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

Abstract

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.

Genetics reveals shifts in reproductive behaviour of the invasive bird parasite Philornis downsi collected from Darwin’s finch nests

“…To assess changes in genetic structure across the sampling period, all specimens collected from nests and traps were analysed (1) for all years pooled, and (2) separately per year (2006,2008,2014,2020), and for each of the 10% and 30% missingness datasets. Samples from a given year are independent generations as previous studies found that P. downsi only survives to the next year's breeding season, but not subsequent breeding seasons, and our sampling periods are 2-6 years apart (Causton et al 2019;Bulgarella et al 2022).…”

“…However, increased multiple mating can decrease female longevity in insects, due to the increased cost of both mating and egg production (Arnqvist and Nilsson 2000;Kawagoe et al 2001;Gotoh and Tsuchiya 2008). Given that females must survive during the arid, non-breeding period on the Galápagos (Causton et al 2019;Bulgarella et al 2022), selection for increased female longevity may be driving the observed decrease in multiple mating. The costs and benefits of polyandry in P. downsi are currently not understood.…”

“…Therefore, large variation in annual P. downsi populations are expected, not only because of the initial colonisation bottleneck, but also given annual reductions in population size after the host breeding season ends (Causton et al 2019). Only relatively small numbers of adults survive to the next breeding season (Causton et al 2019;Bulgarella et al 2022), and therefore only a small proportion of the census population contributes to the next generation.…”

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

“…The fly larvae then hatch and feed on the blood of developing chicks, causing high mortality across bird species ( Fessl et al 2018 ). Since the ramifications of this invasion have been realized, greater attempts have been made to study the population dynamics, ecology, and natural history of this species in its native and introduced range with the hope that these insights can inform more effective control ( Bulgarella et al 2015 , 2022 , Kleindorfer and Dudaniec 2016 , Kleindorfer et al 2016 , Knutie et al 2016 , Causton et al 2019 , Cimadom et al 2019 , Common et al 2021 , Pike et al 2021 ). While much progress has been made in understanding P. downsi , there are still substantial gaps in our knowledge of this species basic ecology and reproductive behavior ( Fessl et al 2018 ).…”