Interannual variation in Protocalliphora blowfly (Diptera: Calliphoridae) parasitism of Tree Swallows (Tachycineta bicolor) breeding in southern Quebec s farmlands: 16 years of data on species composition, morphometry and

Although adult blowflies of the genus Protocalliphora are rarely observed in the wild, their larvae are among the most prevalent ectoparasites of altricial birds across the Holarctic. Yet, the spatial and temporal dynamics of infestations, species composition, and interactions with natural parasitoids remain poorly understood. Here, we present a long-term, multi-site studies of Protocalliphora infestations in 2,673 Tree Swallow (Tachycineta bicolor) nests, based on 16 years of data (2004-2019) collected across a 10,200-km{superscript 2} gradient of agricultural intensity in southern Quebec, Canada. We found that Protocalliphora infestation prevalence and load (number of puparia) in Tree Swallow nests, varied markedly across space and time, with cyclical patterns emerging at approximately 75% of sites. This suggests the influence of both regional and local processes. Probability of hyperparasitism of Protocalliphora puparia by Nasonia parasitoid wasps was high but variable (min [~]48%; max [~]90%; mean [~]70%), likely contributing to the observed temporal oscillations in Protocalliphora prevalence and load. We also documented substantial interannual shifts in the relative abundance of three Protocalliphora species (P. bennetti, P. metallica, P. sialia), emphasizing the importance of species-level resolution in ecological studies of parasitism. Morphometric analyses revealed considerable overlap in puparia size among species, challenging the utility of traditional diagnostic traits for species identification. Finally, we report the first field estimates of Nasonia dormancy and/or mortality within hosts, which were observed in 3% to 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.