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 Trichogramma 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 patches were reduced to the same level of profitability before being left. This appeared to be in accordance to the optimal predictions of the Charnov Marginal Value Theorem. The proximate leaving mechanisms involved were analyzed by means of a Cox proportional hazards model. Each oviposition in a healthy host appeared to have an incremental influence on the patch residence time, whereas each rejection of a healthy host or of a host that was previously attacked by the same female (i.e., self-superparasitism) had a decremental 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 suggest that this set of patch leaving rules seems to provide the females with a sufficient way to reach the predictions of the Charnov model. Among the different mechanisms involved, the incremental effect associated with each oviposition in a healthy host appeared to play the most important role. The relationship between the proximate mechanistic rules adopted by the females and the ultimate prediction of the Charnov model is discussed.