Galapagos Species Checklist

Philornis downsi

Dodge & Aitken, 1968

Domain Eukaryota
Kingdom Animalia
Division Arthropoda
Class Insecta
Order Diptera
Suborder Brachycera
Family Muscidae
Genus Philornis

Philornis downsi  Dodge & Aitken, 1968


English common name: Ectoparasitic fly

Spanish common name: mosca parásita

Taxonomic comments: Syn: Philornis isla Couri 2000: 1.

Name status: Accepted name; taxon occurs in Galapagos.

Description: Similar in size and appearance to the common house fly, and usually dark in coloration. Male adults usually have yellow legs and eyes closer together, whereas females have darker legs. Female antenna plumose. Adult flies are free-ranging, eating fruits and nectar. They lay eggs in bird nests and the fly larvae feed on the blood and tissue of young chicks.

Larvae are free-living external parasites upon nestlings in bird nests (Dodge & Aitken 1968; Fessl et al. 2001), including Darwin’s finches.

Year of first record: 1964

Last updated: 24 Aug 2016


  • In
  • Na

  • Ac

    Taxon accidentally introduced, naturalized in the wild.

  • Cu
  • Er
  • Es
  • Ic
  • AcQ
    Questionable Accidental
  • NaQ
    Questionable Native


Galapagos island groups: Floreana, Isabela, San Cristóbal, Santa Cruz, Santiago.

Present on 13 islands: Champion, Daphne Major, Fernandina, Floreana, Gardner (Floreana), Isabela, Marchena, Pinta, Pinzon, San Cristobal, Santa Cruz, Santa Fe, Santiago.

Preference for altitude zone in Galapagos: Dry zone-humid zone.

Native range: Known from Trinidad & Brasil, but the native range is likely to be wider.

Distribution classification: Eutropical.

Please be aware that this distribution map is automatically generated from database records (CDF and external specimens, literature records, and observations) and may not accurately reflect the currently-known distribution for all species.

General Ecology

Habitat preferences: Found in most habitat types in the Galapagos, from the arid lowland to humid highland zones.

Substrate or host preferences: Larval hosts are mainly passerine birds, but also include Culiciformes.

Trophic role: Pathogenous parasite (a parasite that causes diseases of its host).

Feeding type: Adults feed on fruit and nectar, and larvae are semi-hematophagous (blood and tissue feeding) parasites of birds.

Feeding preferences: Little is known about the feeding habits of adult flies. It is believed that they feed on fruits and nectar. In the laboratory they will feed on a variety of fruits and protein sources.

Reproductive biology: Females lay their eggs in birds’ nests on the nest material surface or on nestlings. Fly eggs have also been found on the nasal openings (nares) of nestlings. The eggs hatch after 1-2 days and 1st instar larvae feed within the nestlings’ nares. The 2nd and 3rd instar stages live in the nest base and emerge at night to feed on the blood and tissues of nestlings either externally or within the nares. Larvae pupate in the nest base after about 7 days, and emerge as flies approximately 14 days later. Genetic studies have shown that female flies can mate with up to 5 males, but mating has not been observed in the nests or anywhere else in the field.

Reproduction mode: Exclusively sexual (through production of gametes and fertilization).

Dispersal mechanism: Active dispersal.

Dispersal propagule: Philornis species are able to travel over large distances. In Galapagos, Philornis downsi are attracted to lights on tourist boats and may also have been dispersed by boats.

Galapagos associated species: Camarhynchus heliobates, Camarhynchus pallidus, Camarhynchus parvulus, Camarhynchus pauper, Camarhynchus psittacula, Certhidea olivacea, Coccyzus melacoryphus, Crotophaga ani, Dendroica petechia aureola, Geospiza fortis, Geospiza fuliginosa, Geospiza magnirostris, Geospiza scandens, Mimus melanotis, Mimus parvulus, Mimus trifasciatus, Myiarchus magnirostris, Pyrocephalus rubinus.

Natural enemies: In Galapagos four species of parasitic wasps have been reared from Philornis pupae (Spalangia endius, Brachymeria podagrica, B. cabira; and the eulophid Aprostocetus [Tetrastichinae]). There are no records of parasitoids of P. downsi in its native range; however, at least three species of wasps are known to parasitize Philornis species.

Invasion Ecology

Invasion risk score: Extreme risk.

Form of introduction: Accidentially introduced.

Impact in Galapagos: At least 16 endemic bird species, one native, and one introduced specie are attacked by P. downsi. The fly’s impact on birds is a serious threat especially to vulnerable and declining species. Philornis downsi parasitism has already been implicated in the decline of endemic, critically endangered species such as the mangrove finch (Camarhynchus heliobates) and the medium tree finch (C. pauper).
In addition to direct mortality (sometimes up to 100% of hatchlings in a nest die from parasitism), studies have confirmed that surviving birds have reduced life expectancy due to deformed beaks, reduced growth rates, and anaemia. Philornis has a greater impact on bird species with small clutch sizes, e.g. tree finch species, than species with bigger clutch sizes.

Related species impact: Philornis species affect at least 115 species of bird, particularly in the neotropics. Larvae of the genus Philornis reside in bird nests and may feed on either nestling feces (coprophagous scavengers), the blood of nestlings (semi-hematophagous parasites), or on nestling tissue and fluid (subcutaneous parasites).


Control history in Galapagos: At this point in time, there are no known techniques to effectively mitigate the threat of P. downsi. In spite of considerable efforts by the Charles Darwin Foundation (CDF) and collaborators there are still substantial gaps in the understanding of the life history and ecology of P. downsi, which has prevented the development of methods to control the fly. Because of this, an international workshop was held by the Galapagos National Park Directorate (GNPD) and CDF in February 2012 to bring together local and international experts to find a solution for the management of P. downsi. Control options that are being investigated include mass trapping using lures, biological control, and spot treatments of nests with low risk pesticides.



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