Using modified trapping regimes to understand the behavioral and spatial ecology of Philornis downsi (Diptera: Muscidae).

The avian vampire fly Philornis downsi (Dodge & Aitken) (Diptera: Muscidae) is native to continental South America and the Caribbean, but invasive in the Galapagos Archipelago. The larvae of P. downsi feed on the blood and tissues of the nestlings of 75% of the small land bird species that are endemic or native to Galapagos, causing high in-nest mortality and severe population declines in some species. Efficient trapping techniques are vital to safeguarding these birds in the short term as well as for monitoring fly populations, but basic information about the ecology of the fly is still needed to help develop a species-appropriate trapping method. In this study, we used a novel trapping regime with a vertical distribution to make inferences about P. downsi's behavioral and spatial ecology and to optimize trap catch. Our results showed that male and female P. downsi were trapped in greater numbers below the canopy (3.1-7.5 m), lower down than other commonly caught insect species (5.1-11.5 m). Notably, the effect of trap height remained consistent across seasons and different weather conditions. These findings suggest that P. downsi tend to move at heights where their hosts nest (at or below the canopy) and do not spend time above the canopy. This also makes it unlikely that strategies such as hill-topping or aerial swarming are being used to locate mates. As such, trapping and control efforts should be focused below the canopy in forests with similar canopy heights to effectively capture P. downsi and reduce bycatch of other insects.

Persistence of the invasive bird-parasitic fly Philornis downsi over the host interbreeding period in the Galapagos Islands.

Many parasites of seasonally available hosts must persist through times of the year when hosts are unavailable. In tropical environments, host availability is often linked to rainfall, and adaptations of parasites to dry periods remain understudied. The bird-parasitic fly Philornis downsi has invaded the Galapagos Islands and is causing high mortality of Darwin's finches and other bird species, and the mechanisms by which it was able to invade the islands are of great interest to conservationists. In the dry lowlands, this fly persists over a seven-month cool season when availability of hosts is very limited. We tested the hypothesis that adult flies could survive from one bird-breeding season until the next by using a pterin-based age-grading method to estimate the age of P. downsi captured during and between bird-breeding seasons. This study showed that significantly older flies were present towards the end of the cool season, with ~ 5% of captured females exhibiting estimated ages greater than seven months. However, younger flies also occurred during the cool season suggesting that some fly reproduction occurs when host availability is low. We discuss the possible ecological mechanisms that could allow for such a mixed strategy.

Identification and Optimization of Microbial Attractants for Philornis downsi, an Invasive Fly Parasitic on Galapagos Birds.

We investigated the role of olfactory cues from actively fermenting yeast (Saccharomyces cerevisiae) in attraction of adult Philornis downsi and identified two synergistically attractive yeast volatiles. Larvae of this invasive fly parasitize the hatchlings of passerines and threaten the Galapagos avifauna. Gas chromatography coupled with electroantennographic detection (GC-EAD), coupled gas chromatography-mass spectrometry (GC-MS), and field trapping experiments were used to identify volatile compounds from a yeast-sugar solution. EAD responses were consistently elicited by 14 yeast volatiles. In a series of field trapping experiments, a mixture of the 14 EAD-active compounds was similarly attractive to P. downsi when compared to the yeast-sugar solution, and we found that acetic acid and ethanol were essential for attraction. A mixture of 0.03 % acetic acid and 3 % ethanol was as attractive as the 14-component blend, but was not as attractive as the yeast-sugar solution. Philornis downsi showed positive and negative dose-responses to acetic acid in the ranges of 0.01 ~ 0.3 % and 0.3 ~ 9 %, respectively. Further optimization showed that the mixture of 1 % acetic acid and 3 % ethanol was as attractive as the yeast-sugar solution. Both mixtures of acetic acid and ethanol were more selective than the yeast-sugar solution in terms of non-target moths and Polistes versicolor wasps captured. These results indicate that acetic acid and ethanol produced by yeasts are crucial for P. downsi attraction to fermented materials on which they feed as adults and can be used to manage this invasive fly in Galapagos.