Making a healthy choice: Tactical host selection behaviour of a parasitoid wasp

Abstract

Host selection behaviour plays a pivotal role in determining the success of parasitoids, especially in environments where host quality varies. In heterogeneous environments, parasitoids frequently encounter hosts that are infected. We conducted experiments to ascertain whether the ecto‐parasitoid Bracon brevicornis can differentiate between healthy caterpillars and those infected with Bacillus thuringiensis (Bt) and examined the implications of this host selection behaviour. In an olfactometer choice assay, B. brevicornis avoided Bt‐infected Spodoptera litura hosts. In a no‐choice assay, the female parasitoid displayed a lower paralysis rate on infected hosts, indicating a strong selection behaviour. Furthermore, we explored the impact of infection duration by exposing female wasps to larvae infected for 24, 48 and 72 h with Bt. Both choice and no‐choice assays demonstrated that female B. brevicornis refrains from ovipositing on infected larvae, regardless of the infection duration. Direct exposure to Bt through consumption did not affect the fitness of the tested parasitoid wasp. However, post‐24‐h Bt infection, host larvae showed an increased total hemocyte density, particularly high phagocytic cell numbers, and enhanced melanization, rendering the host larva unsuitable for parasitoid development. Taken together, our study underscores that parasitoids exhibit robust host selection behaviour by actively avoiding infected hosts. This insight is valuable for devising new pest control strategies in agriculture that safeguard beneficial parasitoids.