Taxonomic name: Philornis downsi Dodge & Aitken, 1968
Organism type: insect
Adult Philornis downsi flies feed on fruit, but larvae are semi-haematophagous (blood and tissue-feeding) parasites of birds. P. downsi larvae were first discovered in finch nests on Santa Cruz Island in 1997, although retrospective examination of insect collections show that the fly was present in the Galapagos Islands as early as 1964. Since then the parasite has spread to 12 of the 13 main Galapagos Islands and its larvae have been found in 64-100% of Darwin’s finch nests. The blood sucking larvae cause mortality in up to 76% of nestlings. For this high impact, it is given the highest risk ranking amongst introduced insects and amongst diseases/parasites.
Eggs: approximately the shape of a rice grain, 2-3mm in length, elongated oval shaped, creamy white in colour. Larvae:1st, 2nd and 3rd instar phases vary in size and development. Creamy colour, soft-bodied, segmented along thoracic region, mouth hooks and other sensory/feeding apparatus at anterior end, spiracles (for breathing) present at posterior and anterior region (anterior spiracles in 2nd and 3rd instar only). Pupae: Light to dark brown in colour depending on duration, elongated barrel-shaped cocoon tapering towards anterior and posterior ends, rounded on one end and with a with cuff-like margin on the other. Adult fly: Similar in size to common house fly, generally dark in colour though colour varies according to size of individual.
For full description of developmental stages see Fessl et al. 2006
Philornis carinatus, Philornis deceptiva, Philornis seguyi
agricultural areas, coastland, natural forests, ruderal/disturbed, scrub/shrublands
In the Galapagos, Philornis downsi occurs in most habitat types, including both arid lowland and humid highland zones. No information is available from Brazil and Trinidad
In the Galapagos Islands, known Philornis downsi fitness costs to Darwin's finches include: high nestling blood loss (18-55%), multiple body wounds and infections, grossly deteriorated nasal openings (Fessl et al, 2006a), reduced haemoglobin levels (Dudaniec et al 2006) and reduced growth rates (Fessl and Tebbich, 2002). Consequently, it is not surprising that P. downsi parasitism has been linked with high brood mortality: 16% to 95% (Fessl and Tebbich, 2002; Fessl et al, 2006a; Huber, 2008), and reduced fledging success (Dudaniec et al, 2007). Species with small clutch sizes, e.g. tree finch species are higher impacted than species with bigger clutch sizes (Fessl and Tebbich, 2002). As well, parasite intensity is higher in islands with highlands (Wiedenfeld et al, 2007).
Impacts of P. downsi parasitism especially threaten small remaining populations of the 'Critically Endangered (CR)' mangrove finch (see Camarhynchus heliobates in IUCN Red List of Threatened Species) with an approximate population of 100 individuals; the 'Critically Endangered (CR)' Floreana mockingbird (see Mimus trifasciatus in IUCN Red List of Threatened Species), and the the 'Critically Endangered (CR)' medium tree finch (see Camarhynchus pauper in IUCN Red List of Threatened Species). The Darwin's medium tree finch
has recently been uplisted from 'Vulnerable (VU)' to 'Critically Endangered (CR)'. Recent estimates put the total population at not more than 1,660 individuals, and it has recently begun declining rapidly owing to the effects of P. downsi (BirdLife International, 2009). No information is available to our knowledge on impacts of P.downsi on other places.
Native range: Philornis downsi is known from the Island of Trinidad (Dodge and Aitken 1968) and from Brazil (Angra dos reis, Rio de Janeiro): (Mendonça and Couri 1999).
Known introduced range: P. downsi has been introduced to the Galapagos Islands (Ecuador), probably firstly to Santa Cruz Island with imported fruit, pigeons or nest material. A retrospective examination of museum specimens found flies collected on Santa Cruz in 1964 (Causton et al, 2006). The fly is now found on the following Islands in Galapagos: Santa Cruz, Floreana, Gardner por Floreana, Isabela, San Cristobal, Champion, Pinzon, Santiago, Marchena, Fernandina, Santa Fe (Weidenfeld et al, 2007), and Daphne (S. Huber, pers comm., Fessl & Lingango, technical report 2008). In Galapagos, larvae have been found in higher densities on islands with a humid highland zone: Isabela, Floreana, Santiago, San Cristobal, Santa Cruz. P. downsi has been found in comparable densities across habitats on Santa Cruz Island, but in higher densities in the humid zone on Floreana (Kleindorfer et al, unpublished data).
Introduction pathways to new locations
Transportation of domesticated animals: Philornis downsi was accidentally introduced from mainland South America. Probably via fruit importation or in nesting material with pigeons
Transportation of habitat material:
Local dispersal methods
Natural dispersal (local): Dispersal of Philornis downsi occurs between islands via boat activities and prevailing winds; for small distances by flying. Probably expanded range across each island after becoming established.
Preventative measures: Quarantine measures to reduce introduction and dispersal (health standards for importing live birds, inspections of cargo).
Chemical: Charles Darwin Research Station (CDRS) and collaborators are trialing fly traps and lures for short term control. Accessible bird nests can be successfully liberated from parasites by applying a 1% Pyrethrin solution to the inside of the nest (without spraying directly on the nestlings, of course) (Fessl et al. 2006b). Currently, CDRS researchers are collecting more biological data on Philornis (e.g. life history, mating behaviour, fly distribution over the year and in different zones). They are also trying to breed the flies in the lab in order to evaluate the possibility of using sterile insect techniques to control the fly.
The adult Philornis downsi fly is vegetarian; its larvae feed on the blood and body fluids of bird nestlings. In Galapagos, documented hosts include Passeriformes and Cuculiformes: Mangove finch (Cactospiza heliobates), Woodpecker finch, (Cactospiza pallida), Warbler Finch (Certhidea olivacea), Small Ground Finch (Geospiza fuliginosa), Medium Ground Finch (Geospiza fortis), Cactus Finch (Geospiza scandens), Small Tree Finch (Camarhynchus parvulus), Medium Tree Finch (Camarhynchus pauper), Large Tree Finch (Camarhynchus psittacula) (Emberizidae); Galapagos Flycatcher (Myiarchus magnirostris), Vermilion Flycatcher (Pyrocephalus rubinus) (Fringillidae); Chatham mocking bird (Mimus melanotis), Galapagos mockingbird (Nesomimus parvulus), Floreana Mockingbird (Nesomimus trifasciatus) (Mimidae), Yellow Warbler (Dendroica petechia) (Parulidae); Dark-billed Cuckoo (Coccyzus melacoryphus), Smooth-billed ani (Crotophaga ani) (Cuculidae). In Brazil, documented hosts include: Rufous-capped Antshrike (Thamnophilus ruficapillus) (Thamnophilidae). In Trinidad documented hosts include: Cocoa Thrush (Turdus fumigatus) (Turdidae); Southern House-wren (Troglodytes musculus) (Troglodytidae); Palm Tanager (Thraupis palmarum) (Thraupidae); Gray-breasted Martin (Progne chalybea) (Hirundinidae); Shiny Cowbird (Molothrus bonariensis), Yellow-rumped Cacique (Cacicus cela) (Icteridae); Tropical Mockingbird (Mimus gilvus) (Mimidae); Piratic Flycatcher (Legatus leucophaius), Tropical Kingbird (Tyrannus melancholicus), Great Kiskadee (Pitangus sulfuratus) (Tyrannidae); Bananaquit (Coereba flaveola) (Coerebidae); Rufous-tailed Jacamar (Galbula ruficauda) (Galbulidae); Smooth-billed Ani (Crotophaga ani) (Cuculidae); Silver-beaked Tanager (Ramphocelus carbo) (Thraupidae); Bare-eyed Thrush (Turdus nudigenis) (Turdidae). (Galapagos references: Fessl and Tebbich, 2002; Fessl et al, 2001, Fessl et al, 2006a, Fessl et al, 2006b, Dudaniec et al, 2007; Wiedenfeld et al, 2007; O’Connor et al, in prep. Brazil reference: Mendonca and Couri, 1999. Trinidad reference: Dodge and Aitkin, 1968).
Adult fly mating behaviour is currently unknown though has not been observed in the nest. Females have been observed depositing eggs in the nesting material (O'Connor, unpublished data) and are known to mate with up to 5 males per laying event (Dudaniec et al, 2008). Captive breeding experiments are currently being carried out at Charles Darwin Research Station.
Female flies lay eggs in the nasal cavities of nestlings or in the nesting material. Larvae pass through 3 instar phases and are principally ectoparasitic feeding on blood and tissue fluids. First and early second instars tend to be subcutaneous feeders, feeding within the nostril of bird nestlings. Later instars are semi-haemotophagous and are free-living within the nest. The larval period in the nest is approximately 5-6 days. Third instar larvae drop to the bottom of the nest where they pupate (Fessl et al, 2006a). Philornis flies are known to emerge from pupae after approximately 2 weeks (Dodge, 1971).
Reviewed by: R. Dudaniec, PhD, Post-doctoral Fellow, University of British Columbia, Vancouver, BC, Canada; B. Fessl, PhD, Mangrove Finch Project, Charles Darwin Foundation, Santa Cruz, Galápagos; C. Causton, PhD, Adjunct Researcher Charles Darwin Foundation for the Galapagos Islands (CDF); S. Kleindorfer, PhD, Senior lecturer Flinders University, Collaborating Scientist (CDRS)
Compiled by: Jody O'Connor, PhD Candidate School of Biological Sciences, Flinders University of South Australia. Adelaide, Australia & IUCN SSC Invasive Species Specialist Group (ISSG)
Last Modified: Tuesday, 5 August 2008