The impacts of climate change on the overwintering activity of black-legged ticks, Ixodes scapularis

Warming winters are increasing opportunities for ticks to become active during a typically dormant season. Increases in winter activity thus increase disease risk to the public if these winter-active ticks carry pathogens, such as the bacteria that causes Lyme disease (Borrelia burgdorferi). However, we know little of how many ticks become active, or what drives activity in the winter beyond crossing a threshold that permits movement. Thus, to predict how changing winters will impact overwintering activity and disease risk, we examined the daily winter activity of adult, female I. scapularis under simulated overwintering conditions over three winters using Trikinetics behavioural monitors. Approximately 50% of these field-collected ticks were infected with B. burgdorferi; however, infection had an inconsistent effect on activity. Large amplitude changes in temperature encouraged increased activity, rather than reaching an absolute temperature threshold. Further, we could classify ticks into three cohorts of activity levels: highlight active ticks, fairweather ticks, and low levels of activity. These profiles suggest that winter behaviour is driven by the physiological state of the individual tick. The impacts of winter climate change on tick physiology (e.g. energy use) are thus likely to impact activity in the winter. It will next be important to understand the drivers underlying these differences in activity to determine the impact on tick survival and disease transmission during this formative season.