Patch leaving decision rules and the Marginal Value Theorem: an experimental analysis and a simulation model

The patch exploitation strategy of females of the insect parasitoid Trichog ramma brassicae was studied on patches containing different proportions of hosts that were previously attacked by conspecific females. On average, T. brassicae females spent more time on patches of higher quality, and all pat ches were reduced to the same level of profitability before being left. Thi s appeared to be in accordance to the optimal predictions of the Charnov Ma rginal Value Theorem. The proximate leaving mechanisms involved were analyz ed by means of a Cox proportional hazards model. Each oviposition in a heal thy host appeared to have an incremental influence on the patch residence t ime, whereas each rejection of a healthy host or of a host that was previou sly attacked by the same female (i.e., self-superparasitism) had a decremen tal effect. These patch leaving mechanisms did not change according to the quality of the patch the females were exploiting. A Monte Carlo simulation was developed around the results of the Cox regression model. The results s uggest that this set of patch leaving rules seems to provide the females wi th a sufficient way to reach the predictions of the Charnov model. Among th e different mechanisms involved, the incremental effect associated with eac h oviposition in a healthy host appeared to play the most important role. T he relationship between the proximate mechanistic rules adopted by the fema les and the ultimate prediction of the Charnov model is discussed.