Winter survival and population dynamics of bean leaf beetle (Coleoptera: Chrysomelidae)

Lam, Wai-Ki
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Ecological study of the population dynamics of bean leaf beetle adults, Cerotoma trifurcata (Forster) (Coleoptera: Chrysomelidae), with special reference to the overwintering populations, was conducted in central Iowa. Ecological habits, spatial distribution, cold tolerance, and winter survival of overwintering beetle adults, were studied for 3-yr (1996 through 1999). Additionally, the population fluctuation of the adult beetles during the growing season was studied for 10-yr, from 1989 through 1998, inclusively. Overwintering beetle adults mostly hibernated in the crop residue of soybean fields ( ≈ 20%) and leaf litter of woodlands ( ≈ 80%). A few beetles (< 1%) overwintered in alfalfa fields, cornfields, and grasslands. The overwintering adults aggregated in the litter of soybean fields and woodlands. Residue in the habitats served as an insulating layer for the overwintering beetles. However, over 50% mortality of the overwintering beetles was observed in both soybean fields and woodlands during the 3-yr study. Laboratory study indicated that most of the overwintering beetles survived more than hundreds of hours when they were kept at --5°C or above, whereas most of the beetles died when they were kept at --10°C or below. This result indicated that cold temperature has a strong effect on the winter survival of the overwintering populations. A model for the estimation of the mortality of overwintering beetle adults, based on the accumulation of the air mean subfreezing winter temperature, was developed. Linear regression and multiple regression stepwise selection procedure analyses were used to relate environmental factors to population fluctuations of bean leaf beetle during the 10-yr study. The analyses showed a strong relationship between the environmental factors, including temperature and precipitation, and beetle population densities. A predictive model for F2 adults in the next late season was developed. The major factors affecting the survival of overwintering populations and population fluctuations of the beetles, estimation of winter survival, and the prediction of the population dynamics of bean leaf beetle during seasonal cycles are discussed.