Cold tolerance of overwintering bean leaf beetles (Coleoptera : Chrysomelidae)

Laboratory studies were conducted during the winters of 1996 and 1997 to de termine the cold tolerance of overwintering adult bean leaf beetles, Ceroto ma trifurcata (Forster). Second-generation adults were collected during ear ly September from soybean fields near Ames, IA. The collected beetles were reared at eec with a photoperiod of 10:14 (L:D) h in the laboratory for 3 w k. The beetles were transferred to a chamber with high relative humidity at 5 degrees C and photoperiod of 9:15 (L:D) h for evacuation of gut contents dangling. After 2 wk at 5 degrees C, the beetles were selected randomly an d subjected to cold baths maintained at temperatures, including -15, -10, - 5, 0, and 5 degrees C. At certain time intervals, beetles were retrieved fr om the cold baths for the observation of mortality. More than 50% of the ad ult beetles survived over hundreds of hours at -5, 0, and 5 degrees C, wher eas most of the beetles died after 15 min at -10 and -15 degrees C. The res ults indicated that the critical temperature range causing significant mort ality of the overwintering beetles would be between -5 and -10 degrees C. M oreover, during the two-winter study the daily leaf litter mean temperature recorded in woodland mainly stayed at above -5 degrees C. The results show ed that hibernating in the leaf litter of woodland allowed the beetles to a void extreme temperature fluctuations during winter.