Mexican Bean Beetle - Vegetables | University of Maryland Extension

Eggs, larva, and adult Mexican bean beetle

Updated: February 20, 2023

Appearance

Eggs: Bright yellow, laid on-end in clusters of 40-60 on lower leaf surfaces.
Larvae: Yellow, oval shaped, covered with small black spines (they appear fuzzy), soft-bodied; 4 instars.
Pupae: Yellow-orange, similar to larvae but smooth and lighter in color, with spiny larval skin pushed down to point of attachment to plant.
Adults: Beetles with convex oval form like that of ladybugs, 1/3" long, orange to copper colored with 16 black spots arranged in three rows of 6-6-4 on the back.

Adults are the overwintering stage and may aggregate under plant debris, in open fields, or in wooded areas.
In the spring, adults emerge and lay eggs on the undersides of leaves.
Larvae hatch out within two weeks and join adults feeding on foliage.
Slow-moving adults will drop to lower leaves or the ground when disturbed and can excrete blood from leg joints as a defense.
Adults are strong flyers.
One to three generations a year.

Legumes, especially lima and snap beans. Also cowpea and soybean. Less preferred are fava bean, lentil, and mung bean.

Mexican bean beetle (MBB) larvae and adults feed on the underside of leaves between the veins, removing the lower epidermis of the leaf. The upper epidermis dies, producing a transparent, lacy look. Damaged tissue falls out, and skeletonized leaves may curl and fall off.
Larvae are particularly damaging to leaves.
Adults may also feed on blossoms, pods, and stems. Bean plants can tolerate up to 20% defoliation before yield is reduced.

Cover entire rows of beans with row cover after planting. You can leave the cover on until harvest.
Crush egg clusters, larvae, and adults by hand.
Purchase and release the beneficial wasp Pediobius faveolatus when larvae are first observed. Most practical in large plantings or community gardens.
For severe infestations, use insecticides such as spinosad, neem, or pyrethrum.
Badly infested plants should be stuffed into plastic bags and sealed. One week later you can compost the plants and dead beetles.
Pull up and bag bean plants after harvest. Adults may fly away, but this will kill any eggs, larvae, and pupae on the plants.
To prevent damage in gardens with high MBB populations: plant a small area with bean seeds in early spring. Overwintered adults will find and feed on these plants. When fully infested, pull plants up and stuff into plastic trash bags to destroy the pests.