Patterns of parasitism by insect parasitoids in patchy environments

Abstract

1. This paper shows how the different spatial patterns of per cent parasitism in patches of different host density can be explained within a single model framework that takes into account the parasitoid's aggregative response, and the factors limiting the degree of host exploitation within patches. 2. Two contrasting laboratory examples are presented in which the distribution of searching parasitoids and the resulting levels of parasitism in different patches are both known for a range of parasitoid densities. 3. A model is described predicting the number of hosts parasitized per patch, in which the number of parasitoids searching is determined from a simple expression allowing different degrees of aggregation. 4. The model generates patterns of parasitism encompassing the two laboratory examples and a wide range of examples from the field.5. The importance of density dependent spatial distributions of parasitism to population stability is briefly discussed.

Direct and indirect effects of landscape structure on a tri‐trophic system within agricultural lands

“…Secondly, as Protocalliphora abundance per nest is positively correlated with bird clutch size (Eeva et al 1994), and levels of parasitism are generally positively correlated with host density (Hassel 1982), we predicted that both Protocalliphora abundance per nest and the levels of parasitism by Nasonia increase with host abundance regardless of the landscape structure. Lastly, as higher trophic levels may be more negatively affected by the strong ''bottom-up'' effects associated with landuse change (habitat loss and reduced host abundance) (Thies et al 2003, Cagnolo et al 2009), we predicted that the negative effects of agricultural intensification should be greater with increasing trophic rank.…”

Handling time promotes the coevolution of aggregation in predator–prey systems

“…Hence, for longer handling times, predator densities and per-capita predation rates are inversely related. This inverse relationship has been observed in studies of the egg parasitoid Trichogramma evanescens and its host, the stored product moth Plodia interpunctella (Hassell 1982) in which 'parasitism (rates are) inversely density dependent, despite the tendency for the parasitoids to aggregate on the leaves with highest host densities' (Hassell 2000, p. 547) .…”

Role ofDiadegma semiclausum(Hymenoptera: Ichneumonidae) in controllingPlutella xylostella(Lepidoptera: Plutellidae): Cage exclusion experiments and direct observation

“…The aggregation response of parasitoids has been identified as an important factor that contributes to host regulation (Hassell & May, 1974;Reeve & Murdoch, 1985;Murdoch et al ., 1987;Ives et al ., 1999). Positive aggregation response by parasitoids should generate a direct density-dependent parasitism (Comins & Hassell, 1979;Hassell, 1982;Waage, 1983;Lessells, 1985). However, empirical studies often fail to detect density-dependent parasitism (Waage, 1983;Smith & Maelzer, 1986), and reveal diverse patterns of parasitism (Lessells, 1985;Stiling, 1987;Walde & Murdoch, 1988).…”

“…At the plot level, A plot 0/number of wasps observed on the high host density plot/total number of wasps observed on the two plots, and D plot 0/number of hosts present in the high host density plot/total number of hosts present in the two plots (which was 0.77). If the parasitoids distribute themselves in proportion to hosts or aggregate more on patches with high host density, i.e., A ]/D, the positive aggregation response by parasitoids should generate a direct density-dependent parasitism (Hassell & May, 1974;Hassell, 1982). The resultant pattern of percent parasitism was compared in relation to the relative degree of aggregation response across the observational dates using Student t -test with the adjusted Sequential Bonferroni Method (Rice, 1989).…”