Bean plants are a favorite food of certain caterpillars. These pests eat the leaves and can, in severe infestations, defoliate the bean plants. Bean-eating caterpillars are larvae of butterflies, moths or beetles. Two types of bean-eating caterpillars can be identified by their black or dark-brown heads and greenish-yellow bodies. Both types are the larvae of a butterfly belonging to the skipper family.
The caterpillar of the long-tailed skipper butterfly (Urbanus proteus) hatches in late summer. It starts out tiny, about a quarter-inch long, growing to more than 1 inch as it feeds on bean leaves. Its black head appears disproportionately large in relation to its striped, greenish-yellow body. The head has two orange spots directly behind the mandibles. This caterpillar cuts flaps from the edges of bean leaves, rolling them over and sticking the flaps down with silk to form a sheltering tent. This habit gives this pest the common name of bean leafroller. In its late life stages, the caterpillar sticks two leaves together to form a shelter where it will pupate.
The long-tailed skipper inhabits the southeastern United States. The adult is a brown butterfly with five to seven white spots on the top side of each forewing. It has a wingspan of about two inches. Each hind wing has a tail-like extension trailing backward, which gives the species its common name. The caterpillars feed for about three weeks and pupate for about 10 days before emerging as adults. The adults feed on nectar until autumn sends them migrating south to Florida, Mexico and Central America where they spend the winter before returning north to lay eggs and start the life cycle over again.
Silver Spotted Skipper
Another dark-headed caterpillar that feeds on bean plants is the larva of the silver-spotted skipper butterfly (Epargyreus clarus). The caterpillars have plump greenish-yellow bodies with faint transverse stripes, dark brown heads that look almost black, and yellow eye patches near the mandibles. These caterpillars also roll leaves to make tents. When the caterpillar is small it only uses the edges of leaves, but when it is bigger -- the caterpillar can grow to 2 inches -- it uses an entire leaf for its tent. The caterpillars are also feed on the foliage, including a variety of cultivated and wild plants in the bean and pea family.
The silver-spotted skipper ranges through states along the East, West and Gulf Coasts, the Great Lakes and the Great Plains. The adult butterflies are brown with a two-inch average wingspans. The forewings have a row of yellow-gold spots and dashes of white on the fringe. The hindwings have a large irregularly-shaped silvery-white patch and a rounded edge. The life cycle varies with the climate. In areas with mild or no winters, this species will go through multiple generations. In areas with severe winters, there may be only one generation until the insect develops a hardened pupa for spending the winter.
Bean leafrollers can be controlled by hand-picking or with insecticides. For minor infestations, inspect your bean plants frequently. Pick off and destroy any caterpillars you see and crush their leaf tents. They won't bite or sting you. Also look for orange or cream-colored spherical eggs laid on the underside of the leaves and crush them. If yours is a large bean patch or you have a major infestation, you can spray or dust with a biological or chemical insecticide. Follow the application directions and cautions of the product you choose. When using any insecticide you should minimize direct contact, wash hands and change clothes after application. You should thoroughly wash treated produce before eating. And if you have a child in the house, keep insecticides under lock and key.