Host Habitat Volatiles Enhance the Olfactory Response of the Larval Parasitoid Holepyris sylvanidis to Specifically Host-Associated Cues

Host foraging of parasitic wasps attacking insects living in stored food may be guided by volatile cues emanating from these postharvest products. However, little knowledge is available as to how habitat odor released from noninfested stored food affects the parasitoid’s response to host-specific chemicals. In this study, we investigated the impact of wheat grist odor on the olfactory host search by the ectoparasitoid Holepyris sylvanidis. This parasitoid attacks larvae of the confused flour beetle Tribolium confusum, a common pest of grain products. Olfactometer bioassays showed that female H. sylvanidis were attracted by volatiles released from host larval feces, whereas odor of noninfested wheat grist was neither attractive nor did it mask the host-indicating cues. We analyzed the odor of host larval feces and wheat grist by coupled gaschromatography—mass spectrometry and recorded the parasitoid’s electroantennographic (EAG) responses to the detected volatiles. Two specifically host-associated components of the fecal odor, (E)-2-nonenal and 1-pentadecene, elicited strong EAG responses. Both components were attractive when tested individually, but less than larval feces. Attraction of parasitoids to these host-specific key compounds was enhanced by addition of (i) noninfested habitat substrate odor or (ii) a blend of 3 EAG-active (but not behaviorally active) volatiles that had been identified in odor of noninfested grist (benzaldehyde, 1-tetradecene, 1-hexadecene), but were also detected in the host fecal odor. The impact of these volatiles ubiquitously released in a food store by noninfested habitat substrate on the parasitoid´s orientation to host-specific volatile cues is discussed.