Spatial and Temporal Distribution Patterns of the Bean Leaf Beetle, Cerotoma trifurcata (Forster) 1 , on Soybeans in Illinois 2

Abstract

Intensive surveys of adult bean leaf beetles, Cerotoma trifurcata (Forster), carried out in 1971 and 1972 suggest that the species has 2 complete generations per year in east-central Illinois. Adults of the first generation start emerging in early July until the middle of August, and adults of the 2nd generation emerge starting in early September. The overwintering population is composed mainly of these 2nd generation adults. There is an apparent diel cycle of distribution with catches of adults being lowest near noon and highest about 1700 h. Dispersion of most adult populations approximated a negative binomial distribution. Dispersion was also analyzed using Morisita's index ( I δ), and Taylor's power law. We found k c = 7.33, and Taylor's b = 1.13, both implying a moderately contagious distribution. These 3 measures of dispersion were compared to determine the relative merits and range of applicability of each. Extensive surveys of soybean fields in 15 counties in Illinois indicated the bean leaf beetle is more abundant south of the 40° N parallel. Multiple regression analysis was used to assess the influence of chlorinated hydrocarbon residues in the soil and texture of the soil on beetle abundance. Pesticide residue levels in the soil can possibly explain the relative abundance of beetles in localized areas, but not the general patterns of distribution along north-south transects. Other latitude-related factors are probably more important in determining these patterns. Bean leaf beetle abundance was inversely correlated with clay content in the soil.