Physiology of Cold Hardiness, Seasonal Fluctuations, and Cryoprotectant Contents in Overwintering Adults of Hypera postica (Coleoptera: Curculionidae)

The alfalfa weevil, Hypera postica (Gyllenhal), is widely distributed in various alfalfa-growing regions in the world. Adults enter the field gradually from hibernation places and overwinter in a nondiapausing state, exhibiting feeding, mating, and oviposition over autumn and winter whenever the weather permits. In this study, the variability of supercooling point (SCP) and cold hardiness in alfalfa weevil was investigated. Adults were collected monthly from alfalfa fields in Karaj, Iran, during autumn, winter, and spring in 2011-2012. Supercooling points and LT50 (exposure at subzero temperatures from -5 to - 13 °C for 24 h and 50% death of the sample) were studied. The average SCP varied from -14.4 ± 0.7 °C in October to - 8.7 ± 0.4 °C in May. Mean SCPs were significantly lower during autumn (-13.4 °C) than winter months (-8.6 °C). The frequency distribution of SCPs in individuals in autumn was bimodal and in winter was unimodal. Insects could survive after exposure to -5 °C in all months, but mortality increased with cold intensity. Percent mortality at -11 °C was ∼89% in November and December and then increased to 100% in January and February. The approximate LT50 values were -5.7 °C and -6 °C in January and February, respectively. Seasonal changes in sorbitol, glycerol, glucose, and trehalose were consistent with a role in freeze protection. However, microhabitat may also play an important role in protecting insects from cold weather and allowing them to tolerate low temperatures and increase their population.