Mind the Gap: the evolution of oviposition site and specialisation in the parasitoid superfamily Chalcidoidea

Parasitoid wasps have contributed significantly to our understanding of ecological specialisation and the evolution of traits linked to host range. Oviposition site is one such trait. Endoparasitoids deposit their eggs inside the host body and tend to be more highly specialised than ectoparasitoids (which lay eggs externally) because they must develop specific strategies to overcome host immune defences. Intermediate to endo- and ectoparasitism is a strategy that we call ‘gap laying’. Gap-laying parasitoids deposit their eggs between the hard outer puparium and the larva of dipteran hosts. This behaviour has received less empirical attention than endo- and ectoparasitism but has important implications for the evolution of specialisation. Using a phylogenetically controlled comparative approach we find that gap-laying species in the hymenopteran superfamily Chalcidoidea exhibit numeric host ranges intermediate to endo- and ectoparasitoids, but these groups exploit a similarly taxonomically related range of hosts. We found that gap-laying can arise from ectoparasitism or endoparasitism, but once it evolves it shows patterns consistent with an evolutionary dead-end compared to other strategies. The results of this study demonstrate how oviposition site, beyond the normal endo-ectoparasitoid dichotomy, influences host specificity, shedding light on the causes and consequences of ecological specialisation in the parasitic Hymenoptera.