Ramification of relaxed thermoregulation by disease vectors under climate change

Understanding how vector species such as ticks respond behaviorally to thermal and host-related cues is critical for predicting the effects of climate change on disease transmission. Ticks employ distinct questing strategies-ranging from static sit-and-wait behavior to active host seeking-that influence their exposure to abiotic stressors. We investigated whether Dermacentor andersoni and Dermacentor similis, two sit-and-wait tick species native to Eastern Washington, USA, exhibit behavioral plasticity in response to temperature gradients and host stimuli. We conducted three experiments to assess (1) thermal preferences of D. andersoni under host (dog) scent conditions, (2) thermotactic responses of both species to infrared (IR) radiation in the presence of dog odor, and (3) D. andersoni's approach behavior toward human hosts at various distances, i.e., a host emitting a combination of IR, CO2, and odors. In thermal gradient assays, D. andersoni showed significantly increased movement toward warmer zones following CO₂ exposure and exhibited wide thermal preferences depending on specific dog odors. However, when exposed to an IR source, we found strong sit-and-wait behaviors by D. andersoni and D. similis regardless of temperature or radiation. The ticks were not attracted to infrared radiation and did not move toward the stationary exposed hand of an observer. Ticks may prioritize optimal locations to encounter potential hosts, over enzymatically optimal temperatures. Rather than evolving to detect hosts at a distance, Haller's organs may have evolved to differentiate warm attachment sites from cooler fur. Our results suggest that Dermacentor questing behavior (remaining on station despite a different preferred temperature) may make them particularly vulnerable to future rises in temperature.