Mexican bean beetles are especially problematic in the rainy areas of the East, and tend to pose fewer problems in dry climates.
Illustration By Keith Ward
This article is part of our Organic Pest Control Series, which includes articles on attracting beneficial insects, controlling specific garden pests, and using organic pesticides.
Mexican Bean Beetles (Epilachna varivestis)
The most common problem encountered when growing beans, Mexican bean beetle (Epilachna varivestis) adults and larvae feed on the leaves and pods of all types of beans. Badly hit plants produce poorly because so many leaves are consumed; bean pods can also be seriously damaged. Organic controls for Mexican bean beetles include crop rotation, handpicking and maintaining good insect balance in the garden so that a wide variety of natural predators are present. For large plantings, imported predatory wasps or infectious fungi also can be used.
What Are Mexican Bean Beetles?
Native to a moist region of southern Mexico, Mexican bean beetles are now found wherever beans are grown. They are especially problematic in the rainy areas of the East, and tend to pose fewer problems in dry climates. Emerging in mid to late spring, just as beans begin growing vigorously, Mexican bean beetle adults look like large brownish-orange lady beetles with black dots over their backs. Adults are often seen lurking in bean foliage, looking for mates. Much more visible are the yellow, soft-bodied larvae that appear a few weeks later on both sides of bean leaves.
What Mexican Bean Beetle Damage Looks Like
Mexican bean beetle larvae are leaf-eating machines. The use their mouthparts to rasp away leaf tissue between veins, so that heavily damaged leaves look skeletonized. In the worst uncontrolled cases, Mexican bean beetles can completely strip plants of all foliage. One bad year often follows another.
Mexican Bean Beetle Life Cycle
Mexican bean beetles overwinter as adults in woods and weeds, and emerge in late spring. After flying to find host plants, mated females lay clusters of yellow eggs on leaf undersides. The clusters may include 40 to 75 eggs, so they are easy to spot. The eggs hatch about a week later, and the larvae feed for four to five weeks before entering their pupal stage. In cool weather the entire Mexican bean beetle life cycle can take 45 days or more, but 30 days is more typical. The second generation that emerges in midsummer causes the most damage to garden beans.
Natural Enemies of Mexican Bean Beetles
Numerous natural predators can assist in the struggle to manage Mexican bean beetles. These include birds, toads, spined soldier beetles, tachinid flies, and several species of tiny parasitic wasps. However, additional interventions are typically needed because native predators are usually few compared to the number of beetles, larvae and eggs present in an infested bean plot.
Mexican Bean Beetle Organic Control
Consider these natural controls for Mexican bean beetles, listed in seasonal order:
- Handpick adults and larvae whenever you see them. Squash egg clusters with your fingers. Once you see a few larvae, handpick daily to achieve good early season control.
- Interplant petunias with beans. Another popular companion planting approach involves growing rows of beans between rows of potatoes.
- Install floating row covers over bush beans in early summer, after the young seedlings have been weeded. Open the covers weekly to check for the presence of any adults or larvae, and remove them. Row covers work beautifully to prevent this bean pest, but lucky individuals occasionally emerge and prosper beneath them.
- If you have a home chicken flock, have the birds work over pulled bean plants before they are composted.
In large plantings of more than a quarter acre, these organic interventions may be worth their substantial cost:
- Spray with a product based on the fungus Beauvaria bassiana (such as Mycotrol) as soon as Mexican bean beetle larvae are seen. When the first generation of larvae becomes infected with this fungus, sufficient spores are usually present to infect later generations.
- Release an imported, commercially-reared parasitic wasp called Pediobus foveolatus after Mexican bean beetle larvae have appeared. State support may be available to growers in New Jersey and Maryland.
More Advice on Organic Mexican Bean Beetle Control
Keep a close watch on your growing beans in spring, and do not allow the first generation of Mexican bean beetles to triple itself by the time your beans grow into big, robust plants. Do all you can to provide food and habitat for beneficial wasps, flies, ladybeetles, and predatory stink bugs. Scout for eggs if adults are seen, using a small hand-held mirror to get a good look at leaf undersides.
Planting plenty of flowers that attract beneficial insects is a sound strategy, along with maintaining seldom-disturbed islands that provide habitat for ground beetles and other beneficials.
More information on organic Mexican bean beetle control is available from the University of Massachusetts, Cornell University and Florida State University.