Interacting effects of mean temperature and temperature variability on <i>Ixodes scapularis</i> (Acari: Ixodidae) overwinter survival and energy use

Within recent decades, the northeastern United States has experienced milder winters, supporting an increase in blacklegged tick (Ixodes scapularis) densities and concomitant increase in reported cases of tick-borne disease. While numerous laboratory and field studies have investigated impacts of cold temperatures on tick overwinter survival, temperature variability also may contribute to tick longevity and remains largely underexplored. In this study, we conducted three experiments. First, we carried out a laboratory experiment in which blacklegged tick nymphs were exposed to different low temperatures and cycles of temperatures typical of the northern geographic range of the blacklegged tick and survival was measured. We found that tick survival was highest under a high temperature, low variability regime (in which ticks were exposed to -5 °C for nine days followed by 4 °C for one day) compared to both lower temperature and higher variability regimes. Second, in a parallel field experiment, we tested overwinter survival of captive nymphal blacklegged ticks held in outdoor enclosures spanning a natural ambient temperature gradient and manipulated snowpack conditions. Again, we found that tick survival was highest at the warmer field site and when snow was allowed to accumulate over the enclosure, which reduced temperature variability. Finally, we conducted an assay to measure ticks' energy usage over the course of two winters and established a link between survival and percent lipid content. We propose that tick mortality may be driven by repeated freeze-thaw cycles, causing ticks to rapidly expend energy. Overall, we anticipate that understanding the interacting effects of cold temperature and temperature variability can aid in forecasting range shift of the blacklegged tick under climate change.