Abstract
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.