A COUNT-DOWN MECHANISM FOR HOST SEARCH IN THE PARASITOID VENTURIA-CANESCENS

1. On the basis of his study of patch time allocation by Venturia cane scens, a larval parasitoid of phycitid flour moths (Lepidoptera: Pyral idae), J. K. Waage proposed a decision mechanism for patch exploitatio n. This putative mechanism would be efficent in habitats, such as gran aries, consisting of patches with heterogeneous host densities. Howeve r, the distribution of hosts in mummified fallen fruits, a common natu ral substrate, differs from their distribution in granary stores, tend ing to be rather uniform with patches containing a few, mostly one, ho st larva. This discrepancy led us to re-examine Waage's mechanism. 2. We investigated V. canescens's decision mechanism in small patches con taining low host densities. Following the previous study, we tested th e relation between the probability per unit time to abandon a patch an d 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. 3. A major component of Waage's mo del is an increase in the tendency to remain in a patch after an ovipo sition. In habitats where hosts are uniformly distributed, and particu larly 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 re sidence time. Ovipositions, however, decrease the amount of time subse quently spent by the parasitoid on the patch. 5. Based on these result s we formulate a post-hoc 'count-down' model for the decision rule for patch leaving in V. canescens in habitats with uniform host distribut ions. 6. There is no evidence for or against the possibility that this parasitoid may be capable of facultative changes in its patch exploit ation rule as a function of host distribution. 7. Although increases i n patch time after oviposition have been found in several other parasi toid species, re-examination of Waage's experiments shows that his res ults do not unambiguously support the existence of such a mechanism in Venturia.