A temporal analysis of horse fly prevalence in Tokachi, Hokkaido, Japan, based on a 3-year entomological survey.

Horse flies (Diptera: Tabanidae) are significant pests that affect both livestock and humans via their blood-feeding behavior and role as biological and mechanical vectors of several pathogens. The Tokachi district in Hokkaido is a major livestock farming region in Japan, but long-term data on horse fly activity have been lacking for over 40 years. To address this, we conducted a 3-year entomological survey (2022-2024) at 11 livestock farms using one commercial stationary horse fly trap (H-trap) per each farm. Horse flies were collected from early June to late October each year and identified morphologically. A total of 56,542 individuals from 18 species and five genera were collected using H-traps. Tabanus nipponicus (67.91%) and Haematopota tristis (29.99%) were the dominant species. Seasonal abundance showed a unimodal peak in late July to early August, with most activity from early July to late August. Compared to data from the 1980s, the appearance of dominant species has shifted approximately one month earlier, likely due to warming temperatures, as supported by degree-day analysis. Furthermore, a shift in dominance from Hae. tristis to T. nipponicus may reflect land use changes in the region. This study provides updated insights into the species composition and seasonal dynamics of horse flies in a key agricultural area of Japan and highlights the importance of long-term monitoring to assess the impacts of climate and environmental change on pest populations.