Ticks in changing times: Drought seems to be a main driver of declining Ixodes ricinus abundance in an urban setting

Urban green spaces promote human well-being, but also represent areas of tick infestation and pathogen transmission risk. In Hanover city, Germany, long-term monitoring of questing tick abundance was initiated in 2017-2018 (Hauck et al., 2020). Here, we combine this data with a six-year follow-up, including the 2018-2020 and 2022 Europe-wide drought periods. Additionally, the density of infected nymphs (DIN) was calculated including pathogen prevalence data from 2020 (Glass et al., 2022). Tick density was observed to decline after drought years with a time lag, falling from an average of 25 and 26 ticks per 100 m² in 2017 and 2018 to 20 ticks in 2019, 16 in 2020 and 6 in 2021, which is a reduction of almost 75%. After the precipitation-rich year of 2021, density began to recover in 2022 (7 ticks/100 m²), but remained low in 2023 (10 ticks/100 m²) and 2024 (8 ticks/100 m²). After the 8-year observation period, tick density has therefore been reduced by almost 70% compared to initial values. Generalized linear mixed modelling showed significant associations between I. ricinus density and climatic variables. The DIN per monitoring site was 0.03-9.76/100 m², 0.00-0.04/100 m² and 0.11-16.81/100 m² for Borrelia spp., A. phagocytophilum, and Rickettsia species. This study strongly indicates that climate change, especially prolonged drought, affects I. ricinus populations in urban settings, and its long-term dataset can serve as a reference for future ecological and public-health risk assessments under a changing climate.