A hitch-hiker’s guide to parasitism: the chemical ecology of phoretic insect parasitoids

Phoretic arthropods use other animals as vehicles to migrate to new environments. Among insect parasitoids, phoresy is almost exclusively restricted to minute wasp species that develop in or on the smallest and most inconspicuous life stage of their host: the egg. Females of about 35 egg parasitoid species are known to hitch-hike with adult hosts to reach their egg-laying sites. Recent studies suggest that phoretic parasitoids strongly rely on chemical espionage to locate their transporting host. These wasps have evolved intriguing ways to exploit cues that are part of their host ’ s communication system, including sex, anti-sex and aggregation pheromones. Such a ‘chemical-espionage-and-ride’ strategy can be innate but it can also be learned. The extent and mechanisms by which hosts might avoid exploitation are poorly understood. Here we discuss why we expect phoresy to be much more widespread among egg parasitoids than is known so far. It is expected to be adaptive, especially in those species that have limited ability for directed fl ight, have a short time window available for parasitism, have a narrow host range, and parasitize abundant hosts that lay large eggs (or eggs in groups) with a large distance between them. We review some recently published examples of chemical espionage by phoretic egg parasitoids and discuss to what extent phoretic wasps represent a selective force against the use of chemical cues by their hosts. At the end of the chapter we identify unexplored aspects of the chemical ecology of phoretic insect parasitoids that warrant further investigation. In addition to the fundamental interest, research into the ways that phoretic parasitoids have evolved to locate their host may help improve the effi cacy of using parasitoids as biological control agents against insect pests.