Variation in the supercooling points of laboratory-reared Canadian Dermacentor variabilis (Acari: Ixodidae)

The geographical range of the american dog tick, Dermacentor variabilis (Say, 1821), in Canada is expanding northwards into areas where ticks experience colder temperatures during winter. Understanding the ability of D. variabilis to cope with freezing temperatures is important for determining what limits the distributional range of this species. In this study, the supercooling point (SCP = the sub-zero temperature at which freezing occurs) was measured for 318 laboratory-reared D. variabilis individuals (109 nymphs and 209 adults). These ticks were reared from questing adult ticks collected from an area adjacent to Blackstrap Provincial Park in Saskatchewan, Canada. The results showed that there was no relationship between the SCP and body weight of adult ticks, and no difference in the SCP of female and male ticks. However, the mean SCP of nymphs (-23.6°C) was significantly lower than that of adults (-21.6°C). The biological significance of this finding needs to be explored further given that D. variabilis larvae and adults are the primary overwintering stages at northern latitudes.