Life Cycle of the Bean Leaf Beetle, Cerotoma trifurcata (Coleoptera: Chrysomelidae), in Southern Minnesota

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Abstract
In southern Minnesota, bean leaf beetles, Cerotoma trifurcata (Forster), were first collected from emergence cages and alfalfa in mid-May. Beetle emergence peaked both years between late May and early June, which corresponded with soybean planting. Two seasonal peaks of bean leaf beetle adult abundance on soybeans were recorded. The first peak corresponded to overwintered, colonizing adults and the second to first-generation adults. Teneral adults of the first generation appeared in mid-July during soybean bloom. First-generation adult abundance peaked in late August when soybeans were beyond pod set. Beetles were present in the field until late September, when soybeans were at harvest maturity. Female beetles from both the colonizing and first generations were collected on soybeans or alfalfa and dissected. Only colonizing adults were mated and showed ovarian development. High fat content was found in early colonizing adults and first-generation adults, but fat content decreased during the period of ovarian development of colonizing females. The bean leaf beetle has a single generation in southern Minnesota and mating and ovarian development occur after emergence from overwintering sites.

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