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

Abstract

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. 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, and particularly when patches contain a single host, this behaviour would not be adaptive. 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. Based on these results we formulate a post-hoc 'count-down' model for the decision rule for patch leaving in V. canescens in habitats with uniform host distributions. 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. 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.