First record of Philornis (Diptera: Muscidae) parasitizing the threatened Saffron-cowled Blackbird in South America with a description of a new Philornis species

Bird parasitism by Philornis flies imposes negative selection pressures on avian hosts. Philornis species are known to attack many threatened and non-threatened bird species in the Neotropics. The Saffron-cowled Blackbird (Xanthopsar flavus Gmelin, 1788) is an endangered South American grassland species in decline due to habitat loss. To our knowledge, this study constitutes the first report of parasitism of X. flavus by two Philornis species, Philornis downsi Dodge & Aitken, 1968 and a new species. We described the new species from larvae, puparia, and adults collected from parasitized X. flavus nests and nestlings. We identified P. downsi from puparia collected directly from nests. We named the new species Philornis aguapey sp. nov., identified integrating morphological and molecular data. We compared fly morphology and proposed new characters to distinguish P. aguapey sp. nov. from other recognized species. We presented data on Philornis prevalence within the breeding range of X. flavus in Argentina. Our morphological and molecular approach to identifying Philornis species highlighted the taxonomic challenges of this genus, emphasizing the need for integrative approaches for species identification. Philornis parasitism of X. flavus, mainly in Corrientes province, raises concerns about its potential additive impact on this already endangered bird population. Nothing is known of the effects of this fly parasitism on development and mortality of X. flavus nestlings. Taxonomic gaps and inconsistencies exist in the genus Philornis, especially at its southern distribution, where information remains scarce. Thus, we encourage further research to address some of the knowledge gaps in this host-parasite system.