Cucumber Beetles

Cucumber beetles are frequently abundant in bean fields, but damage is usually insignificant. The western spotted cucumber beetle is yellowish-green, 0.25 inch (6 mm) long, and has eleven black spots on its wing covers. The whitish wormlike larvae feed on roots of corn, legumes, and grasses. When the surrounding areas dry up, the adults will swarm into the bean field.

A close relative, the western striped cucumber beetle is yellowish and has three black lines down its back. The larvae live in the soil where they feed on roots, while the adults are foliage and flower feeders. For further assistance with identification, see A Field Key to the Cucumber Beetle Species Found in California.

Although western cucumber beetles feed on all ages of dry bean plants, they cause the most damage to young plants, resulting in seedling stand losses. On mature plants, they tend to feed primarily on the older foliage and therefore seldom cause economic damage to established dry beans.

Specific treatment thresholds have not been established for cucumber beetles. Start inspecting plants for cucumber beetle damage when the crop emerges. Continue monitoring through the vegetative growth and flower bud to bloom stage. If there are damaging numbers during the early-season with excessive foliage loss, an insecticide application may be warranted to prevent crop loss. Mid- to late season infestations are not associated with crop loss and do not require insecticide application.