Wolbachia manipulates host pre-imaginal learning in a parasitoid wasp

The Hopkins host-selection principle (HHSP) suggests that organisms at higher trophic levels demonstrate a preference for the host species on which they developed during larval stage. Although investigated in many herbivorous and predatory insects, the HHSP has, to our knowledge, never been tested in the context of insects hosting selfish endosymbiotic passengers such as the maternally inherited bacterium Wolbachia pipientis. Here, we investigate the effect of Wolbachia infection on host pre-imaginal learning in the parasitoid wasp Trichogramma brassicae (Hymenoptera: Trichogrammatidae). We compare host-choice in Wolbachia-infected and uninfected adult female parasitoids after rearing them on two different Lepidopteran hosts, namely the flour moth Ephestia kuehniella Zeller (Lepidoptera: Pyralidae) or the grain moth Sitotroga cerealella (Lep.: Gelechiidae). We show that in T. brassicae, Wolbachia affect the pre-imaginal learning ability of female wasps. Wolbachia infected wasps do not show any host preference and easily switch hosts in the laboratory, while uninfected wasps significantly prefer to lay eggs on the host species they developed on. We discuss how the facilitation of a generalist strategy by Wolbachia may allow T. brassicae to escape intraspecific competition with their uninfected counterparts, and may have important evolutionary consequences for the host and its symbionts.