Alien Invasion: Biology of Philornis Flies Highlighting Philornis downsi, an Introduced Parasite of Galápagos Birds

Abstract

The muscid genus Philornis comprises approximately 50 described species of flies, nearly all of which are obligate parasites of nestling birds. Philornis species are native to the Neotropics and widely distributed from Florida to Argentina. Most research on this group has focused on P. downsi, which was introduced to the Galápagos Islands in the late twentieth century. Although Philornis parasitism kills nestlings in several native host species, nowhere do the effects seem more severe than in P. downsi in the Galápagos. Here, we review studies of native and introduced Philornis in an attempt to identify factors that may influence virulence and consider implications for the conservation of hosts in the Galápagos.

Host tolerance and resistance to parasitic nest flies differs between two wild bird species

“…Alternatively, studies have found that larger‐bodied host species can withstand the effects of parasitism of Philornis spp. better than smaller‐bodied host species (Knutie et al, ; McNew & Clayton, ). Smaller‐bodied hosts have a higher surface area to volume ratio and higher metabolic requirements than larger‐bodied hosts (Furness & Speakman, ; Schmidt‐Nielson, ).…”

“…However, swallows sustain fewer parasites per gram of body mass compared to bluebirds, which is likely because swallows resist the parasite with an immunological response. This resistance in swallows could be because they are unable to tolerate similar parasite densities as bluebirds due to their body size because smaller‐bodied hosts probably suffer a higher cost of parasitism (Cardon, Loot, Grenouillet, & Blanchet, ; McNew & Clayton, ). Other studies have also found that bluebirds had higher parasite densities than swallows but the effect of the parasite on the hosts differs based on location and year (Table ).…”

“…One possible explanation for why nonparasitized nestlings are producing an immune response is that there are other parasites in the system, such as endo-or intracellular parasites, that may not be affected by the experimental manipulation (Pedersen & Fenton, 2015;Shutler et al, 2004). Several studies have shown that a reduction of the target parasite resulted in an increase in a nontarget parasite species (Knowles et al, 2013;Pedersen & Antonovics, 2013 (Knutie et al, 2016;McNew & Clayton, 2018). Smaller-bodied hosts have a higher surface area to volume ratio and higher metabolic requirements than larger-bodied hosts (Furness & Speakman, 2008;Schmidt-Nielson, 1984).…”

“…We then tested whether bluebirds and swallows had effective defenses against P. sialia . Previous studies found that eastern bluebirds have higher parasite densities compared to tree swallows (Table ), and larger‐bodied bird species, such as bluebirds, may be able to tolerate parasites more than smaller‐bodied bird species (Heimpel et al, ; McNew & Clayton, ). Because swallows have lower parasite densities than bluebirds, we predicted that swallows would be resistant to P. sialia compared to bluebirds.…”

Environmentally cued hatching in the bird‐parasitic nest fly Philornis downsi

“…In this study, we consider whether ECH occurs in the fly Philornis downsi Dodge & Aitken (Diptera: Muscidae), a neotropical fly with obligate bird ectoparasitic larvae (Dodge & Aitken, 1968;Fessl & Tebbich, 2002;Dudaniec & Kleindorfer, 2006;Fessl et al, 2006). Philornis downsi is native to mainland South America but it has gained notoriety due to the devastating effect that it has had on the endemic passerine fauna of the Gal apagos archipelago where it was first recorded in 1964 (Causton et al, 2006;Fessl et al, 2010Fessl et al, , 2018Kleindorfer et al, 2014;Kleindorfer & Dudaniec, 2016;Koop et al, 2016;McNew & Clayton, 2018). Adult P. downsi females lay their eggs in the nests of passerine birds and on hatching the larvae feed externally and in the nares of the altricial nestlings, rasping at the skin and consuming the blood and lymph from the resulting wounds (Fessl et al, 2006(Fessl et al, , 2018.…”