Cold-hardiness of Dermacentor marginatus (Acari: Ixodidae).

The cold-hardiness of Dermacentor marginatus using laboratory-reared offspring of ticks collected in Germany was characterized. Investigations of unfed stages revealed that adult ticks suffered 50% mortality at -10 degrees C after 4-5 months, but larvae and nymphs suffered mortality within few days, whereas -15 degrees C was lethal for all stages within a very short period. Larval hatch and moulting of engorged larvae and nymphs did not occur at < or = 10 degrees C. Embryonic development of eggs with larval hatch was considerably reduced by exposure of eggs to < or = 10 degrees C. Engorged females did not lay eggs at < or = 10 degrees C, the oviposition capability, however, persisted over 6 months at 10 degrees C, 5 months at 5 degrees C, 3 months at 0 degree C and 2 months at -10 degrees C without substantial decrease of the oviposition capacity or reduction of viable eggs. These results present evidence that unfed adult ticks are the ecoepidemiologically most effective stages, which are capable to tolerate long and extremely cold winters without substantial impairment of the population density. It is also considered that engorged females interrupt their oviposition at low and subzero temperatures delaying it for months and so contribute in bypassing winter conditions. None of the stages survived supercooling indicating that D. marginatus is freeze intolerant. Mean supercooling point (SCP) ranged between -26 degrees C in eggs and -12, 6 degrees C in engorged females. Compared with eggs, the SCP of the other stages was significantly higher. In conclusion, the SCP is considered to have no predictive value in the context with cold-hardiness.