Parasitic insects use chemical cues to locate their hosts, and prior experiences can modify their responses to these odours1-4. Females of the parasitic wasp Microplitis croceipes experienced by contact with host faeces, orient and fly upwind to odours from their hosts, larvae of the moth Heliothis zea5. We use flight tunnel studies to show that associative learning occurs during encounters with host faeces. When females touch the faeces with their antennae they learn to recognize and subsequently fly to various volatile odours, even novel and otherwise unattractive odours like vanilla, associated with the faeces. They link these volatile odours with a water extractable nonvolatile chemical in the faeces, evidently a host-specific recognition cue. The association of tracking cues with host by-products, without the need for direct contact with the host, is a valuable adaptation for locating cryptic and evasive hosts.
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