The black-legged tick Ixodes scapularis detects CO2 without the Haller's organ.

Both male and female ticks have a strong innate drive to find and blood-feed on hosts. Carbon dioxide (CO2) is considered a critical behavioral activator and attractant for ticks and an essential sensory cue to find hosts. Yet, how CO2 activates and promotes host-seeking in ticks is poorly understood. We studied CO2 responses in the black-legged tick Ixodes scapularis, the primary vector for Lyme disease in North America. Adult males and females were exposed to 1, 2, 4, or 8% CO2, and changes in walking behavior and foreleg movement were analyzed. CO2 is a potent stimulant for adult Ixodes scapularis, even at lower concentrations (1%). Behavioral reactions depended on the animal's state: Walking ticks increased their walking speed, while stationary ticks started to wave their forelegs and began to quest - both behaviors resembling aspects of host-seeking. Only in sporadic cases did stationary animals start to walk when exposed to CO2, supporting the hypothesis that CO2 acts as an activator rather than an attractant. Furthermore, Ixodes scapularis did not show a clear concentration preference and was not tuned more robustly to breath-like CO2 concentrations (∼4%) than the other concentrations tested. Moreover, we provide convincing evidence that the foreleg Haller's organ is not necessary for CO2 detection. Even with disabled or amputated Haller's organ, Ixodes scapularis responded robustly to CO2, signifying that there must be CO2-sensitive structures important for tick host-seeking that have not yet been identified.