Climatic stress decreases tick survival but increases rate of host‐seeking behavior

Abstract

AbstractTicks are vectors of many diseases and are expanding in geographic distribution. However, how ticks will fare in their new environments, where they may experience stressful climatic conditions at the expansion front, remains unclear. Since there is a trade‐off in ticks between behaviors that promote longevity and behaviors that promote reproduction, we hypothesized that extreme climatic stress reduces the survivorship of ticks but increases the frequency of tick host‐seeking behavior, or questing. Here, we used a novel method to simulate climatic stress on individual ticks of three species—Amblyomma americanum, Dermacentor variabilis, and Ixodes scapularis—to evaluate their survival, physiology, and questing behavior. The first experiment involved placing 144 adult ticks of each species in two temperature ranges (15–25°C and 25–35°C) and three relative humidity (RH) treatments (32%, 58%, and 84% RH). We assessed the ticks daily for survivorship and questing, and we measured water loss by comparing the mass of each tick when it died to when it was fully hydrated. In this first experiment, ticks in warmer and less humid conditions generally died faster than those in cooler and more humid conditions. Ticks of all three species were more likely to quest shortly before their death and consistently died after losing approximately 50%–56% of their total body water content, but Ixodes reached that threshold much faster than the other two species. The second experiment involved placing 18 ticks of each species at 35°C and 32% RH. We assessed the ticks every 3 h for survivorship, questing, and water loss. Ticks again were more likely to quest shortly before their death. With frequent checks, we were able to measure the dehydration tolerance more accurately and the rate of water loss. Ticks of all three species consistently died after losing approximately 51% of their total body water content. However, Ixodes lost water approximately 5 times faster than Amblyomma and 11 times faster than Dermacentor. These results demonstrate that severe climatic stress tilts the trade‐off toward higher questing rates but not higher overall questing time because of reduced survival rates.