CONICET | Buscador de Institutos y Recursos Humanos

For parasitoids, host finding is a central problem that has been solved through a variety of behavioural mechanisms. Among species in which females do not make direct contact with hosts, as is the case of many dipteran parasitoids, eggs must be laid in an appropriate part of the host habitat. The asilid fly Mallophora ruficauda is a pestiferous species common in the open grasslands of the Pampas region of South America. Adults are predators of insects and larvae are solitary parasitoids of larvae of scarab beetles (Coleoptera: Scarabaeidae). Females lay eggs in clusters on tall grasses. Upon eclosion, pollen-sized larvae fall and parasitize soil-dwelling scarab beetle larvae. In contrast with most studied host-parasitoid interactions, host searching by M. ruficauda is carried out by both larvae and adults. We hypothesized that wind dissemination of M. ruficauda larvae is important in the host finding process, and that females lay eggs at heights that maximize parasitism of its concealed host. Through numerical and analytical models, resembling those used to describe seed and pollen wind dispersal, we estimated an optimal oviposition height in 1.25-1.50 m range above the ground. Our models took into account host distribution, plant availability and the range over which parasitic larvae search for hosts. We found that the results of the models matched heights at which egg clusters of M. ruficauda are found in the field. For parasitoids that carry out host searching at immature stages rather than as adults, plants are part of a dissemination mechanism of larvae that, as with minute seeds, uses wind and a set of simple rules of physics to increase offspring success.

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