Onion fly and little house fly host finding selectively mediated by decomposing onion and microbial volatiles

Responses of onion flies,Hylemya antiqua (Meigen), to various synthetic onion and microbial volatiles as well as volatiles from microbial cultures and decomposing onions were tested to characterize the most effective host-finding stimuli. Of nine onion and microbial volatiles tested individually, only the known attractant,n-dipropyl disulfide, caught significant numbers of flies. However, a blend of these volatiles attracted more flies than any single chemical, includingn-dipropyl disulfide. In another experiment, agar plates inoculated with microorganisms from decomposing onions did not attract onion flies. However, cut onions inoculated with microorganisms and conditioned 4 days caught more onion flies than freshly cut onions andn-dipropyl disulfide. These results suggest that a blend of chemicals, rather than a single key chemical, is the more effective host-finding stimulus, and that microbial activity enhances the attractancy of a blend of onion volatiles. Large numbers ofFannia canicularis (L.), the little house fly, responded to the microbial cultures, demonstrating the existence of a potent attractant for this muscid.