| Summary: | Parasitoids are organisms that kill their host before completing their development. Typical parasitoids belong to Hymenoptera, whose females search for the hosts. But some atypical Diptera parasitoids also have searching larvae that must orientate toward, encounter, and accept hosts, through cues with different levels of detectability. In this work, the chemical cues involved in the detection of the host by parasitoid larvae of the genus <i>Mallophora</i> are shown with a behavioral approach. Through olfactometry assays, we show that two species of <i>Mallophora</i> orient to different host species and that chemical cues are produced by microorganisms. We also show that treating potential hosts with antibiotics reduces attractiveness on <i>M. ruficauda</i> but not to <i>M. bigoti</i> suggesting that endosymbiotic bacteria responsible for the host cues production should be located in different parts of the host. In fact, we were able to show that <i>M. bigoti</i> is attracted to frass from the most common host. Additionally, we evaluated host orientation under a context of interspecific competence and found that both parasitoid species orient to <i>Cyclocephaala signaticollis</i> showing that host competition could occur in the field. Our work shows how microorganisms mediate orientation to hosts but differences in their activity or location in the host result in differences in the attractiveness of different cues. We show for the first time that <i>M. bigoti</i> behaves similar to <i>M. ruficauda</i> extending and reinforcing that all <i>Mallophora</i> species have adopted a parasitoid lifestyle.
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