“…Due in part to the very high activity of hosts and cuckoo wasps, the number of chrysidids that entered the observation patch could not be counted. Instead, we determined the maximum number of chrysidids that were simultaneously present during an interval of 5 min (similar to Rosenheim et al 1989). To obtain an activity index for an hour, we calculated the mean of the respective 5-min intervals.…”

“…In aculeate Hymenoptera, little is known about the anti-parasitic significance of temporal patterns of nesting activity (e.g. Rosenheim 1987;Rosenheim et al 1989). The activity of brood parasitoids but not of hosts might, for example, be constrained by low temperature since brood parasitoids are usually considerably smaller and probably more dependent on high ambient temperature (see Willmer 1985;Rosenheim 1990).…”

Do digger wasps time their provisioning activity to avoid cuckoo wasps (Hymenoptera: Crabronidae and Chrysididae)?

“…Because these natural enemies primarily exploit the host food provisions, one of the possible ways for the host to reduce parasitic pressure is to shift its provisioning so as to avoid times or places with high enemy activity (Rosenheim 1988;Rosenheim et al 1989;Strohm et al 2001), a set of behaviors generally called "enemy-free space strategy".…”

Temporal and spatial dynamics of predation in a robber fly (Efferia staminea) population (Diptera: Asilidae)

“…Aggregations of a predator in areas of high resource abundance have been noted for some insect predators and parasitoids (e. g . , Rosenheim et al 1989), but have not been documented for robber flies. In fact, Shelly (1984) presented evidence that foraging robber flies (Atractia marginata) did not cluster in areas of high prey density.…”