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 parasitoidTrichogramma brassicae was studied on patches containing differentproportions of hosts that were previously attacked by conspecificfemales. On average, T. brassicae females spent more time onpatches of higher quality, and all patches were reduced tothe same level of profitability before being left. This appearedto be in accordance to the optimal predictions of the CharnovMarginal Value Theorem. The proximate leaving mechanisms involvedwere analyzed by means of a Cox proportional hazards model.Each oviposition in a healthy host appeared to have an incrementalinfluence on the patch residence time, whereas each rejectionof a healthy host or of a host that was previously attackedby the same female (i.e., self-superparasitism) had a decrementaleffect. These patch leaving mechanisms did not change accordingto the quality of the patch the females were exploiting. AMonte Carlo simulation was developed around the results of theCox regression model. The results suggest that this set of patchleaving rules seems to provide the females with a sufficientway to reach the predictions of the Charnov model. Among thedifferent mechanisms involved, the incremental effect associatedwith each oviposition in a healthy host appeared to play themost important role. The relationship between the proximatemechanistic rules adopted by the females and the ultimate predictionof the Charnov model is discussed.