The nymphal and adult life stages of the tick Ixodes pacificus Cooley & Kohls are the primary vectors of the causative agent(s) of Lyme disease in the far-western United States. In contrast to I. pacificus adults, data on the extent of spatial and temporal variation in the density of the nymphal stage have been scarce. Therefore, we compared the density of I. pacificus nymphs from 1997 to 1999 at a small rural community at high risk for Lyme disease (CHR) and the University of California Hopland Research and Extension Center (HREC), Mendocino County, CA, I. pacificus nymphs were collected readily by drag sampling in leaf litter but not from low vegetation. The mean number of questing nymphs per 100 m2 in leaf/fir-needle litter areas from late April to early June differed significantly among years at both the CHR and the HREC. Climatic data from the HREC suggested that yearly nymphal densities in this area may be positively correlated with the amount of rainfall and negatively correlated with maximum temperatures from March to May. Further, nymphal density was two to four times higher at the CHR than the HREC in all 3 yr. Yearly mean nymphal density generally differed two- to four-fold (HREC) and 10- to 20-fold (CHR) among individual sampling areas. Also, two- to three-fold differences in mean nymphal density were frequently observed between continuous litter and litter edges bordering on other habitats within sampling areas at the HREC, and between sampling areas <100 m apart at the CHR. The distributions of nymphs within individual areas were generally aggregated at both the HREC and the CHR, at the 15-m sampling transect scale we used. Environmental factors with some potential to predict the density of I. pacificus nymphs at different spatial scales included climatic conditions, topographic exposure, and presence of habitat edges or logs. However, stepwise multiple regression analysis revealed that only 6.2–18.0% (HREC) or 14.9–27.9% (CHR) of the yearly variation in nymphal densities was explained by the abiotic or biotic traits measured in this study, at the 15-m transect scale. Thus, other abiotic or biotic traits that we did not examine (e.g., local densities of tick hosts) must account for most of the spatial variation in nymphal density.
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