Persistence of the invasive bird-parasitic fly Philornis downsi over the host interbreeding period in the Galapagos Islands

Abstract

Many parasites of seasonally available hosts must persist through times of the year when hosts are unavailable. In tropical environments, host availability is often linked to rainfall, and adaptations of parasites to dry periods remain understudied. The bird-parasitic fly Philornis downsi has invaded the Galapagos Islands and is causing high mortality of Darwin's finches and other bird species, and the mechanisms by which it was able to invade the islands are of great interest to conservationists. In the dry lowlands, this fly persists over a seven-month cool season when availability of hosts is very limited. We tested the hypothesis that adult flies could survive from one bird-breeding season until the next by using a pterin-based age-grading method to estimate the age of P. downsi captured during and between bird-breeding seasons. This study showed that significantly older flies were present towards the end of the cool season, with ~ 5% of captured females exhibiting estimated ages greater than seven months. However, younger flies also occurred during the cool season suggesting that some fly reproduction occurs when host availability is low. We discuss the possible ecological mechanisms that could allow for such a mixed strategy.

Figure 1

Pterin relative fluorescence (in thousands) of female and male P. downsi adults held under laboratory conditions until death at specified ages. Linear regression for females: Relative fluorescence = 0.21*Age + 18.46; r² = 0.7462; p < 0.001. Linear regression for males: Relative fluorescence = 0.13*Age + 20.71; r² = 0.6741; p < 0.001.

Figure 2

Estimated ages (in days) of adult female and male P. downsi flies captured at a lowland site on Santa Cruz Island, Galapagos between February 2015 and March 2016. Box plots show medians, upper and lower 75% quantiles and outliers. Numbers indicate sample size (number of flies) and the grey box indicates months that typically do not support nesting of most P. downsi hosts.

Figure 3

Frequency distributions of estimated ages of female (n = 238; A) and male (n = 143; B) P. downsi flies captured at a lowland site on Santa Cruz Island, Galapagos from February 2015 through March 2016. The vertical dotted line marks 210 days, which corresponds to the length of a typical interbreeding season, during which host availability for P. downsi is low.

Figure 4

(A) The number of mature and immature eggs found from dissections of P. downsi females captured between September 2015 and March 2016 (means ± SE). (B) The proportion of P. downsi females captured between September 2015 and March 2016 that were found to contain sperm in at least one spermatheca. The grey box indicates the months of the host interbreeding period for both panels.