Fly or die: Environmental conditions, not trap design, influence Philornis downsi capture rate in the Galapagos

Abstract

The avian vampire fly, Philornis downsi, is one of the greatest threats to the landbirds of the Galapagos. In its larval stages P. downsi parasitizes on many of the archipelago’s endemic and native birds, resulting in nestling death or lifelong deformities. Because P. downsi mating habits and biology are not well understood, live trapping wild flies is integral to growing a lab population. Currently, McPhail and tephri traps are used to capture flies, however, they are often able to escape. This study assessed the effectiveness of four trap designs over a two month period in the summer of 2024. We found that an insignificant numberof flies escaped from traps, and different trap types did not affect P. downsi daily catch rates. Additionally, an alternative trap that included an extended entrance aimed to minimize the number of escapees had lower catch rates compared to traps currently in use. Catch rates were positively correlated with temperatures and negatively correlated with humidity. Our results suggest the optimal strategy for live capture of P. downsi should focus on increasing the quantity of traps during periods of increased temperature and decreased humidity. Using McPhail and Tephri traps with mesh lining for capture of live vampire flies will further our scientific understanding of this threat.