Climate-driven variation in the phenology of juvenile Ixodes pacificus on lizard hosts

Background: Ectothermic arthropods, like ticks, are sensitive indicators of environmental changes, and their seasonality plays a critical role in tick-borne disease dynamics in a warming world. Juvenile tick phenology, which influences pathogen transmission, may vary across climates, with longer tick seasons in cooler climates potentially amplifying transmission. However, assessing juvenile tick phenology is challenging in arid climates because ticks spend less time seeking for blood meals (i.e., questing) due to desiccation pressures. As a result, traditional collection methods like dragging or flagging are less effective. To improve our understanding of juvenile tick seasonality across a latitudinal gradient, we examine Ixodes pacificus phenology on lizards, the primary juvenile tick host in California, and explore how climate factors influence phenological patterns.

Methods: Between 2013 and 2022, ticks were removed from 1,527 lizards at 45 locations during peak tick season (March-June). Tick counts were categorized by life stage (larvae and nymphs) and linked with remotely sensed climate data including monthly maximum temperature, specific humidity, and Palmer Drought Severity Index (PDSI). Juvenile phenology metrics, including tick abundances on lizards, Julian date of peak mean abundance, and temporal overlap between larval and nymphal populations, were analyzed along a latitudinal gradient. Generalized Additive Models (GAMs) were applied to assess climate-associated variation in juvenile abundance on lizards. 

Results: Mean tick abundance per lizard ranged from 0.17 to 47.21 across locations, with the highest in the San Francisco Bay Area and lowest in Los Angeles, where more lizards had zero ticks attached. In the San Francisco Bay Area, peak nymphal abundance occurred 25 days earlier than peak larval abundance. Temporal overlap between larval and nymphal stages at a given location varied regionally, with northern areas showing higher overlap. We found that locations with higher temperatures and increased drought stress were linked to lower tick abundances, though the magnitude of these effects depended on regional location.

Conclusion: Our study, which compiled 10 years of data, reveals significant regional variation in juvenile I. pacificus phenology across California, including differences in the abundance, peak timing, and temporal overlap. These findings highlight the influence of local climate on tick seasonality, with implications for tick-borne disease dynamics in a changing climate. 

Methods
The data for this study was collected from 2013 to 2022, primarily during the peak juvenile activity months of Ixodes pacificus ticks, which are March through June. This dataset reflects the collective efforts of various lab groups engaged in ecological research, which included both lizard sampling and tick burden assessments in California, United States. This aggregated dataset includes 45 unique sampling locations and encompasses a total of 253 sampling days. Of the 45 locations, 93% were sampled multiple times, with 84% being sampled three or more times. Various subsets of this data have been published previously by Swei et al 2011, MacDonald et al. 2018, Sambado et al. 2024, Copeland et al. 2025.