Alfalfa weevil, Hypera postica Gyllenhal, is a key pest of alfalfa and widely distributed in alfalfa fields of Iran. In early autumn, the first adults of H. postica arrive in the fields where they steadily increase their populations during autumn and following winter. To evaluate the impact environmental factors including temperature on the survival of the weevils, this study was conducted to measure the levels of variability in supercooling point and cold hardiness in the weevils. Overwintering adults were collected monthly from alfalfa fields on the outskirts of Karaj during the autumn 2011and winter 2012. The supercooling points and survivorship were studied at sub-zero temperatures from -5 to -13 ºC for 210 minutes. The average supercooling point ranged from -14.4 ± 0.68 in October to -8.8 ± 0.80 in March. Mean supercooling points for H. postica adults were significantly lower in autumn (-13.4 ºC) comparing winter (-8.6 ºC). Insects survived after being exposed to -5 ºC in all months. The mortality of the adults increased with cold intensity. The percent mortality for adults treated at -11 ºC was approximately 89% in November and December and reached 100% in January and February. The range of LT50 values varied from -5.7 ºC (highest) to -8.6 ºC (lowest). Results showed that adults could not withstand the temperatures below the supercooling point. It is found that the cold intensity around supercooling point could cause 80% mortality. It can be concluded that as the ambient temperature in the field approaches the supercooling point, it tends to be detrimental on H. postica population
