Abstract
To evaluate the cold-hardiness of the rice water weevil (RWW), Lissorhoptrus oryzophilus, supercooling points were measured, and RWW's mortality after continuous and intermittent exposure to moderately low temperatures was investigated using field-collected overwintering adults. RWW was freezing intolerant. The mean supercooling points in dry and wet conditions were −15.2°C and −10.5°C, respectively. One-time exposure of RWW to −10°C for 3 h made no difference in the supercooling points. The effect of repeated exposure to −10°C for 3 h differed greatly between dry and wet conditions. The mortality rate was 30.5% after 12 treatments under dry conditions, yet it reached 100% after 7 treatments under wet conditions. In wet conditions, the mortality of adults exposed continuously to −3°C or −5°C for 10 to 120 days was at most 7.5%, while exposure to −10°C for 20 days resulted in 100% mortality. These results reveal that RWW adults suffer much higher mortality in wet conditions than in dry conditions and that overwintering would be difficult for RWW in wet conditions if they were exposed repeatedly or continuously for a long period to −10°C. Improvement in the forecasting of RWW occurrence is discussed on the basis of the results obtained.
