A Comparison of Questing Substrates and Environmental Factors That Influence NymphalIxodes 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.

Revealing large‐scale parasite ranges: An integrated spatiotemporal database and multisource analysis of the winter tick

“…This finding could be for several reasons, including already significant changes in climate since the 1960s, which have shifted the average isopleth north (Zhang et al, 2019), or possible differences in the calculation methods of the degree‐days themselves (e.g., length of growing season used) (Sridhar & Reddy, 2013). However, it is also likely that factors influencing winter tick survival at the population level act at a much smaller spatial scale than is fully captured by thermal isopleths, as with other tick species (Dobson et al, 2011; Dumas et al, 2022; Hacker et al, 2021). Winter ticks have a short but critical window of opportunity for important life‐cycle processes to occur during their off‐host life stages.…”

Effects of Climate on the Variation in Abundance of Three Tick Species in Illinois

“…Monthly total precipitation directly strongly influences both maximum VPD and dewpoint, resulting in an indirect effect of precipitation within those models as well. Previously, it has been suggested that precipitation is associated with tick abundance due to the impact it has on humidity, a factor that has been demonstrated to decrease the likelihood of Ixodes tick desiccation ( Hahn et al 2016 , Hacker et al 2021 ). Our results support these findings.…”

Ecology and Epidemiology of Lyme Disease in Western North America

“…Both active and passive surveillance have observed low infection prevalence in I. pacificus adults, ranging from 0.6 to 7.3% [21,22,33,[52][53][54][55][56][57][58]. Nymphal infection prevalence is similarly low, although slightly higher than in adults, ranging from 1.4 to 9.9% [31,33,40,53,55,56,[58][59][60][61]. These rates are considerably lower when compared with the infection prevalence for I. scapularis nymphs and adults, which can be as high as 23% [62] and 47% [63], respectively.…”

“…The seasonal abundance of I. pacificus varies in western North America. Based on active [27,[40][41][42][43][44][45][46][47][48][49][50][51] and passive [22,33,34,52] surveillance efforts in Northern California, Oregon, and Washington, I. pacificus nymphs are most abundant during the spring and early summer, while I. pacificus adults are most abundant during winter to early spring. In Southern California, both adult and nymphal I. pacificus have a shorter period of abundance compared to those in Northern California [50].…”

“…In general, environments that are moderately warm and moist support greater numbers of I. pacificus ticks in the west [40,51,113]. Specifically, moderately warm temperatures can activate host-seeking activities and accelerate the development of all three life stages of I. pacificus, leading to an increase in overall numbers for the tick [51,114].…”