State dependent superparasitism in a solitary parasitoid: egg load and survival

Abstract

In solitary parasitoids, superparasitism (the allocation of an egg to an already parasitized host) has a payoff, measured in offspring produced and costs, measured in eggs and time invested. Solitary parasitoids that are capable of host discrimination must adopt the strategy that ensures the best use of both their egg load and available lifetime. In this paper, we develop a state-dependent model defining the optimal strategy of superparasitism for a solitary parasitoid species with overlapping generations. The fitness measure we use is based on the growth rate of the number of genotype copies. The model predicts that the tendency to superparasitize should increase as the egg load of the parasitoid increases, or as its life expectancy decreases. The model also predicts that under particular conditions wasps should show partial preferences for parasitized hosts. These predictions were tested with the parasitoid Venturia canescens (Hymenoptera: Ichneumonidae). The tendency of the wasps to superparasitize in the presence of both healthy and parasitized hosts was correlated to egg load and access to food before the experiment A complementary experiment, where parasitized hosts were given sequentially to parasitoids, showed that V. canescens exhibits partial preferences toward superparasitism. These experimental results and a previous work support the predictions of the model.