In last week's Bulletin (no. 20, August 7), Kevin Steffey focused on soybean defoliation. As we move into the latter half of August and into early September, our attention turns toward soybean pod feeders. Bean leaf beetles, grasshoppers, and stink bugs are all capable of inflicting yield losses to soybeans during the pod-fill stage of development.|
The last generation of bean leaf beetles will begin to feed on soybean pods after the leaves become too old (begin to lose some green coloration). The beetles scrape off the green tissue on the pods (Figure 1) but do not chew through the pod wall. The resulting scars on the pods provide an opening 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.
Figure 1. Bean leaf beetle defoliation and pod injury.
Bean leaf beetle injury to soybean pod (on plant).
Bean leaf beetle injury to soybean pods (on table).
Bean leaf beetle injury to soybean pods (on table, pods opened).
Grasshoppers cause more direct injury to the soybean seeds. Because they have strong chewing mouthparts, grasshoppers often chew directly (Figure 2) through the pod wall and take bites out of or devour entire seeds. If more than 5 to 10 percent of the pods are injured by grasshoppers or bean leaf beetles, an insecticide application may be warranted.
Most individuals tend to overlook stink bugs and the potential injury they can cause, even though they may be the most important pod feeders in the state. Watching for stink bugs, especially in the southern half of the state, should be a high priority for soybean producers.
Figure 2. Grasshopper injury to soybean pods.
Grasshopper on soybean pod.
Green stink bugs are believed to migrate northward from overwintering sites (wooded areas under 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 roughly 45 days from egg hatch to adult emergence (Figure 3). There is usually only one generation of green stink bugs per year in Illinois.
Figure 3. Life cycle of a stink bug.
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.
Stink bugs feed directly on pods and seeds; however, their injury is difficult to assess because their mouthparts leave no obvious feeding scars. Stink bugs use their mouthparts to penetrate pods and puncture the developing seeds (Figure 4). They inject digestive enzymes into seeds, and the feeding wound provides an avenue for diseases to gain entry into the pod. Seed quality is also reduced by stink bug feeding, and beans are more likely to deteriorate in storage.
Figure 4. Stink bug injury to soybean pods.
Other species of stink bugs also occur in soybeans. The brown stink bug has feeding habits and a biology similar to those of the green stink bug. The brown stink bug should not be confused with the beneficial spined soldier bug. These two species can be distinguished from each other if you look at the feeding beak and underside of the abdomen. The beak of the brown stink bug is slender and embedded between the lateral parts of the head. The base of the beak of the spined soldier bug is stout and free from the lateral parts. In addition, the spined soldier bug has a dark round spot located centrally on the underside of its abdomen (belly). Be sure you are aware of the species present in a soybean field before making a control decision.
An insecticide application for control of stink bugs may be warranted when the level of infestation reaches one adult bug or large nymph per foot of row during pod fill. Suggested insecticides are Asana XL at 5.8 to 9.6 ounces per acre, Lorsban 4E at 2 pints per acre, Penncap-M at 1 to 3 pints per acre, and Warrior 1EC at 3.2 to 3.84 ounces per acre. Use of Asana, Penncap-M, and Warrior is restricted to certified applicators only.