Bean Leaf Beetles--Waiting for Soybeans

May 7, 1999
Each crop in Illinois has its early insect invaderscutworms in corn, alfalfa weevils in alfalfa, several insects in wheat, and bean leaf beetles in soybeans. Although very few soybeans have been planted yet, the presence of bean leaf beetles is worth noting. As soon as soybeans emerge from the soil, any bean leaf beetles in the vicinity will locate them to begin feeding and laying eggs. Bill Brink, Extension Educator/Crop Systems at the Springfield Extension Center, has observed bean leaf beetles in soybean stubble from last year, and several other folks have seen bean leaf beetles in alfalfa, one of their favorite "hang-outs" in the spring. However, in both situations, the bean leaf beetles are just biding their time, waiting for soybeans to emerge.

Bean leaf beetles overwinter as adults in protected areas such as woodlots. When they leave the overwintering habitats, they move to other suitable habitats (such as alfalfa) to mate and lay a few eggs. However, the females reserve most of their complement of eggs for soybeans. Larvae that hatch from eggs laid in soybean fields feed on roots and nodules, but the injury is not economic.

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 bee-tles 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 pre-sent behind the "neck-like" prothorax.

When adult bean leaf beetles enter soybean fields in the spring, they feed on the young leaves, leaving small, round holes. Most soybean growers become pretty worked up over defoliation when soybean plants are in stages V1 to V2. Small plants with lots of holes in the leaves often cause people to overreact to the injury. However, research from the University of Nebraska has shown that seedling soybeans compensate very well for early season injury by bean leaf beetles. Most entomologists now concur that early season management of bean leaf beetles usually is not necessary. Densities of 16 beetles per foot of row in the early seedling stage or 39 per foot of row at stage V2+ are necessary for economic injury. These large densities usually are not reached during most years. So our advice is to try to ignore the injury to seedlings that you might observe, unless the numbers of bean leaf beetles are extraordinarily high. There is no need to spend money for an insecticide treatment that will cost more than the injury inflicted by the beetles, especially when profit margins are so tight.--Kevin Steffey

Author: Kevin Steffey