Conspecific cueing or cooperative feeding?—Foraging stable flies are visually attracted to conspecific flies

Stable flies, Stomoxys calcitrans, blood-feed daily on livestock hosts. The presence of conspecific flies on host animals may signify host suitability ('conspecific cueing') or may signal density-dependent feeding benefits ('Allee effects'). However, stable flies captured on sticky traps reportedly render traps less attractive to conspecifics. We investigated whether this deterrent effect is context-dependent, and whether host-foraging stable flies are attracted to, rather than deterred by, conspecifics. In two-choice laboratory bioassays, we offered food-deprived, CO2-stimulated stable flies a choice between paired landing platforms that were baited or not (control) with conspecific flies ('bait flies'). The presence of bait flies-irrespective of their abundance, sex or relative position on the platform-prompted attraction and landing of foraging flies. Even heterospecific flies and fly-look-alike decoys were attractive, provided they visually resembled stable flies. As shown in separate bioassays, chemical cues of bait flies on their own were not attractive to foraging flies. Our data suggest that foraging stable flies may aggregate on hosts. Aggregated blood-feeding may be adaptive in that aggregated conspecifics on a host animal may signal the presence of a suitable food resource, and group-feeding may help suppress immune responses of host animals.