Given its size and spots, you might mistake this pest for a ladybug. Though related to ladybugs, Mexican bean beetles are far from beneficial. Their distinctive bronze background color gives away their identity. Mexican bean beetles are fairly common in the United States, but are most prominent in the southeast and rarest in the northwest. Gardeners east of the Rockies are most familiar with this pest.
The adult beetle features sixteen black spots on its back. Larvae are fat, hump-backed spiny yellow grubs about 1/3 inch long. Both adults and larvae feed on foliage, leaving a skeleton of veins.
Adults overwinter on plant debris, emerging in late spring or early summer to lay clusters of yellow eggs on the undersides of leaves. There are one to four generations per year depending on the climate.
Use floating row covers over seedlings to prevent egg laying. Check leaf undersides for masses of yellowish eggs, and squish any you spot. Hand-pick adult beetles and larvae. Neem oil will deter feeding adults; horticultural oil and insecticidal soap are useful against the larvae. Pedio wasps (Pediobius faveolatus) parasitize adult bean beetles. Toads, birds, and spined soldier bugs are general predators. Clean up plant debris in the garden at the end of the season to reduce the number of overwintering adults. Where these beetles are a severe problem, look for varieties of beans that are naturally resistant.
Photography by Whitney Cranshaw, Colorado State University, Bugwood.org