The role of direct chilling injury and inoculative freezing in cold tolerance of Amblyomma americanum, Dermacentor variabilis and Ixodes scapularis

Supercooling points and chill tolerance were compared among nymphs and adults of the ixodid ticks Dermacentor variabilis, Amblyomma americanum and Ixodes scapularis (Acari: Ixodidae). Supercooling points in the range of <-- 22 to -18 degrees C were observed for nymphs, and -22 to -8 degrees C for adults. The lower lethal temperatures observed under dry conditions, -14 to -10 degrees C, were warmer than the supercooling points, but still much colder than -4.8 degrees C, the lowest temperature recorded from a likely tick habitat in southwestern Ohio. Based on our experiments, spontaneous freezing and direct chilling injury are not significant mortality factors in these species in the field. Mortality was observed between -5 and -3 degrees C for A. americanum and D. variabilis nymphs chilled for 2 h while in direct contact with ice. This mortality is probably due to inoculative freezing. Given the requirement for a rather humid microhabitat for off-host survival, these findings suggest that inoculative freezing is an important cause of overwintering mortality in these medically important species.