MODELING/GIS, RISK ASSESSMENT, ECONOMIC IMPACT Climate-Based Model Predicting Acarological Risk of Encountering the Human-Biting Adult Life Stage of Dermacentor andersoni (Acari: Ixodidae) in a Key Habitat Type in Colorado

We exploited an elevation (climate) gradient ranging from 1,700 to 2,500 m in Poudre Canyon of LarimerCounty, CO, to determine climatic correlates of abundance per 15-s drag sampling time unit (hereafter referred to as abundance) of the human-biting adult life stage of the Rocky Mountain wood tick, Dermacentor andersoni Stiles (Acari: Ixodidae), in a key risk habitat for tick exposure: south/west-facing, rocky hillsides with mixed grassÐbrushÐconifer vegetation. The relationship between elevation and abundance was parabolic, with peak tick abundances occurring at mid-range elevations (2,200Ð2,400 m) and tick abundances approaching zero at 2,100 and 2,500 m. Regression modeling demonstrated that abundance of host-seeking adult ticks in south/west-facing exposures was accurately predicted by several climate variables related to temperature (e.g., mean annual minimum temperature, maximum temperature, and base 10 C growing degree-days, and median length of annual freeze-free period; r values ranging from 0.771 to 0.864), whereas mean annual precipitation, snowfall, or relative humidity were uninformative in this respect (r values ranging from 0.020 to 0.316). Abundance of D. andersoni adults peaked at a mean annual maximum temperature of 10 C and a mean annual growing degree-day value of 650. Relationships between climate variables and abundance of D. andersoni adults were used to create geographic information system (GIS)-based models for predicted tick abundance in south/west-facing exposures in Larimer County. This is the Þrst GIS-basedmodel developed for spatial patterns of abundance ofD. andersoni. Finally, preliminary data fromPoudreCanyon indicate a shift toward peak abundances ofD. andersoni adults occurring in sheltered northern/eastern exposures, rather than in drier and hotter southern/ western exposures, at elevations below 2,100 m.