Mexican Bean Beetles
By William A. Kolbe B.C.E.
New Jersey Gardeners are reporting sightings and damage on beans (especially lima and snap beans) from Mexican Bean Beetles (MBB). Activity levels and ensuing damage by these plant feeders will vary this year due to the cool spring and water amounts from rainfall (and lack of rain). They tend to pose fewer problems in dry climates. Host plants are usually legumes.
Eggs of the MBB are bright yellow, laid on-end and in clusters of 40-60 on lower leaf surfaces. Larvae are yellow, oval shaped, covered with small black spines (they appear fuzzy) soft-bodied. Larvae go through 4 instars. Pupae are yellow-orange, similar to larvae but smooth and lighter in color, with spiny larval skin pushed down to the point of attachment to plants. Adults are convex oval form similar to lady bugs, one-third inch long, orange to copper colored with 16 black spots arranged in three rows of 6-6-4 on the back.
MBB larvae and adults feed on the underside of leaves between the veins, removing the lower epidermis of the leaf. The upper epidermis dies, producing a transparent, lacy look. Damaged tissue falls out and skeletonized leaves may curl and fall off. Larvae are particularly damaging to leaves. Adults may feed on blossoms, pods and stems.
Adults are the overwintering stage and usually aggregate under plant debris. They can be in open fields or in wooded areas. Adults emerge in the spring and lay eggs on the undersides of leaves. Larvae hatch out in two weeks and join adults feeding on foliage. Adults are strong flyers. In cool weather the entire Mexican bean beetle life cycle can take 45 days or more, but 30 days is more typical. There are one to three generations per year.
Early monitoring is helpful in preparing for this pest. Inspect leaf undersides for yellow egg clusters. Brilliant yellow larvae and coppery adults are easy to spot.
Tips for Prevention
Control. 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. Cover entire rows of beans with floating row cover after planting. You can leave the cover on until harvest. Crush egg clusters, larvae, and adults by hand. Purchase and release the beneficial wasp Pediobius faveolatus when larvae are first observed. Most practical in large plantings or community gardens. For severe infestations requiring insecticides use, consider organic methods such as spinosad, neem, or pyrethrum. Contact Rutgers Extension Agents for recommendations. Pull up and bag bean plants after harvest. Plant resistant cultivars such as “Wade and “Logan” snap beans and “Black Valentine” lima beans. Leave a few flowering weeds between rows to attract native predators and parasites, or interplant with flowers such as Queen Anne’s lace or yarrow, and herbs such as dill to encourage beneficial insects such as spined soldier bugs (Podisus maculiventris) and parasitic wasp Pediobius foveolatus.
Conclusion: 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.
Images of Mexican Bean Beetle can be found at various internet sites. IPM images is one such site: http://www.ipmimages.org/search/action.cfm?q=Mexican%20bean%20beetle
Editor’s Note: William A. Kolbe, BCE is a Board Certified Entomologist for Viking Pest Control based out of Warren, NJ. He has a Bachelor’s Degree in Entomology with a minor in Ecology from the University of Delaware. He is a member of The Denville NJ Community Gardens. He can be reached at 800-618-2847 or visit www.vikingpest.com