Interactions between the invasive parasite, Philornis downsi, and its Darwin’s finch hosts: a matrix modeling approach

Abstract

Philornis downsi, a parasitic fly native to mainland South America, has become invasive in the Galapagos Islands where it causes severe mortality in several species of Darwin’s finches. Female flies lay eggs in bird nests, and larvae feed on the blood of nestlings. Both the percentage of infested nests and the number of larvae per nest are higher in Galapagos than in the fly’s native range, resulting in high nestling mortality and raising serious conservation concerns. To better understand the factors limiting P. downsi population growth, we developed a stage-structured matrix model incorporating empirically estimated survivorship and duration for each life stage as well as two forms of density-dependent dynamics: (1) density-dependent oviposition behavior constrained by the maximum number of immature flies that finch nests can support, and (2) density-dependent mortality of P. downsi larvae due to competition among larvae. These additions provided ecological realism and allowed population simulation outcomes to be interpreted at per-nest scales. Finally, we introduced a range of density-independent mortality values for P. downsi eggs, larvae, pupae and adults to identify which life-stage mortality most strongly influences finch survival. Our comparison indicates that increasing pupal mortality has the strongest effect on the per-nest abundance of P. downsi (and thus finch survival), consistent with pupation occurring after larval density-dependent mortality. We discuss the implications of these effects for management of P. downsi, focusing on the potential importation of pupal parasitoids as biological control agents.