Stable flies preferentially oviposit on sites offering conspecific eggs with egg-derived bacterial semiochemicals

In various dipteran taxa, selection of oviposition sites by gravid female flies is informed by the presence of adult conspecific flies, their eggs, or their larval offspring. Working with blood-feeding stable flies, Stomoxys calcitrans (Diptera: Muscidae), we tested three hypotheses: (1) stable flies preferentially oviposit on sites offering conspecific eggs with egg-derived airborne semiochemicals (message-bearing chemicals), (2) egg-borne bacteria elicit oviposition, and (3) egg- or egg bacteria-derived semiochemicals attract flies. Two-choice laboratory experiments, testing for fly attraction and oviposition, revealed that female flies were attracted to, and preferentially oviposited on, sites with deposits of conspecific eggs. Semiochemicals emitted from bacteria isolated from stable fly eggs, and grown on agar, attracted flies and prompted oviposition. Of the eight bacterial isolates, Serratia marcescens and Sporosarcina sp. were most attractive to gravid female flies. Gas chromatographic-mass spectrometric analysis of S. marcescens headspace volatile extracts identified 6 volatiles, and gas meters recorded elevated levels of carbon dioxide and ammonia in the headspace of S. marcescens. These two gases in binary combination, liberated from aqueous sodium bicarbonate and ammonium hydroxide, attracted flies and prompted oviposition. The attraction and oviposition-inducing effects of these gases were not enhanced by the 6-volatile blend. Our data suggest that gravid stable flies engage in conspecific cueing to locate and select suitable sites for oviposition and development of larval offspring. Unraveling all semiochemicals that mediate stable fly attraction and oviposition site selection may improve stable fly control tactics.