The bird-parasitic ?y, Philornis downsi, was ?rst recorded in the Galápagos Islands in 1964 where it likely invaded from mainland Ecuador. This muscid ?y is now the leading cause of recent declines in endemic landbird populations as its larvae feed on the nestlings of at least 19 bird species in the Galápagos, including many species of Darwin’s ?nches. As yet, no long-term control method has been implemented for P. downsi, but importation (also known as classical) biological control may be a viable option.