Be on the lookout for bean leaf beetles

If you’ve got beans growing in the garden, now is the time to be on the lookout for the bean leaf beetle (Cerotoma trifurcata).  This small beetle (~1/4 inch) is one of the more important soybean crop pests in the United States, but it can prove to be a real headache in the home garden.  The bean leaf beetle can vary in color, from a showy red with black markings to a drab tan with little to no markings.  They are quite wary and will often drop to the ground when disturbed and remain motionless or simply fly away.  Adults feed on bean foliage, especially the younger leaves, creating circular feeding holes.

8015Bean Leaf beetle damage.    Photo by David Rhodes.

Large numbers of beetles can easily defoliate plants and reduce plant vigor and yield.  They will also feed on the developing beans, creating entry points for fungal diseases.  These beetles are also a vector of bean pod mottle virus, which can stunt plants and cause a mosaic pattern on leaves.  Additionally, developing bean leaf beetle larvae will feed on plant roots.  As with several garden pests, the adult beetles overwinter in nearby weeds and soil debris, emerging in spring to feed and lay eggs.  The spring eggs hatch relatively quickly and by July, perhaps when your beans are doing really well in the garden, new adults emerge to start the cycle all over again.  It is important to control these mid to late-summer adults as they are the ones that will be laying additional eggs, overwintering, and emerging in the spring.  A few beetles in your garden this season can easily become too many in the garden next season.  Eliminating overwintering sites (the weeds!) can help to control next season’s population.

ImageThe bean leaf beetle can have variable patterns and colors, but they will always have the black triangle on the upper part of the abdomen.

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