## 4 Discussion
While the most attractive bacterium in our cage assays (i.e., KO) seemed to produce a repellent compound (i.e., isoamyl alcohol), this may be explained by the fact that behavioral assays were conducted with bacterial Tenax(r) extracts, which did not contain isoamyl alcohol, although isoamyl alcohol was detected in KO through SPME. The low degree of behavioral response observed in the cages, reflected by the high proportion of flies not choosing between the bacterial extracts and the control within 24 h, may be explained by the origin of the bacterial strains or the pH at which those extracts were collected. Even though the Enterobacteriaceae species used in this study were previously described to be present in QFF [13; 16], they were not sourced directly from the QFF's gut, which could have influenced the production of MVOCs and, consequently, behavioral attraction [56]. Furthermore, the lack of behavioral attraction to the bacterial extracts may be linked to the origin of the tested flies, which were factory mass-reared and fed with a standard diet based on yeast extract rather than bacteria [58]. As such, behavioral assays with natural QFF populations and screening several strains of live bacteria should be considered. Most bacteria display optimum growth rates at slightly acidic-neutral pH, but Enterobacteriaceae are also found in the midgut of _B. tryoni_, at a pH of 1.4-2.0 [59; 60]. During oviposition, the transfer of gut bacteria along eggs as previously described, from a confined acidic environment to a more neutral external environment (i.e., oviposition sites such as fruits [13; 61], may trigger an increase in MVOC production. Thus, we suggest that the presence of naturally occurring KO on host-plants, associated with higher release rates of 2-phenylethanol and indole at alkaline pH, may be linked to greater attractiveness to female QFF. Similar findings were reported in _C. capitata_, where an elevation of pH, from acidic to neutral, was correlated with increased release rates of ammonia, resulting in higher bait efficiency [53; 62]. Changes in the production of MVOCs by the bacteria transferred to the oviposition site, may provide a new source of chemical information (e.g., timing of oviposition) [13; 63], potentially affecting female attraction. Yet, even though female QFF have been observed to reuse previous oviposition sites [64; 65], there is still no evidence of MVOCs used as markers of time and quality of oviposition site by this species. Given that only virgin females were used in our study, we cannot confirm that the variation in MVOCs with pH conditions play a specific role in oviposition site selection by mated females. However, future oviposition assays using the MVOCs and bacteria tested in this study should be considered. We also strongly advocate for further comparisons of behavioral modulation between virgin and mated females as a result of mating-induced switches in olfactory preferences previously demonstrated in flies [6; 66]. For instance, small alcohol molecules, such as isoamyl alcohol, were recently found to enhance the capture of mated female QFF in the field [46], while we described this compound as repulsive to virgin females.