Host-Seeking Phenology of Ixodes pacificus (Acari: Ixodidae) Nymphs in Northwestern California in Relation to Calendar Week, Woodland Type, and Weather Conditions

Local knowledge of when humans are at elevated risk for exposure to tick vectors of human disease agents is required both for the effective use of personal protection measures to avoid tick bites and for implementation of controlmeasures to suppress host-seeking ticks. Here, we used previously published data on the seasonal density of host-seeking Ixodes pacificus Cooley and Kohls nymphs, the primary vectors of Lyme disease spirochetes in the far western USA, collected across a broad habitat and climate gradient in northwestern California to identify predictors of periods of time within the year when questing nymphal density is elevated. Models based on calendar week alone performed similarly to models based on calendar week and woodland type, or meteorological variables. The most suitable model for a given application will depend on user objectives, timescale of interest, and the geographic extent of predictions. Our models sought not only to identify when seasonal host-seeking activity commences, but also when it diminishes to low levels. Overall, we report a roughly 5-7 month period in Mendocino County during which host-seeking nymphal densities exceed a low threshold value.