The ontogeny of host-seeking behaviour in a parasitoid dipteran.

The robber fly Mallophora ruficauda is one of the principal apiculture pests in the Pampas region of Argentina. As adults, they prey on honeybees and other insects, while as larvae they are ectoparasitoids of third-instar scarab larvae. Females of M. ruficauda lay eggs in tall grasses. After being dispersed by the wind, larvae drop to the ground, where they dig in search of their hosts. It is known that M. ruficauda larvae exhibit active host-searching behaviour; however, it is unknown which instars are involved in this search. We carried out experiments in the laboratory to determine which larval stages are involved in host location. We report here that the second instar of M. ruficauda orientates specifically toward a source of Cyclocephala signaticollis odour, while first larval instar is indifferent to the host cues. Furthermore, we have determined that second instar larvae are more motivated to initiate exploratory movements than larva of the first stage. So far as we know, this is the first case among parasitoids, where the second instar is responsible for host location. Here we provide relevant information of this parasitoid's host-searching strategy, increasing the available knowledge of this significant apiculture pest.