Rates of Attack and Control of the Offspring Sex Ratio in the Parasitic Wasp Lariophagus Distinguendus in an Environment Where Host Quality Varies

Abstract

Lariophagus distinguendus (Hymenoptera, Pteromalidae) is a solitary (one egg per host) parasite of the larvae of many weevils; the common granary weevil (Sitophilus granarius) was used as host in our experiments. Policy rules for female wasps on how to respond to different host sizes under different conditions of host density, in order to maximize reproductive success, are defined by a number of optimality models. We carried out experiments 1) to test predictions derived from such models and 2) to investigate the nature of the proximate causal factors in control of parasitisation behaviour. Our results support several predictions: effects of different host densities on accept/reject decisions, differential use of hosts for male and female offspring, use of relative instead of absolute host measures. However, host discrimination was never absolute, and seemingly erroneous decisions were made more often than could be accounted for as occasional errors. Parasitisation decisions (reject, accept for a male egg, accept for a female egg) appeared to depend (at least partly) upon experience with (a) previous host(s). Therefore, the procedures seem to involve storage of information in, and retrieval from a memory; the question is how much detail is required to account for the observed performances? We propose a hypothesis for the proximate control of parasitisation decisions. On the basis of this hypothesis we developed a model which was simulated on a computer. A simple two-factor version of the model proved sufficient for generating all essential properties of parasitisation behaviour. A detailed memory store is not a necessary requirement ; this factor could be reduced to a one-dimensional, scalar variable associated with the number of mature eggs available for oviposition upon an encounter with a host.