Bean Leaf Beetle Plant Damage and Control

Bean leaf beetles (Cerotoma trifurcata) range in size from 1/5 to 1/4 of an inch and may be red, tan, green, tan, or brown. They are also variously marked, frequently by a black border around the margin of wing covers, and by two pairs of spots along the inner wing margin. In some individuals, the spots are missing, in others the spots are run together and resemble splotches.1

To the bean leaf beetle, "bean" means just about any legume: soybeans, cowpeas, green beans, and leguminous clovers and alfalfa. Sometimes they even feed on nonlegumes such as nettle and wild Euonymous. But they lay no eggs in association with any of these plants, waiting until legume seedling emerge from the soil.2

Adult bean leaf beetles leave their winter quarters in April. Female bean leaf beetles bury their eggs in the soil at the base of a bean plant. These eggs, the first generation of the season, mature to become the parents of a secon generation, which in the fall retreats to overwintering sites. The larvae live in the soil at the base of a plant and feed on the roots. Adults beetles feed on the underside of the leaves chewing round holes.

Leaf damage caused by bean leaf beetles has been associated with several fungal deseases such as Alternaria leaf spot and pod necrosis. Alternaria atrans is a weak parasite that infects leaves through wounds made by a mechanical injury such as puncture. Infection occasionally occurs on young plants, but Alternaria leaf spot is generally a disease of plants nearing maturity.5



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Bean leaf beetles also transmit a number of plant viruses including cowpea mozaic virus (CPMV) and cowpea chlorotic mottle virus (CCMV). Leaves infected with CPMV have pale green/yellow and necrotic lesions, while those infected with CCMV are mottled, slightly crinkled, and tend to stand upright. Mottling is most intense on the diseased younger leaves. Soybean plants are stunted and produce fewer seeds which are small in size and reduced in quality.5

Control

Interplanting potato plants with bean plants may help keep the beetles away. Plant marigolds, summer savory, rosemary, radishes, and garlic for bean protection. Garlic spray will repel them, and, as a last resort, pyrethrum, neem, or rotenone will kill them. Be sure to spray on the underside of the leaves where adult beetles feed. Clean all debris from the garden at the end of the season to eliminate habitat for the overwintering beetles. Handpicking the adults, larvae, and even orange-yellow eggs will help reduce the population. Natural predators include praying matises, ladybugs, and spined soldier bugs. The nymphs of spined soldier bugs eat hatching bean beetles, and the adults move over to eat cabbageworms.3

Picture of bean leaf beetle

References

  1. Ninety-nine Gnats, Nits, and Nibblers by May R. Berenbaum
  2. Insights From Insects: What Bad Bugs Can Teach Us. Gilbert Waldbauer
  3. Tiny game hunting: environmentally healthy ways to trap and kill the pests ... By Hilary Dole Klein, Adrian M. Wenner
  4. Ortho home gardener's problem solver By Denny Schrock
  5. Field crop diseases By Robert F. Nyvall



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