Temporal and spatial variation in sex-specific abundance of the avian vampire fly (Philornis downsi)

Understanding the range and behaviour of an invasive species is critical
to identify key habitat areas to focus control efforts. Patterns of range
use in parasites can differ temporally, across life stages and between
sexes. The invasive avian vampire fly, Philornis downsi, spends the larval
stage of its life within bird nests, feeding on developing nestlings and
causing high levels of mortality and deformation. However, little is known
of the ecology and behaviour of the non-parasitic adult fly life stage.
Here, we document sex-specific temporal and spatial patterns of abundance
of adult avian vampire flies during a single Darwin’s finch breeding
season. We analyse fly trapping data collected across 7 weeks in
the highlands (N = 405 flies) and lowlands (N = 12 flies) of Floreana
Island (Galápagos). Lowland catches occurred later in the season, which
supports the hypothesis that flies may migrate from the food-rich
highlands to the food-poor lowlands once host breeding has commenced. Fly
abundance was not correlated with host nesting density (oviposition site)
but was correlated with distance to the agricultural zone (feeding site).
We consistently caught more males closer to the agricultural zone and more
females further away from the agricultural zone. These sex differences
suggest that males may be defending or lekking at feeding sites in the
agricultural zone for mating. This temporal and sex-specific habitat use
of the avian vampire fly is relevant for developing targeted control
methods and provides insight into the behavioural ecology of this
introduced parasite on the Galápagos Archipelago.