While they are not a pest on every farm, some farms report significant damage from these pests and have to take action to prevent crop loss.They may be pests on snap beans, lima beans, and, more recently, soybeans. Soybeans are a less suitable host for development and survival compared to snap beans, but crop damage can ocur on both crops. Using biological control can reduce the need for insecticides; see below for details.
Mexican bean beetle adults are coppery brown with black spots. They look very much like large ladybeetles and in fact are closely related – but unlike lady beetles they feed on leaves, not other insects. Shortly after adults arrive in a bean field, they lay yellow-orange egg masses on the underside of bean leaves. These hatch into bright yellow, spiny, oval larvae, which feed, molt several times as they grow, and pupate on the underside of leaves. Feeding damage from adults and larvae can reduce yield and injure pods if numbers are high.
Mexican bean beetles overwinter as adults, and seek out beans for feeding and reproduction. Overwintering adults colonize beans in June. There are 2-3 generations per season, usually increasing in numbers with each generation. The complete life cycle takes 30-40 days in the summer months, but closer to 60 days when temperatures are cooler.
Adults and larvae feed primarily on leaves, but when numbers are high they will also damage pods. Feeding occurs on the underside of leaves, but causes death of leaf tissue in a lace-like pattern. Feeding damage over 10-20% can cause yield loss. Plants are most sensitive in the pod fill stage.
Monitoring & Thresholds:
Search plants, especially the underside of leaves, for adults, eggs, larvae and feeding damage.
For information on biological control of Mexican Bean Beetle by the parasitic wasp Pediobius foveolatus, please see our article Mexican Bean Beetle, Biological Control.
Crops that are affected by this insect:
--R. Hazzard, A. Brown and A. Cavanagh , University of Massachusetts Vegetable Program