Citizen science tick observations as a proxy for human exposure: Ecological, behavioural and weather drivers in Finland

Citizen science is an important source of large-scale data on disease vectors, but observations are influenced by both vector availability and human behaviour. We analysed nearly 300 000 citizen science records of two tick species in Finland ( Ixodes ricinus and Ixodes persulcatus ) using generalized additive models to assess how seasonality, weather, and human activity jointly shape daily tick–human encounter reports. Nonlinear seasonal patterns were observed, with species-specific differences consistent with tick phenology. Human population increased the probability and intensity of reporting, while weekend effects and reduced reporting during rainfall indicated the effects of human activity. Temperature responses differed between species, suggesting that encounter patterns vary according to tick species and environmental conditions. Consequently, citizen science tick observations should not be interpreted as measures of tick abundance alone. Rather, they integrate vector ecology, environmental conditions and human behaviour, allowing elevated encounter risk to be identified based on tick species and weather conditions.