Mexican Bean Beetle
The Mexican bean beetle gets into beans all over the country, with exception of the Pacific states. They attack beans, peas, clover, and alfalfa.
The adult mexican beetle overwinters in hedgerows, ditchbanks, and woodlands. The beetle is 1/3 of an inch long and mustard brown to copper in color with black spots. It lays clusters of 40 to 60 lemon yellow eggs on the undersides of bean leaves. If you see single eggs that are pale orange and opaque, leave them alone. This egg belongs to the Mexican beetles nicer cousin, the aphid eating Ladybug.
The Mexican beetle larvae are yellow and covered with dark spines. They do not favor caterpillars. They are more oval with the spines growing out of the body like whiskers. They hide on the undersides of leaves and when feeding, leave huge holes that really stand out taking on a lace-like appearance.
Handpicking and soapy water are the best methods of control. Usually once a week will keep the population under control. Since the larvae are very slow, it takes awhile for the damage to actually become a problem to the plant and the harvest. Adults are also slow moving and easy to pick off. Also, plant early maturing plants that will miss the warmer beetle season in later summer months. If you must use chemicals, use foliar sprays on the underside of leaves and granular types in the soil at time of planting.
Bean Leaf Beetle
The bean leaf beetle can affect plants in the same areas as the Mexican beetle, however, the most prominent areas of destruction are Indiana, Michigan, and Wisconsin.
The beetle is 1/4 of an inch long, reddish or yellow, and fly like crazy on warm days. Some have spots and some don't, but all have a black triangle behind their heads where their wings come together. The beetle will lay eggs in the soil beneath your plants. Those eggs hatch into slender white larvae up to 1/3 of an inch long. The larvae feed on plant roots.
If you notice weak plant that seems chronically thirsty, pull up the plant to see if larvae are feeding on it's roots. Keep as much soil on the root ball as you can and plunge it into a bucket of water. Within seconds the larvae float to the top. You can't save the plant but at least you know you have a problem. If you'd had a larvae problem in the past, treat the soil with beneficial nematodes. They will destroy bean beetle larvae as well as cutworms if they can find them. Nematodes only work in warm weather however. To apply, mix nematodes in water and spray the solution on the soil where the pests are feeding in early evening. Then water the plant a little so they get down in the soil.
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