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

Mariana Bulgarella,Roger D Moon,Deborah A Freund,Jonathan D Oliver,Andrea Cahuana,Charlotte E Causton,M Piedad Lincango,James R Miksanek,George E Heimpel,Alyssa J Colwitz,Ismael E Ramírez,Roxanne Sage,Paola F Lahuatte

doi:10.1038/s41598-022-06208-5

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.

Highlights

A common challenge faced by parasites is the seasonal availability of their hosts
If adult P. downsi persist through the host interbreeding period, and there is no recruitment of new adults, flies captured during that season would become progressively older
Of the puparia that did not eclose (959), 924 were dissected and contained either undistinguishable remnants of P. downsi larvae or pupae (48.6%) or adults (51.4%), either in formation or fully formed. None of these were alive upon dissection and we found no evidence for diapause in this group of P. downsi puparia

Summary

Introduction

A common challenge faced by parasites is the seasonal availability of their hosts. In tropical and subtropical habitats, host supply can be limited by seasonally resource-poor conditions. Philornis downsi has persisted in Galapagos since at least the 1960s and is known to attack 21 bird species (including 11 of 17 species of Darwin’s finches) from most habitats on at least 15 islands[21,23,24] This is despite the fact that almost all known hosts in Galapagos exhibit a distinct breeding season during the hot period (January through May), which is associated with sporadic rain showers and an increase in insect food for developing n estlings[25,26]. Outside of these months (i.e. during the cool season), and especially in the dry lowland habitats, nesting by bird species known to be hosts of P. downsi appears to be very limited[26]. If fly reproduction continues during the host interbreeding period, young flies would be present throughout the host interbreeding period

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