Adult Mexican bean beetles are oval-shaped, yellowish-brown, 1/4-inch beetles with 16 black spots on wing covers (They are related to lady beetles). Their larvae are fat, yellowish orange, 5/16-inch grubs with long, branching spines. Mexican bean beetle eggs are yellow ovals laid on end on the undersides of leaves. This insect pest is found in most states east of the Mississippi River; also Texas, Arizona, Utah, Colorado, and Nebraska.
Both larvae and adults skeletonize leaves of cowpeas, lima beans, snap beans, and soybeans. They feed from the undersides of leaves, leaving characteristic lacy damage; severely defoliated plants may be killed. Beetles are most abundant in weedless fields.
Adults overwinter in leaf litter in nearby fields; in spring, females lay eggs on beans. Eggs hatch in 5 to 14 days, larvae feed 2 to 4 weeks, then pupate on leaves. One to three generations per year.
Plant early-season bush beans to avoid main beetle generations; plant soybeans as trap crops, destroy them when infested with larvae; handpick larvae and adults daily in small bean patches; scout for and destroy eggs; cover plants with floating row cover until plants are large enough to withstand damage; attract native predators and parasites by leaving flowering weeds between rows or by interplanting flowers and herbs; dig in crop residues as soon as plants are harvested; release spined soldier bugs (Podisus maculiventris) to control early generation; release parasitic wasps Pediobius foveolatus as soon as possible after egg clusters appear on your plants; apply insecticidal soap, especially to leaf undersides; as a last resort, spray with neem.
Photo: Clemson University, USDA Cooperative Extension Slide Series, Bugwood.org