Dartmouth College Hanover, New Hampshire, United States
This study is examining the effects of environmental conditions on survival and host-seeking behavior of the tick Ixodes scapularis, a primary vector for Lyme disease. I. scapularis spend the majority of their life cycle off-host, with periods of time spent in development or inactivity and the rest in pursuit of a host. To search for a host, I. scapularis moves from a saturated microclimate in the leaf litter to a drier, more exposed layer. However, this drier horizon imposes a physiological constraint and, while a tick can quest for hosts for some period of time, it ultimately must return to the litter to reabsorb water. Tick host-seeking strategies must therefore balance physiological constraints with maximizing the probability of encountering a host. We are testing how tick survival and behavior change with the physiological stress of dry conditions by exposing unfed adult female I. scapularis to different daily durations of low relative humidity (0, 4, 8, or 12 hours) alternating with near-saturated conditions in laboratory conditions. Our pilot experiment focused on survival, but we are now measuring individual tick behavior with a Drosophila Activity Monitor (DAM) and will quantify survival, wet mass, and energy reserves at the end of each experiment.
Results to date suggest that I. scapularis is very sensitive to the duration of exposure to low relative humidity (RH). For example, in the preliminary experiment focusing on survival over a 21 day period, there was a 53% decrease in the survival of unfed adult female I. scapularis under prolonged dry conditions: only 46.6% of ticks were still alive after 3 weeks of exposure to 25% RH for 12 hours per day. By contrast, 100% of the control ticks kept at near saturation throughout the experiment survived. Ticks exposed to 8 hours at 25% RH had intermediate survival in that 73% were still alive at the end of the experiment. Ongoing experiments are assessing more fine-scale behavioral responses using the DAM to test the hypothesis that ticks adjust their host seeking strategies to avoid prolonged exposure to adverse conditions. We expect that this work will contribute to our understanding of the fine-scale decision-making of an individual adult tick, including its choices on when and how to seek a host as a function of energetic reserves, water balance, and the physiological stress of the current abiotic environment.