Cold hardiness and influences of hibernaculum conditions on overwintering survival of American dog tick larvae

Understanding how ticks survive the multitude of stresses experienced during winter is integral to predicting population dynamics and transmission of tick-borne pathogens. The American dog tick (Dermacentor variabilis), a predominant vector of Rocky Mountain spotted fever, overwinters in any of its post-egg life stages. In this study, we characterized the cold tolerance of larval D. variabilis and examined the effect of various acclimatory conditions on cold hardiness. Cold-shock survival during 2h exposure to various subzero temperatures was assessed and a lower lethal temperature of -20°C and a 50% mortality temperature near -16°C was established. Larvae exposed to -5°C showed high survival (∼70%) after 14 d and near 50% for up to 56d at -5°C. Larvae cycled between supra- and subzero temperatures showed better long-term survival than at constant -5°C. The temperature of crystallization (T_c) was ∼-23°C and no larvae survived freezing after reaching their T_c. Larvae exposed to inoculative freezing survived brief, mild treatments (70% survival of -5°C for 2h) but survival was reduced compared to larvae cooled in the absence of ice. Reduced photophase, rapid cold hardening, dehydration, and long-term thermal acclimation all improved larval cold hardiness to varying degrees. Survival data were compared to measurements of hibernacula conditions, and we conclude that larvae face little threat from cold-induced mortality but inoculative freezing does pose a risk, and the geographic distribution of D. variabilis is only minimally influenced by the ability of larvae to survive low temperature exposure.