Bean Leaf Beetles--Another Pest to Watch for in Soybeans

July 23, 1999
Another soybean pest has made its presence known recently in soybean fields in central Illinois. Adults of the first generation of bean leaf beetles have begun to emerge and are feeding on soybean leaves. Charlie Helm, research entomologist in the Center for Economic Entomology in the Illinois Natural History Survey, observed extremely high numbers of bean leaf beetles feeding in one of his research trials on Monday, July 19. He had looked at his trial on Friday, July 16, and had seen very few beetles. Apparently emergence occurred suddenly during the weekend. Charlie and one of his assistants surveyed some soybean fields in Champaign (four fields) and Piatt counties (five fields) on July 20 and found the following numbers of bean leaf beetles: 18 and 21 beetles per 20 sweeps in 30-inch-row soybeans in Champaign County; 13 and 60 beetles per 20 sweeps in drilled soybeans in Champaign County; 16, 17, and 34 beetles per 20 sweeps in 30-inch-row soybeans in Piatt County; and 14 and 16 beetles per 20 sweeps in drilled soybeans in Piatt County. These numbers do not reflect the numbers that Charlie found in his plots, but at least we know the beetles are there.

The bean leaf beetles we observed in the spring were the adults that had left overwintering sites to find sources of food. After feeding for a while and laying eggs in soybean fields, those adults died. The larvae have been feeding on soybean roots and nodules for the past few weeks, although economic losses attributed to this feeding have not been documented. The larvae then pupated in the soil, and adults have emerged, ready to start the cycle all over again.

Color and pattern variations of bean leaf beetle adults.

Bean leaf beetle adults are about 1/4 inch long with considerable variation in color. The background color of most bean leaf beetles is light yellow to tan; however, some bean leaf beetles are green, and others are red. Their wing covers usually have four main black spots and stripes along the edges, but these markings may be absent. A black triangle is always present behind the "necklike" prothorax.

Bean leaf beetle and injury to a soybean leaf.

Defoliation by bean leaf beetles in soybeans appears as small, round holes in the leaves, quite different in appearance from the ragged, edge-of-the-leaf defoliation caused by grasshoppers, green cloverworms, and woollybear caterpillars, and also quite different in appearance from the lacy defoliation caused by Japanese beetles. Remember, thresholds for all soybean defoliators are based on percentage defoliation: 20 percent or more defoliation from bloom through pod fill, 30 to 40 percent or more defoliation during vegetative growth. With the current value of soybeans, control measures may not be warranted unless higher levels of defoliation occur.--Kevin Steffey

Author: Kevin Steffey