Interannual variation in Protocalliphora blowfly (Diptera: Calliphoridae) parasitism of Tree Swallows ( Tachycineta bicolor ) breeding in southern Québec’s farmlands: 16 years of data on species composition, morphometry and hyperparasitism by Nasonia wasps (Hymenoptera: Pteromalidae)

Protocalliphora are among the most prevalent ectoparasites of altricial birds across the Holarctic. Yet, their spatial and temporal dynamics of infestations, species composition, and interactions with parasitoids remain poorly understood. We present a 16-year (2004-2019) multi-site study of Protocalliphora infestations based on 2,673 Tree Swallow ( Tachycineta bicolor ) nests collected across a 10,200-km² gradient of agricultural intensity in Québec, Canada. Protocalliphora nest infestation prevalence and load varied markedly across space and time, and showed cyclical patterns at ∼75% of sites, suggesting the influence of regional and local processes. Yearly rates of hyperparasitism of Protocalliphora puparia by Nasonia parasitoid wasps were also high but variable (48-90%), likely contributing to the temporal oscillations in Protocalliphora prevalence and load. Substantial interannual shifts in the relative abundance of three Protocalliphora species ( bennetti , metallica , and sialia ) emphasized the importance of species-level resolution in blowfly ecological studies. Considerable overlap in puparia size among species challenged the utility of traditional diagnostic traits for species identification. Finally, dormancy or mortality of Nasonia occurred in 3-16% of Protocalliphora puparia depending on year. These findings highlight the importance of long-term, multi-trophic, and spatially explicit monitoring to unravel the complex ecological drivers of host-parasite-parasitoid dynamics in increasingly human-modified environments such as agroecosystems.