Christian Stenseth1
(1) Div. of Entomology, Norwegian Plant Protection Institute, Vollebekk, Norway

Summary The effect of temperatures below 0° C on different stages of the two-spotted spider mite, and in particular on active adult females, has been investigated.
Survival of active females at-5° was higher at approximately 100% relative humidity than at a lower degree of humidity. Higher rates of survival were also caused by acclimation of active females and larvae for one day at +5° before storage at-15°. No differences in cold-hardiness were found in active females from an organophosphorous resistant strain and a susceptible strain, and the ability to survive depended on the time-temperature exposures (see Fig. 3).
Eggs were killed very rapidly at-15° C, while larvae survived longer exposures than active females. Compared at the LT50 level females in diapause survived three times the exposure endured by acclimated active females.
Supercooling points measured at a rate of cooling of 2° C per minute were highest in diapausing females and lowest in eggs. Acclimation had no effect on the supercooling points of active females. All stages were killed by freezing. It seems likely that the difference in survival between acclimated and not acclimated active females at-15° is caused by different ability to live in a supercooled state.