Issue No. 21, Article 5/August 12, 2005
Reminder About Soybean Pod Feeders
Insect activity in soybeans this summer has focused on spider mites and now soybean aphids, but don't forget about grasshoppers, bean leaf beetles, and stink bugs. While we've yet to receive news on significant feeding caused by these insects, they are known for the potential to reduce soybean yields by feeding on soybean pods.
Grasshoppers have been abundant in many fields this summer. The hot, dry weather has been very good to these defoliators. They began moving into fields earlier this summer, feeding on both corn and soybean foliage ("Grasshopper Nymphs Moving into Field Edges," issue no. 14, June 24, 2005). However, grasshoppers may also cause direct injury to developing soybean pods. They 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 devour developing seeds. Control of grasshoppers may be warranted when 5% to 10% of pods are damaged.
Grasshopper on soybean.
Pod injury caused by grasshopper feeding.
Injury caused by bean leaf beetles is also often associated with defoliation, but they may feed on pods as well late into the summer. The last generation of bean leaf beetles may move from feeding on soybean leaves to feeding on soybean pods when leaves begin to lose color and nutritional quality. Bean leaf beetles do not chew completely through soybean pod walls but may scrape tissue from the pod. The resulting scars provide openings for entry of spores of various fungal diseases that are normally blocked by the pericarp. Mild infection results in seed staining; severe infection may result in total seed contamination. Control of bean leaf beetles may be warranted when 5% to 10% of pods are damaged.
Bean leaf beetle.
Pod injury caused by bean leaf beetles.
Though stink bug is not an insect commonly thought of as a pest of soybean, when feeding pressure of adults and nymphs is heavy, soybean quality may be lowered. Some stink bugs are predators of other insects. Others are plant feeders that remove sap from bean pods with their sucking mouthparts.
Stink bugs are first found in soybean fields during August. 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. They undergo incomplete metamorphosis (immature bugs resemble the adults), which requires approximately 45 days from egg hatch to adult emergence. 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. Illinois usually has only one generation of green stink bugs per year.
Green stink bug.
The brown stink bug has feeding habits and biology similar to 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.
Brown stink bug.
While feeding, stink bugs insert digestive enzymes into the pod that may cause deterioration of the bean pod. Assessing injury by stink bugs 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 nymph on soybean.
Table 2 lists insecticides labeled for control of these pests. Please read labels for recommended rates and follow all label instructions.--Kelly Cook