Bacteria in stable fly (Diptera: Muscidae) feces inform foraging decisions of conspecific flies

Blood-feeding stable flies, Stomoxys calcitrans (L.), are reportedly attracted to conspecific feces. We investigated whether stable flies are attracted to fly feces-derived bacteria, and whether attraction varies with bacterial species and growth substrate, and the sex and reproductive status of foraging flies. We isolated seven bacterial species from fecal deposits of stable flies. In laboratory two-choice bioassays with paired adhesive-coated traps baited with sterile agar or bacteria-inoculated agar, the seven bacterial species in combination repelled flies. However, the bacterial congeners Serratia marcescens and Serratia surfactantfaciens attracted flies when tested singly. Analyses of the headspace volatiles of S. marcescens and S. surfactantfaciens by gas chromatography-mass spectrometry revealed near-identical odor profiles, which may explain their comparable attractiveness to flies. Flies were more attracted to S. marcescens grown on trypticase soy agar than to S. marcescens grown on bovine blood agar, likely due to substrate-dependent odor profiles. Feces-dwelling bacteria appear to function in the context of foraging rather than oviposition given that S. marcescens attracted both female and male stable flies, and both gravid and non-gravid females. That agar plates inoculated with all seven bacterial species failed to attract foraging flies could mean that the microbial community on agar did not sufficiently resemble that in fly feces, perhaps due to the limitations of culture-dependent isolation methods. Alternatively, additional feces-derived odors may also be necessary for fly attraction. Further investigation is warranted to identify the semiochemicals, and possibly other cues, produced by S. marcescens that make this bacterium distinctively attractive to stable flies.