What's Bugging You? Mexican bean beetle


COMMON NAME: Mexican bean beetle

SCIENTIFIC NAME: Epilachna varivestis

Adults - These small (1/4 to 1/3 inch), roundish beetles look very much like their "good-guy" cousins, the ladybugs. The bean beetle is pale-yellow to copper-brown and will have 16 black spots on the wing covers, arranged in three rows. The ladybug will often have varying numbers of spots and more markings on the thorax (the small area between the wing covers and the head). The Mexican bean beetle will not have thoracic markings.

Pupae - The pupae will look like a slightly-rounded larvae with all the "spines" pushed down to one end. Larvae - The larvae are elongated-oval in shape, 1/3 to 1/2 inch in length and yellow-orange in color. They are covered with numerous black tipped, branched spines.

Eggs - The longish yellow eggs are laid in clusters (usually 30 - 50) on the underside of a leaf on the host plant.

LIFE CYCLE: The adults overwinter in weeds or plant litter. In spring they emerge and feed on foliage for a couple of weeks before laying eggs on the host plants. The eggs hatch in 1 - 2 weeks and the larvae feed voraciously and grow to their full size. When fully mature, they attach their back end to a leaf and pupate. In 7 - 10 days the new adult will emerge and gradually darken and develop the tell-tale spots on its back. There can be 3 or 4 generations of this pest per year in the warmer parts of the country.

HOST PLANTS: All kinds of beans, cowpeas, and reportedly even squash.

TIME OF YEAR: Late spring to late fall (approx. April to Oct.). My personal experience is to have the most problems with them in July, August, and September.

WHAT TO LOOK FOR: If you walk out to your garden one morning and find that your bean plants have turned into vegetative "lace" you can be pretty certain that the Mexican bean beetle is at work. The adults and larvae eat away the leaf, leaving only the veins. Looking closely, especially on the underside of the leaves, you can easily see egg clusters, larvae, pupae and adult beetles.

PROBLEMS AND DAMAGE: A severe infestation of this pest can totally defoliate your entire crop in a day or two. What plants that don't die will probably be weakened, produce a reduced crop, and be more susceptible to other pests and disease. In addition to skeletonizing the foliage, the beetles will feed on the stems and pods too.

CULTURAL CONTROLS: Practice crop rotation (3-5 year cycles) and plant resistant varieties when available. Clean up garden debris to destroy overwintering habitat. Immediately compost the debris or put it into sealed plastic bags and let it "cook" in the sun for a week or so to destroy any remaining pests. Use "timed planting", early crops tend to avoid the main outbreaks of the beetle. If damage is noted, remove the leaf or carefully cut away the damaged area. This way, if you notice more damage later, you will know that you still have the pest present rather than thinking that it is the same damage from before. Plant several smaller blocks of beans in varying locations rather than one large planting. The beetle may get to one or more areas but other areas might escape.

COMPANION PLANTING AND REPELLENTS: Interplant your beans with potatoes, garlic, savory, or nasturtiums. Not only does the potato repel the Mexican bean beetle, but the beans return the favor by repelling the Colorado potato bug. Other reported repellents include strong smelling marigolds, rosemary, and petunias. A repellent spray can be made by steeping cedar chips in hot water.

TRAP PLANTS: Bean beetles seem to be particularly fond of lima beans. A small planting of these, away from your other legumes, can possibly draw the pest away from your main crop and also act as an "early warning system".

MECHANICAL CONTROLS: Early in the season cover your plants with an agricultural fleece (Remay, etc.), fine netting, or cheesecloth to exclude the pest and keep it from laying eggs in your garden. Remove the covering when the temperature gets too high or for pollination. Handpicking of the adults, pupae, and larvae is very effective. Drop them into a container of water topped with a layer of kerosene or oil. Squash any egg clusters you find.

NATURAL CONTROLS: Put a birdbath in your garden and invite all your neighborhood feathered friends to dine. Bean beetles are also favorite prey for various assassin bugs, spined soldier beetles, and ladybugs. They can be parasitized by a commercially available wasp, Pediobius foveolatus.

BIOLOGICAL INSECTICIDES: These are not totally harmless, but used correctly, they can be easier on the environment than some other methods. Use pyrethrum at the first sign of the pest and spray weekly thereafter. As a last resort, you can use rotenone. Be sure to apply the pesticide to the undersides of the leaves and the interiors of the bushes. Spray in the evening, when the honeybees are not around, and follow directions carefully.

CHEMICAL CONTROLS: Call your County Extension Agent for current recommendations. Check at your favorite garden supply store or nursery to see what they have available. Remember to check the label to make sure that the Mexican bean beetle is a listed target pest, and FOLLOW THE DIRECTIONS EXACTLY! When it comes to pesticides, more is NOT better! Wear protective clothing, watch out for non-target plants, pets, children, and other living things. Wash your skin and clothing after application and take care not to get the substance into your eyes, mouth, or breathing passages.

T.J. Martin
July, 1990