Mexican bean beetle adults and larvae feed on the undersides of leaves of several plants, including garden beans, cowpeas and soybeans, leaving the leaves skeletonized with a lace-like appearance. Although feeding occurs mainly on the leaves, young pods and even the stems may be attacked and destroyed. Severely affected leaves often dry up and drop off the plant, reducing photosynthetic efficiency and yield. Serious infestations may result in the death of young plants.

Description:

The Mexican bean beetle is one of the few harmful members of the lady beetle family. The copper-colored adults resemble large lady beetles. They are about 6 mm (1/4 inch) long and 5 mm (1/5 inch) wide, with 8 small black spots on each wing. Immature larvae are yellow and are covered with large spines. Yellow eggs 1 mm (1/20 inch) in length are laid in groups of 40-60 on the lower leaf surfaces.

Mexican bean beetle adults defoliating leaf (R. A. Casagrande)	Mexican bean beetle larvae (R. A. Casagrande)

Life Cycle:

Adult beetles overwinter on the ground in leaf litter and other sheltered areas in fence rows and field edges. Adults begin emerging from these protected areas when beans begin sprouting and continue to emerge for up to two months. The adults feed for approximately two weeks before depositing their eggs on the underside of leaves. Females may deposit an egg-mass every two to three days. Eggs hatch in 5-24 days and larvae feed for two to five weeks before pupation. The greatest amount of injury occurs in July and August, and the adults begin to disappear in late summer.

Control:

- Plant early and fast-maturing varieties to reduce damage and crop loss.

- Plant bush beans instead of pole beans and pick pods as soon as they mature.

- Plants should be plowed under as soon as the crop is harvested to eliminate late-developing beetles and to prevent a within-field overwintering site.

- "Floating-row" covers such as Remay work well as protective barriers.

- If necessary, plants may be protected by insecticides applied according to label directions.

By Richard A. Casagrande and David B. Wallace, 1999