A mechanistic model explains variation in larval tick questing phenology along an elevation gradient.

Abstract

Many tick-borne pathogens are maintained in enzootic cycles passing from nymphs of one tick cohort to larvae of the next via vertebrate hosts. As such, the phenology of larval and nymphal host-seeking, questing, partially determines pathogen persistence. Across the range of the blacklegged tick (Ixodes scapularis), the timing of larval phenology varies due to differences in climate and local adaptation in the timing of temperature-independent diapause. In this study, an elevation gradient was used to isolate climate as temperature varies with elevation over small geographic scales where local adaptation should be absent. The ability of a mechanistic, temperature-driven, literature-parametrized model to explain variation in larval I. scapularis phenology was tested. Over 7 years, I. scapularis ticks were collected using drag-cloth sampling along a > 500 m elevation gradient in western Vermont, USA. At low elevation, more larval ticks quested in late summer, while at high elevation, more quested in early summer. The literature-parametrized model reproduced these differences better than competing models. This validated model provides an explicit, mechanistic connection between temperature and larval phenology, a key determinant of tick-borne disease persistence.