A Comparison of Questing Substrates and Environmental Factors That Influence Nymphal Ixodes pacificus (Acari: Ixodidae) Abundance and Seasonality in the Sierra Nevada Foothills of California

Abstract

In California, the western blacklegged tick, Ixodes pacificus Cooley and Kohls, is the principal vector of the Borrelia burgdorferi sensu lato (sl) complex (Spirochaetales: Spirochaetaceae, Johnson et al.), which includes the causative agent of Lyme disease (B. burgdorferi sensu stricto). Ixodes pacificus nymphs were sampled from 2015 to 2017 at one Sierra Nevada foothill site to evaluate our efficiency in collecting this life stage, characterize nymphal seasonality, and identify environmental factors affecting their abundance and infection with B. burgdorferi sl. To assess sampling success, we compared the density and prevalence of I. pacificus nymphs flagged from four questing substrates (logs, rocks, tree trunks, leaf litter). Habitat characteristics (e.g., canopy cover, tree species) were recorded for each sample, and temperature and relative humidity were measured hourly at one location. Generalized linear mixed models were used to assess environmental factors associated with I. pacificus abundance and B. burgdorferi sl infection. In total, 2,033 substrates were sampled, resulting in the collection of 742 I. pacificus nymphs. Seasonal abundance of nymphs was bimodal with peak activity occurring from late March through April and a secondary peak in June. Substrate type, collection year, month, and canopy cover were all significant predictors of nymphal density and prevalence. Logs, rocks, and tree trunks had significantly greater nymphal densities and prevalences than leaf litter. Cumulative annual vapor pressure deficit was the only significant climatic predictor of overall nymphal I. pacificus density and prevalence. No associations were observed between the presence of B. burgdorferi sl in nymphs and environmental variables.

Keywords: Borrelia; Sierra Nevada; environment; nymph; tick.

Fig. 1.

Nymphal Ixodes pacificus density (nymphs per meter sampled) by substrate type from February 2015 to July 2017 at Salmon Falls, Folsom Lake State Recreation Area.

Fig. 2.

Nymphal Ixodes pacificus counts and density from February 2015 to July 2017 at Salmon Falls, Folsom Lake State Recreation Area, California. Nymphal density was calculated using the kernel density tool in ArcGIS.

Fig. 3.

Nymphal Ixodes pacificus density (nymphs per meter sampled) for all substrates in relation to cumulative vapor pressure deficit. Cumulative vapor pressure deficit is calculated as the sum of the daily average vapor pressure deficits from 1 January to the collection date. Note that y-axis scales vary by substrate type to better visualize the relationship between cumulative vapor pressure deficit and nymphal density.