Issue No. 21, Article 4/August 13, 2004
Don't Forget to Scout Soybean for Pod Feeders
The greatest loss to soybean can be caused by insects feeding on pods. As of August 8, the Illinois Agricultural Statistics Service reports that nearly 82% of soybean in Illinois is setting pods. As we continue through August and move into September, don't forget to scout maturing soybean fields for pod feeders. Bean leaf beetles, grasshoppers, and stink bugs are all insects that can potentially cause yield loss in soybean fields.
Concern of bean leaf beetle injury is typically associated with defoliation of soybean leaves. The last generation of bean leaf beetles, however, may move from feeding on soybean leaves to feeding on soybean pods when soybean leaves begin to lose color and nutritional quality. Bean leaf beetles scrape tissue from the pod but do not chew completely through the pod like other pod feeders. Open feeding scars may predispose the pod to potential fungal diseases that would normally have been prevented by the pod wall. Mild infection caused by these pathogens may cause seed staining, whereas severe infection may lead to total seed contamination.
Bean leaf beetle adults.
Bean leaf beetle feeding on soybean pods.
Grasshoppers are another soybean defoliator that may cause direct injury by feeding on soybean pods. Grasshoppers possess chewing mouthparts that readily consume host plant tissue. Unlike bean leaf beetles, when grasshoppers feed on pods, they chew completely through the pod wall and may take bites out of or completely eat developing seeds. Control of grasshoppers or bean leaf beetles may be warranted when 5% to 10% of the pods are damaged.
Grasshopper on soybean.
Grasshopper feeding on soybean pod.
Though not an insect that is commonly thought of as a pest of soybean, when feeding pressure of stink bug adults and nymphs is heavy, soybean quality may be lowered. Watching for stink bugs, especially in the southern half of the state, should be a high priority for soybean producers.
Green stink bugs are believed to migrate northward from overwintering sites (wooded areas beneath leaf litter) as adults. During the early months of summer, the adults feed on berries in trees, especially dogwoods. Stink bugs are first found in soybean fields during August. They undergo incomplete metamorphosis (immature bugs resemble the adults), which requires approximately 45 days from egg hatch to adult emergence. Illinois usually has only one generation of green stink bugs per year.
Immature stink bugs (nymphs) have a flashy display of black, green, and yellow or red colors and short, stubby, nonfunctional wing pads. The adults are large (about 5/8-inch-long), light green, shield-shaped bugs with fully developed wings. Both adults and nymphs have piercing and sucking mouthparts for removing plant fluids.
Some stink bugs are predators of other insects. Others are plant feeders that remove sap from bean pods with their sucking mouthparts. The brown stink bug has feeding habits and biology similar to those of the green stink bug. The brown stink bug should not be confused with the beneficial spined soldier bug. Adult brown stink bugs are brown and have a yellow or light green underside, and the "shoulders" are rounded. Spined soldier bugs also are brown and have a white to cream-colored "belly." However, their shoulders are sharp-pointed. Be sure you are aware of the species present in a soybean field before making a control decision.
While feeding, stink bugs insert digestive enzymes into the pod that, while beneficial to them, may cause deterioration of the bean pod. Assessing stink bug injury is difficult because they leave no distinct feeding scars. Stink bug feeding may also reduce seed quality and provide an avenue for disease entry. Soybean pods are susceptible to stink bug injury until they reach maturity. Treatment for stink bug injury to pods may be warranted when adult bugs or large nymphs reach 1 per foot of row during pod fill.
Stink bug eggs.
Table 1 lists insecticides labeled for control of these pests.