A difficult choice for tiny pests: Host-seeking behaviour in Xenos vesparum triungulins

The first-instar larvae of strepsipteran parasites, commonly referred to as triungulins, are the host-seeking stage: they must locate, invade and successfully develop in the new host, in order to start their parasitic cycle. Little information is available about the behaviour of Xenos vesparum triungulins. They emerge in batches from the endoparasitic female infecting Polistes dominulus, the primary host, and reach the nest through a vector (a foraging wasp or the parasitised wasp itself). Once there, they have the possibility to penetrate into wasp immatures at different developmental stages. In this study, we performed preliminary analyses aimed to investigate which cues are important to direct triungulin movements during their brief stay in wasp nests. In laboratory conditions we selectively presented different stimuli to Xenos larvae: apparently, the host larva itself is attractive in an open arena, but not inside a confined space, nor are epicuticular compounds of wasp larvae able to control triungulin movements. These are more likely oriented by their gregarious behaviour, whereas light (positive phototaxy) may at a previous stage enhance their emergence via the brood canal opening in the female cephalothorax.