Behavioral responses of the invasive fly Philornis downsi to stimuli from bacteria and yeast in the laboratory and the field in the Galapagos Islands

Philornis downsi (Diptera: Muscidae) is a nest parasitic fly that has invaded the Galapagos archipelago and exerts an onerous burden on populations of endemic land birds. As part of an ongoing effort to develop tools for the integrated management of this fly, our objective was to determine its long and short-range responses to bacterial and yeast cues associated with adult P. downsi. We hypothesized that the bacterial and yeast communities will elicit attraction at distance through volatiles, and appetitive responses upon contact. Accordingly, we amplified bacteria from guts of adult field-caught individuals and bird feces, and yeasts from fermenting papaya juice (a known attractant of P. downsi), on selective growth media, and assayed the response of flies to these microbes or their exudates. In the field, we baited traps with bacteria or yeast and monitored adult fly attraction. In the laboratory, we used the Proboscis Extension Response (PER) to determine the sensitivity of males and females to tarsal contact with bacteria or yeast. Long range trapping efforts yielded two female flies over 112 trap nights (one in extracts from bird faeces and one in extracts from gut bacteria from adult flies). In the laboratory, tarsal contact with bacterial stimuli from gut bacteria from adult flies elicited significantly more responses than did yeast stimuli. We discuss the significance of these findings in context with other studies in the field and identify targets for future work.