Laboratory studies were conducted during the winters of 1996 and 1997 to determine the cold tolerance of overwintering adult bean leaf beetles, Cerotoma trifurcata (Förster). Second-generation adults were collected during early September from soybean fields near Ames, IA. The collected beetles were reared at 24°C with a photoperiod of 10:14 (L:D) h in the laboratory for 3 wk. The beetles were transferred to a chamber with high relative humidity at 5°C and photoperiod of 9:15 (L:D) h for evacuation of gut contents dangling. After 2 wk at 5°C, the beetles were selected randomly and subjected to cold baths maintained at temperatures, including −15, −10, −5, 0, and 5°C. At certain time intervals, beetles were retrieved from the cold baths for the observation of mortality. More than 50% of the adult beetles survived over hundreds of hours at −5, 0, and 5°C, whereas most of the beetles died after 15 min at −10 and −15°C. The results indicated that the critical temperature range causing significant mortality of the overwintering beetles would be between −5 and −10°C. Moreover, during the two-winter study the daily leaf litter mean temperature recorded in woodland mainly stayed at above −5°C. The results showed that hibernating in the leaf litter of woodland allowed the beetles to avoid extreme temperature fluctuations during winter.
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