Mexican Bean Beetle
EN009
Mexican Bean Beetle Epilachna varivestis Mulsant
Kimberly Stoner
Department of Entomology
Connecticut Agricultural Experiment Station
123 Huntington Street
P.O. Box 1106
New Haven, CT 06504-1106
Telephone: (203) 974-8480 Fax: (203) 974-8502
E-mail: Kimberly.Stoner@ct.gov
When and Where to Look
The adult beetles start to attack beans in mid-June. There are usually small numbers of adults at first, and the populations gradually build up over the season. The adults, eggs, larvae, and pupae are found mostly on the underside of bean leaves, where they feed in small patches between the leaf veins, giving the damaged leaf a lacelike appearance. There are two or three overlapping generations a year, and the adults spend the winter under leaf litter and in weedy hedgerows.
Description
Adult: The Mexican bean beetle is closely related to beneficial lady beetles (ladybugs), so the adult must be examined carefully to distinguish it as a pest. The adult beetle varies from light orange to copper in color, with eight small black dots on each wing cover that form three rows across the body when the wings are at rest. Both the head and the pronotum (the part between the wing covers and the head) are also copper colored, which distinguishes the Mexican bean beetle from other lady beetle adults. It is about 1/3 of an inch long.
Eggs: The eggs are yellow and laid in large clumps of 40 or more on the underside of the leaves.
Larva: The larva is yellow, up to 1/3 inch long, and covered with six rows of long, branching, black-tipped spines.
Pupa: The pupa changes from yellow to copper colored as it matures, pushing the larval skin with its spines to the back as it develops.
Damage: Because the Mexican bean beetle feeds in small patches between the leaf veins from the underside of the leaf, the damage looks at first lacelike, then 'skeletonized,' and then, as the plant loses moisture through the damaged leaves, dried out and brown. The beetles occasionally feed directly on the bean pods, but most of the yield loss is due to the loss of leaf area.
Control Methods
Natural: A parasitic wasp, Pediobius foveolatus, has been used successfully to control the Mexican bean beetle in both commercial farming and community garden settings. This tiny wasp lays its eggs in Mexican bean beetle larvae. When new adult wasps emerge, they kill the bean beetle. Unfortunately, the adult wasps do not survive the winter in Connecticut, so they have to be reintroduced each year. Because the adult wasps have a short life span, it is important to introduce them when mid-sized bean beetle larvae are already present on the plants. Some commercial growers plant one plot of snap beans earlier than usual to build up a small population of Mexican bean beetle larvae and introduce the wasps before the main population of Mexican bean beetles develops. The adult wasps feed on nectar at flowers, and benefit from composite (sunflower family) or umbellifer (carrot family) flowers near the bean plot. Pediobius foveolatus is available from several mail-order companies that sell biological control agents.
Mexican bean beetle populations build up more rapidly on plants where the beans have been left on the plants to mature than on plants where the young beans are picked off frequently. So, keep picking your snap beans before they get large and mature. (They taste better and are more productive that way, anyway). In a test of 14 bush snap bean varieties, Blue Lake 274 and Idaho Refugee produced the most consistent high yield, whether or not they were treated with insecticides. Destroy bean plants as soon as they are no longer productive to deny the last generation of beetles the plant material and overwintering sites they provide.
Control of Mexican bean beetles on dry beans may be more difficult. Row covers can be used for dry beans, and do not have to be removed at flowering, because beans are self-pollinating. Locating new plantings away from the previous year’s beans and interplanting beans with other crops may make them more difficult for the beetles to locate.
Handpicking is relatively easy in small gardens because the adults, eggmasses, and larvae are all together on the underside of the leaves and none of these stages is camouflaged. If you pick off the adults by hand, make sure you actually crush them or drown them in soapy water, don't just knock them off the plant because they will crawl back.
Chemical: Sevin, methoxychlor, rotenone, and pyrethrum are available in formulations labeled for control of Mexican bean beetle. Follow instructions on the label. Make sure the underside of the leaves is thoroughly covered, because that is where most of the beetles live and feed.
Summary
Mexican bean beetles are pests of beans (snap beans, dry beans, and soybeans). Both the adults and the larvae feed on bean leaves, eating green tissue and causing the leaves to have a lacy or "skeletonized" appearance. A parasitic wasp is an effective natural control, but does not overwinter, and thus has to be re-introduced each year. Gardeners may also use handpicking or row covers. Any insecticides applied must reach the underside of the bean leaves, where both the adults and larvae are usually found.
(revised 1/98)