Spring fever: early spring predicts Ixodes pacificus (Acari: Ixodidae) activity in northwestern California

In the far western United States, the bacterial agent of Lyme disease, Borrelia burgdorferi sensu lato, is primarily transmitted to humans by the nymphal stage of the western black-legged tick, Ixodes pacificus. Predicting nymphal tick abundance would benefit public health but is complicated by the tick's complex multi-year life cycle and data limitations. To address this, we used readily available climate data, a long-term dataset from northwestern California, and time lags based on the tick's life cycle. Our model showed that warmer early spring temperatures during non-drought conditions the year prior predicted higher nymphal tick densities, while hot, dry springs the year prior were linked to earlier peaks in their abundance. Incorporating human-induced climate change projections, we predicted earlier peaks of nymphal tick activity over the next century, with the potential for an initial doubling in questing nymph numbers. This approach provides a valuable tool for public health and offers insights into the changing dynamics of Lyme disease ecology in the far-western US.