Searching for and Parasitization of Larvae of Drosophila Melanogaster (Dipt.: Drosophilidae) By Leptopilina Heterotoma (Hym.: Eucoilidae): a Monte Carlo Simulation Model and the Real Situation

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Abstract
A simulation model for the interaction between a parasitic wasp and its host was built 1) to check whether all relevant characteristics of the interaction were already known, 2) to have a tool for making predictions about the outcome of this interaction under different conditions and 3) to test alternative hypotheses on 'optimal foraging behaviour'. An important advantage of the use of such a model is the possibility of preliminary testing ideas about the influence of particular components of the interaction. This may facilitate the choice of the components to be studied in experiments. Instead of a mathematical-analytical model, which is only suitable for simple pro- cesses, a Monte Carlo simulation model was chosen, which provides possibilities for the gradual introduction of new components. The decision to build the model step-by-step on the basis of the results of extensive studies on the parasitization behaviour enabled immediate experimental testing before further extension of the model. It was first tried to model the parasitization of Drosophila-larvae by the parasite Leptopilina heterotoma on the basis of a limited number of parameters: patch size, host size, host number, ovipositor stabbing frequency and handling time (probing, ovipositing, standing still, preening). The number of suitable larvae was kept constant by replacing parasitized larvae by unparasitized ones. The model assumed that Lep- topilina does not pursue its hosts, but stabs randomly in the yeast. However, it appeared that the real wasps were less efficient than the simulated ones. Several refinements had to be made in the parameter values used (e.g. not all the available time is used for searching or handling hosts, the size of living hosts is smaller than the values used, and that were obtained from dead ones). After the values were corrected, the model with its assumption of random search ap- peared to provide a good description of the process. To check its applicability in other conditions, host density was varied and, in another experiment, host discrimination (i. e. the ability to distinguish between parasitized and non-parasitized hosts) was in- troduced. In both cases, the results from the experiments agreed very well with the predictions from the model.

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