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Bean Leaf Beetle

Cerotoma trifurcata

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Multiple generations of bean leaf beetles may attack snap beans and other legumes in the home garden.
Bean leaf beetle.

Bean leaf beetle.

© Herman H. Giethoorn, Used with permission. www.flickr.com/photos/hgiethoorn/

Description: Adult bean leaf beetles come in several colors, from yellow-green to red, and their markings can vary as well. Regardless, all bean leaf beetles have a characteristic black triangle mark at the front of the elytra, just behind the pronotum. Generally only the adult beetles will be visible, as all other forms live in the soil. The eggs are oval and orange-red in color. Larvae are white with black ends. The pupae are ghostly white copies of the adults.

Life cycle: Adult bean leaf beetles overwinter in leaf litter or in the soil, usually preferring wooded areas for shelter. As soon as temperatures begin to warm up in the spring, the first adults emerge to feed and mate. Females lay about a dozen eggs at a time in the soil under legume hosts. After several weeks of feeding on the roots, larvae pupate in the soil. Adults emerge to repeat the cycle. In southern areas, bean leaf beetles can produce multiple generations within a growing season.

Crops damaged: Snap beans, soybeans, and other legumes. Adults feed on both the foliage and pods, while larvae feed on the roots.

Signs and symptoms: Round holes in foliage, within the leaf margins. Stunted plant growth due to larvae feeding on roots. Cosmetic damage to pods late in season.

Control measures:

  • If you notice feeding damage, hand pick adult beetles and drop them in soapy water to destroy them. Beetles are most active in the afternoon, so monitor your plants at this time.
  • Young seedlings are most susceptible to bean leaf beetle damage. Be vigilant when plants are young.
  • If you've had problems with bean leaf beetles in past growing years, consider planting snap beans later in the season to avoid early emerging adults in spring.

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