Spatially heterogenous distributions of dipteran abundances identify priority areas for haemosporidian parasite surveillance

How changes in one facet of biodiversity affect others remains poorly understood. Changes in dipteran populations, for example, might impact the spread of haemosporidian parasites. Using biomonitoring data from 934 Lithuanian freshwater sites (2013 − 2022) and a hierarchical Bayesian GLM with spatial correlation, we examined ecological quality, elevation, land-use, and climate as drivers of the abundance of dipteran families containing pathogen vectors. As most taxa were identified only to family or genus, vector and non-vector species could not be distinguished. Higher abundances were associated with better ecological quality, higher elevation, increased precipitation, and warmer temperatures, and were up to ~ 780% higher in the east, where dense forests and better water quality prevail. As a case study illustrating how environmental suitability and dipteran abundance might translate into local infection dynamics, rather than as model validation, we compared haemosporidian prevalence in juvenile blue tits sampled on the Curonian Spit in 2003/2004 and 2018/2019. Prevalence increased, yet haemosporidians were sampled in the west, where dipteran abundances were lower, suggesting a disconnect between haemosporidian sampling locations and where potential vectors are common. This study illustrates the under-utilized potential of biomonitoring datasets for vector and pathogen ecology and identifies priority areas for future parasitological research.