Mexican Bean Beetle
Newly emerged adult Mexican bean beetles are round, about 1/4 inch long, and yellow. Their color changes to copper with age. They are marked by sixteen black spots, eight on each wing cover. Six branching spines are present on each segment of the bright yellow, 1/3 inch-long larvae. Pupae are also bright yellow and have only remnants of larval spines. Eggs are yellow and oval shaped.
Adults overwinter along fence rows, woodlots, or in stubble and can usually be found within 1/4 mile of the host plants. The beetles become active in the spring, when they fly to host plants, feed for a week or two, and then mate. Normally, 400 to 500 eggs (but occasionally three times this number) are laid in clusters on the undersides of leaves over a period of 3 to 6 weeks. Larvae hatch in 5 to 14 days and feed for 2 to 5 weeks before pupating on the undersurfaces of leaves. Adults emerge 7 to 10 days later and live from 4 to 6 weeks. There are approximately three generations per year in Illinois.
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.
The growth stage of the insect (young larvae are easiest to control), cost of treatment, market price of the crop, growth stage of the crop, and percentage of defoliation should all be considered when making control decisions. In soybeans, treatment may be warranted when defoliation reaches 30 percent before bloom and 7 or more adults and larvae can be found in one foot of beans in a row. For blooming or postbloom beans, treatment may be warranted when defoliation reaches 20 percent and S or more adults and larvae are present in one foot of beans per row. To determine the number o£ Mexican bean beetles per foot of row, spread a ground cloth between two rows of beans. Shake the plants overhanging the cloth in one row only, and count the number of adults and larvae on the cloth. You can vary the width of the ground cloth by unrolling it to accommodate differences in space between the rows. The typical length of a ground cloth is one yard. Thus, in dividing the count by three, one can easily determine the number of Mexican bean beetles per foot of row. As the practice of double cropping soybeans after wheat increases, problems with the Mexican bean beetle may increase. Leaves of conventionally planted beans turn yellow before those of double-cropped beans. Thus, double-cropped soybeans and late-maturing varieties readily attract the beetles. Supplemental food sources such as these increase the survival of overwintering beetles.