Host size selection and progeny sex determination in Hymenoepimecis bicolor (Hymenoptera, Ichneumonidae, Pimplinae)

Abstract

Some hymenopteran parasitoids induce intricate behavioral alterations in their spider hosts. Despite these refined adaptations, the success of their attacks and larval development relies on careful host selection. Large hosts will certainly assure the biomass required for the parasitoid larva to pupate, but the selection of large hosts is constrained by the risks involved in the process of host subduing and immobilization.Temporal oscillations in the availability of hosts within the optimal size range may compose another important component of selection. To investigate these issues, we tested the hypothesis that Hymenoepimecis bicolor consistently attacks, and has the reproduction period synchronized with medium-sized individuals of its host, the spider Trichonephila clavipes. We observed that (i) medium-small hosts are preferential victims, (ii) the period of oviposition of H. bicolor is synchronized with the availability of host individuals within these size classes, and (iii) these preferences can be strong enough to remain stable, regarding the temporal variation in the availability of the most suitable hosts. We also reported that (iv) parasitoid females destinate unfertilized eggs to small hosts, which produce male larvae. Data on these aspects allowed us to discuss why there is a preference for specific host sizes. The factors involved include the larval demands, the relative size between the adult wasps and spiders, and the capacity of the hosts to acquire food during larval development.