Larval thermal characteristics of multiple ixodid ticks.

Temperature limits the geographic ranges of several tick species. Little is known about the thermal characteristics of these pests outside of a few studies on survival related to thermal tolerance. In this study, thermal tolerance limits, thermal preference, and the impact of temperature on activity levels and metabolic rate were examined in larvae for six species of ixodid ticks. Tolerance of low temperatures ranged from -15 to -24 °C with Dermacentor andersoni surviving the lowest temperatures. High temperature survival ranged from 41 to 47 °C, with Rhipicephalus sanguineus sensu lato having the highest upper lethal limit. Ixodes scapularis showed the lowest survival at both low and high temperatures. Thermal preference temperatures were tested from 0 to 41 °C. The majority of species preferred temperatures between 17 and 22 °C, while Dermacentor variabilis preferred significantly lower temperatures, near 12 °C. Overall activity was measured across a range of temperatures from 10 to 60 °C, and most tick species had the greatest activity near 30 °C. Metabolic rate was the greatest between 30 and 40 °C for all tick species and was relatively stable from 5 to 20 °C. The optimal temperature for tick larvae is likely near the thermal preference for each species, where oxygen consumption is low and activity occurs that will balance questing and conservation of nutrient reserves. In summary, tick species vary greatly in their thermal characteristics, and our results will be critical to predict distribution of these ectoparasites with changing climates.

Dehydration and starvation yield energetic consequences that affect survival of the American dog tick.

Ticks are obligate hematophagous arthropods, but may have to endure extended time (1-2years) between feedings. During these off-host periods, ticks must contend with a multitude of environmental stresses including prolonged or repeated exposure to desiccating conditions. In this study, we measured the energetic consequences of single and repeated bouts of dehydration of American dog ticks, Dermacentor variabilis, and examined the impact of energy reserves on tick survival during dehydration. Recently molted ticks exposed to a single period at 0% relative humidity (RH) for 5d lost ∼26% of their body water and showed 1.3- and 1.7-fold reductions in protein and lipid, respectively. These reduced energy reserves coincided with increased O2 consumption in dehydrated ticks. Exposure to repeated cycles of dehydration (0% RH, 48h) and rehydration (100% RH, 24h) also reduced energy reserves; however, ticks were able to fully recover their body water after 12 cycles of dehydration/rehydration and endured >20 cycles. Starvation of ticks, in the absence of dehydration, for 18 or 36weeks resulted in the loss of ∼20-40% of protein and 60% of lipid reserves. When ticks were exposed to continuous dehydration at 0% RH, their survival after 18weeks of starvation was only minimally impacted; however, individuals starved for 36weeks succumbed to dehydration much more rapidly than recently fed ticks. Both single and repeated dehydration exposures resulted in substantial energetic costs and ticks with limited energy reserves were more susceptible to dehydration-induced mortality, indicating that adequate energy reserves are critical for tolerance to dehydration stress and long-term success of ticks.

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 (Tc) was ∼-23°C and no larvae survived freezing after reaching their Tc. 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.