Wai-Ki F. Lam

A Predictive Model for the Survival of Overwintering Bean Leaf Beetles (Coleoptera: Chrysomelidae)

Abstract Survival of overwintering bean leaf beetle, Cerotoma trifurcata (Förster), populations in woodland and soybean habitats was studied for three consecutive winters from 1996 through 1999. Four locations of soybean fields and three locations of woodlands were studied at Iowa State University farms near Ames, IA. In the winter of 1996–1997, woodlands and soybean fields were sampled monthly from mid-November through mid-April of the following year, whereas in the winters of 1997–1998 and 1998–1999, both habitats were sampled twice a month from mid-October through mid-April. On each sampling date, three sampling units (1 m2) of leaf litter or crop residue was randomly collected from soybean fields and woodlands at each location. The collected samples were bagged and returned to the laboratory for estimates of overwintering beetles. The beetles obtained from the samples were kept in individual sample bags at 24°C for 100 h to assess their survival. The beetles were then kept in a freezer at −15°C for investigation of potential fungal pathogens and ectoparasitic mites. During the three-winter study, the mortality of overwintering beetles in soybean fields (77.04–88.89%) was higher than those in woodlands (48.9–82.29%). The effects of winter temperature on beetle mortality and a predictive model for the overwintering beetle survivorship are discussed.

Population Dynamics of Bean Leaf Beetles (Coleoptera: Chrysomelidae) in Central Iowa

Abstract Linear regression and multiple regression stepwise selection procedure analyses were used to relate population fluctuations of bean leaf beetle, Cerotoma trifurcata (Förster), in soybean, Glycine max (L.) Merrill, to climatic factors. The study was conducted at Iowa State University Johnson farm near Ames, Iowa for 10 consecutive years (1989 through 1998, inclusively). Bean leaf beetle adults were sampled twice weekly from emergence to mature stages of soybean. Linear regression showed a strong relationship between first- and second-generation adult densities during the growing season, whereas multiple regression analyses indicated a high correlation between second-generation adult densities and environmental factors, including temperature and precipitation. The analyses demonstrated that field history of beetle density is important for pest management. The major factors that affected the population dynamics of bean leaf beetle and regression models for the beetle density prediction are discussed.

Effect of Trichome Density on Soybean Pod Feeding by Adult Bean Leaf Beetles (Coleoptera: Chrysomelidae)

Abstract The role of soybean, Glycine max (L.) Merrill, pod trichomes on feeding by adult bean leaf beetles, Cerotoma trifurcata (Förster), was evaluated under laboratory and field conditions during 1997 and 1998. Three Clark isolines and ‘Corsoy 79’ were used to compare the feeding preference of the beetle on pods with different trichome densities. The three Clark isolines, including densely pubescent, sparsely pubescent, and glabrous, were isogenic except for trichome density. The trichome densities on the pods of Clark densely pubescent, Clark sparsely pubescent, and Corsoy 79 were significantly different. In no-choice tests, under both laboratory and field conditions, the feeding on pods of Clark densely pubescent was significantly lower than that on the other isolines. In the choice test of Corsoy 79 conducted in the laboratory and field, when the trichomes on one of the two pods were shaven off, the feeding on the shaven pods was significantly higher than that of the intact ones. In the choice test among Clark isolines under laboratory condition, the result was significant, with the lowest feeding on the densely pubescent pods. This study demonstrates that densely pubescent soybean has the potential to resist bean leaf beetle feeding on pods.

Cold Tolerance of Overwintering Bean Leaf Beetles (Coleoptera: Chrysomelidae)

Abstract 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.