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 american um 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 supercooli ng points, but still much colder than -4.8 degrees C, the lowest tempe rature recorded from a likely tick habitat in southwestern Ohio. Based on our experiments, spontaneous freezing and direct chilling injury a re not significant mortality factors in these species in the field. Mo rtality 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 re quirement for a rather humid microhabitat for off-host survival, these findings suggest that inoculative freezing is an important cause of o verwintering mortality in these medically important species.