The fly, Philornis downsi Dodge & Aitken, was first collected in 1964 on the Galápagos Islands and is now widespread across the archipelago. Virtually nothing is known about the behaviour and ecology of the fly as well as for the genus in general. Here, we describe all larval instars for the first time, and discuss infection intensity and impacts of parasitism on nestling survival of Darwin's finches. Adult P. downsi are non-parasitic free-living flies, whereas the larvae are obligate blood-feeding parasites on nestling birds. The larvae show a marked shift in their host site specificity – a novel finding for the genus Philornis: the first and early second larval instars live as agents of myiasis in finch nostrils and other tissues, while the older second and third instar larvae reside in the nest material and feed externally on the blood of nestlings, leading to blood losses in nestlings of 18–55%. Pupation occurs in the bottom layer of the nest. The combined effects of tissue damage by the endoparasitic instar larvae and anaemia by nest-dwelling haematophagous instar larvae account for the high nestling mortality (76%) due to Philornis parasitism. This represents the highest mortality by Philornis reported in the literature and emphasizes the extremely serious threat this parasite poses for the endemic passerine fauna of the Galápagos Islands.

Little is known about the genus Philornis (comprising ~50 species), a group of muscid flies that parasitise birds and may be highly detrimental to host nestlings. Philornis species affect at least ...

Understanding host-parasite interactions requires that the multi-faceted relationships among ecological, behavioral and molecular processes be characterized and integrated. The introduced parasitic fly of birds, Philornis downsi (Diptera: Muscidae), was accidentally introduced to the Galápagos Islands circa 1960s and the consequences of its introduction represent one of the most challenging conservation battles faced within host-parasite ecology today. Larvae of the fly were first discovered in nests of Darwin’s finches (Passeriformes: Thraupidae) in 1997. The parasitic fly larvae are killing ~55 % of Darwin’s finch nestlings within nests by consuming the blood and tissue of developing birds, whereby the number of parasitic fly larvae in nests has increased by 46 % from 2000 to 2013. As nestlings increasingly die at an earlier age, which signifies earlier resource termination for the parasite, ~17 % of fly larvae also die. Several local finch populations have become extinct but some host species have increased in abundance, and as a consequence, may be sustaining P. downsi populations along with other non-finch hosts. The high mortality in both Darwin’s finches and P. downsi larvae creates opportunity and imperative to understand the coevolutionary dynamics of this newly evolving host-parasite system. This review examines the rapid changes in host and parasite ecology, behavior, and genetics since P. downsi larvae were first discovered in Darwin’s finch nests. The aim of the review is to synthesize what is known about this lethal host-parasite system and to describe rapid changes in host and parasite biology that characterize the novel association. An over-arching goal is the implementation of conservation outcomes that are informed by evolutionary biological understanding of invasive species.