Vampires fight back: Host defense by the avian vampire fly with implications for biological control

Gregarious pupation is a common defense mechanism against parasitism. This is seen in Philornis downsi, the avian vampire fly, a bird nest parasite that feeds on nestling blood as larvae, and then pupates in tight clusters within the nest. In their native range of mainland Ecuador, the prevalence of P. downsi is relatively low presumably in part because they are attacked by parasitoid wasps, but in the Galapagos islands where they are invasive P. downsi populations are extremely high, causing drastic declines of endemic Galapagos bird populations. I hypothesized that in mainland Ecuador where the flies are less prevalent, their pupal clusters may be small or absent, but in Galápagos, where hundreds of pupae are more likely to be present in a single nest, these clusters would be larger. Additionally, I hypothesized that pupae may be less accessible to parasitoids in larger clusters, resulting in lower parasitism rates in nests with higher densities of P. downsi pupae. This is relevant as two parasitoid species are currently under evaluation as potential biological control agents of P. downsi in Galápagos. I compared P. downsi clustering behavior in Galápagos vs mainland Ecuador bird nests and conducted laboratory experiments to determine whether cluster size affects parasitism by both parasitoid species. We found larger cluster sizes in Galápagos nests. Additionally, cluster size did not impact parasitism rates but we found that together the wasps provide subadditive control of P. downsi and both parasitoid species have different strategies for accessing hosts at varying positions within clusters.