A Count-Down Mechanism for Host Search in the Parasitoid Venturia canescens

Abstract

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. British Ecological Society is collaborating with JSTOR to digitize, preserve and extend access to Journal of Animal Ecology. Summary 1. On the basis of his study of patch time allocation by Venturia canescens, a larval parasitoid of phycitid flour moths (Lepidoptera: Pyralidae), J. K. Waage proposed a decision mechanism for patch exploitation. This putative mechanism would be efficient in habitats, such as granaries, consisting of patches with heterogeneous host densities. However, the distribution of hosts in mummified fallen fruits, a common natural substrate, differs from their distribution in granary stores, tending to be rather uniform with patches containing a few, mostly one, host larva. This discrepancy led us to re-examine Waage's mechanism. 2. We investigated V. canescens's decision mechanism in small patches containing low host densities. Following the previous study, we tested the relation between the probability per unit time to abandon a patch and the following variables: the concentration of the contact kairomone produced by host larvae, the time elapsed since the first patch entry and the occurrence of ovipositions. A major component of Waage's model is an increase in the tendency to remain in a patch after an oviposition. In habitats where hosts are uniformly distributed, andparticularly when patches contain a single host, this behaviour would not be adaptive. 4. Our results confirm that V. canescens spends more time on patches with higher concentrations of contact kairomone and that the probability per unit time of leaving the patch increases with patch residence time. Ovipositions, however, decrease the amount of time subsequently spent by the parasitoid on the patch. 5. Based on these results we formulate apost-hoc 'count-down' model for the decision rule for patch leaving in V. canescens in habitats with uniform host distributions. 6. There is no evidence for or against the possibility that this parasitoid may be capable of facultative changes in its patch exploitation rule as a function of host distribution. 7. Although increases in patch time after oviposition have been found in several other parasitoid species, re-examination of Waage's experiments shows that his results do not unambiguously support the existence of such a mechanism in Venturia. the rate at which hosts are found exceeds the rate at which new hosts develop. The time spent in each patch Parasitoids searching for patchily distributed hosts increases the number of offspring realized there, but experience diminishing returns within patches when reduces the search time that can be spent on other patches. In consequence, the mechanisms determining

Factors affecting the patch-leaving decision of the parasitic wasp Diadegma semiclausum (Hymenoptera: Ichneumonidae)

“…The wasps stayed longer in the infested patch, irrespective of the upwind odor source, indicating that the presence or absence of infochemicals is a major factor in making the decision. Unlike other studies that have focused on the resident time of parasitic wasps (Waage, 1979;Driessen et al, 1995;Vos et al, 1998), the resident time of D. semiclausum did not change with respect to oviposition success. In this paper, we used newly infested patches.…”

Gene flow between arrhenotokous and thelytokous populations of Venturia canescens (Hymenoptera)

“…First, many of these studies made use of a few commonly shared laboratory strains. The second reason is that many of the hosts of V. canescens are stored product pests and strains were typically collected from bakeries, granaries, and dried fruit warehouses (Waage, 1979;Press et al, 1982;Cline et al, 1983;Driessen et al, 1995;Harvey and Vet, 1997;Bonsall and Hassell, 1998). These places provide a relatively constant environment and ample supply of pyralid hosts (Freeman, 1980;Goater, 1986;Harvey and Thompson, 1995).…”

An experimental investigation of patterns of parasitism at three spatial scales in an aphid-parasitoid system (Hymenoptera: Aphidiidae)

“…Van Steenis et al, 1996;Driessen & Bernstein, 1999) tend to leave the patch after a certain number of successful ovipositions, or after a cer tain number of encounters with previously attacked hosts. If so, this "count-down" mechanism (Iwasa et al, 1981;Driessen et al, 1995) will cause the proportion of para sitised aphids to decrease rapidly with increasing aphid density (Shaltiel & Ayal, 1998). Moreover, in dense aphid colonies in small patches, parasitoid interference (Hassell, 1971), and the alarm and escape reactions of aphids (Bowers et al, 1972) can cause decreases in host mortality with increasing host density.…”