Mexican bean beetles (Epilachna varivestis) are part of the ladybird beetle family but unlike most of their counterparts (who are beneficial insects) are notorious agricultural pests and feed on plants rather than insects. They are found throughout Mexico and the eastern United States, and are more prominent in wetter areas. They do not tolerate extremely dry areas.
Adult Mexican bean beetles are oval-shaped, yellowish brown to copper-colored, 1/4 inch long with 16 black spots on their wing covers. Since they look very much like lady beetles, distinguish them from the lady beetles by looking out for their larvae.
Larvae are fat, yellowish orange, 1/3 inch long with no legs and rows of long, branching spines protruding from their body.
The Mexican bean beetles’ eggs are small, yellow and oval and are laid on the underside of leaves.
Adults overwinter in the soil and emerge in spring. The females lay their eggs on host plants and the eggs hatch in 5 to 14 days. The larvae feed for 2 to 5 weeks, pupate and emerge about a week later as young yellowish-colored adults that soon mature. 1 to 3 generations per year.
Cow-pea, lima bean, snap bean, green bean, string bean, bush and pole beans, black-eyed pea, mung, adsuki, velvet bean, alfalfa, clover and soybean can all get affected. Both the adults and larvae predominantly feed on the tender parenchyma on the underside of leaves giving the leaves a skeletonized appearance. Adult beetles may also eat the fruits and flowers of the plant. Larvae tend to do more damage than adults. If left unchecked, Mexican bean beetles can completely defoliate plants, thereby severely ruining yields.
Organic Control and Prevention Methods:
Handpicking: For small plantations, handpicking adults and larvaeis a viable option. Also seek and destroy eggs and pupae. If done regularly, damage can be considerably controlled.
Use Crop Covers: Floating row covers with edges buried in the soil is one of the easiest ways to prevent Mexican been beetles from infesting your plants. Use the covers when you plant the seedlings after weeding and check weekly to ensure no lucky individuals are prospering beneath.
Cultural Control: Cultural control includes planting early-season bean varieties that can help avoid main beetle generations, tilling, removing debris, digging up and getting rid of crop residues of previous seasons to remove overwintering sites.
Trap Crops: Planting soybeans as a trap crop is a common practice. Destroy the plants that are infested.
Biological Control: Beneficial insects, such as ladybugs, lacewings, eulophid wasps and minute pirate bugs are avid predators of both the eggs and young larvae. Toads, birds and spined soldier bugs (Podisus maculiventris) are some other general predators. The commercially-reared parasitic wasps Pediobius foveolatus are a proven biological control method of Mexican bean beetles and is highly recommended.
Companion Planting: Interspersing bean plants with companion plants allows one to take advantage of natural components to either attract beneficial insects or repel pests. Marigold, Nasturtium, Petunias and Rosemary help deter Mexican bean beetles. You can attract native predators and parasites by allowing a few nectar-rich flowering plants such as daisies, sweet alyssum or yarrow grow nearby. Another common approach is planting a row of beans in between a row of potatoes.
Organic Insecticides: Neem oil has been reported to deter adult bean beetles and offset the feeding of larvae but may also harm some beneficial insects too. Organic insecticidal soaps also help control beetle populations and must be applied thoroughly, especially on the undersides. Beauveria bassiana is a fungus that acts as a parasite on various bugs including the Mexican bean beetle. Use a product based on this fungus as soon as you see an infestation.
Other Methods: Spreading diatomaceous earth on the surface next to the plant and on the plant is not too pleasing to beetles and may deter them from infesting your plants. Chickens are known to feed on all sorts of insects including beetles. Let them loose near your bean plants only once the bean plants are tall. Aluminum foil mulch works by reflecting sun rays which discourages flying pests from landing on host plants. Ask your local nursery for varieties of beans that are naturally resistant to the Mexican bean beetles in your area.
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