Mexican Bean Beetle

Mexican Bean Beetle, Epilachna varivestis Mulsant, is the most injurious insect pest of beans. They have spread throughout the western region of the United States, damaging dry beans, okra, eggplant, alfalfa, and clover.

The adult Mexican bean beetle is about 1/4 inch (6 mm) long, very convex and short in form. It varies in color from yellow when newly emerged from the pupal stage to a coppery-brown when mature or in the overwintering stage. Each wing cover had eight black dots in three rows across the back when the wings are at rest. When disturbed, the adults fall from the plant and exude a yellow liquid from their leg joints. The adult beetles pass the winter under trash and other ground cover along hedgerows and in similar protected places.

The yellow egg is about 1.3 mm long and elliptical in shape. Larva – The yellow larva is about 8.5 mm long and covered with dark, branched spines. Color plate. Pupa – The yellow or copper colored pupa is about 6 mm long. It moves very little, has fewer spines than the larva, and is most commonly found on the lower half of the soybean plant.

A native of Mexico, the Mexican bean beetle is now present throughout the United States except in the Pacific Coast states. Damage to soybeans has been most prevalent along the East Coast from Maryland south to Georgia, in southern Indiana, and in some parts of Kentucky. In North Carolina, the Mexican bean beetle occurs on soybeans throughout the growing season.

The Mexican bean beetle feeds readily on many varieties of beans (bush, soybean, lima and pole) but may also infest other plants such as alfalfa, clover, cowpea, and kudzu.

Life Cycle :The adult Mexican bean beetle overwinters under leaves or other debris in grassy, weedy areas and around fence rows or trees. The adults move into the bean fields and gardens soon after the bean plants emerge. The adults feed for a week or two before laying their yellow egg masses on the underside of the leaves.

The eggs hatch in five to 14 days. The bright yellow larvae are oval-shaped with six rows of branched spines. The larvae feed for two to five weeks. Larvae and adults feed on all types of beans and are an occasional pest of soybeans.

They generally feed on the underside of leaves, removing all of the leaf tissue except the clear layer on the upper side of the leaf, called the epidermis. This damage, called “window-paning,” gives the leaves a lace-like or skeletonized appearance. The remaining leaf tissue turns brown in a couple of days, giving the field a burnt cast. New pods and stems are often attacked, and severely damaged plants may die prematurely.

Damage :The Mexican bean beetle is one of only two North American species of destructive insects in an otherwise beneficial family (ladybird beetles) that contains over 400 species. Adult Mexican bean beetles feed on seedlings early in the season. The larvae feed on leaves; in their early growth stages, they feed exclusively on the lower surface of the leaf.

Bean pods may also be scarred, but this damage is seldom considered economic. Soybeans near woodlots, alfalfa fields, and fields where residues have not been plowed are most likely to incur damage. Though the Mexican bean beetle has mandibles that are typical of chewing insects, it does not swallow bits of food. Rather, it masticates its food and consumes the resultant juices.

The foliage of garden beans such as snap, kidney, pinto, and lima are preferred, but Mexican bean beetles can also be serious pests of soybeans. The beetles also feed on alfalfa, clover, peanut, okra, eggplant, squash, and various weeds. Both larvae and adults impart a skeletonized or lacy appearance to leaves by consuming the leaves’ epidermal layers. Heavily infested soybean fields take on a dusty appearance as leaves shrivel and turn brown.

Sampling Method : If during sampling Mexican bean beetle feeding damage is evident, determine the percentage defoliation level for individual plants in 5 separate areas of the field and estimate the average percentage defoliation level for the field as a whole. Ascertain the size of the population by taking 20 sweep net samples in each of the 5 areas of the field. Record the number of Mexican bean beetle adults and larvae separately.

If beetles are feeding on pods, randomly select 2 plants in each of 5 areas of the field and count the number of pods per plant and the number that show insect damage (10 total plants).

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