Don't Underestimate Potential Impact of Insect Injury to Soybean Pods

August 13, 1999
The second half of August is the time to begin checking soybean fields for pod injury that can be caused by a variety of insect pests. Bean leaf beetles, grasshoppers, and stink bugs are all able to cause yield losses in soybean fields during the pod-fill stage of development. So, although it has been a long summer already, keep scouting soybeans now and well through early September for these insect pests.

The last generation of bean leaf beetles will begin to feed on soybean pods after the leaves become too old (when they 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.

Bean leaf beetles.

Bean leaf beetle injury to pods.

Diseased beans caused by bean leaf beetle injury to pods.

Grasshoppers cause more direct injury to the soybean seeds. Because they have impressive 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. Insecticides that are labeled for both bean leaf beetles and grasshoppers in soybeans include Asana XL,* dimethoate, Lorsban 4E, Penncap-M,* Sevin XLR Plus, and Warrior T* or 1E* (insecticides followed by an asterisk are restricted-use products and may be applied only by certified applicators). Please consult the product labels for the amounts of product that may be applied for the control of each insect pest.

Direct pod injury caused by grasshopper or cricket.

Crickets can also cause direct injury to developing beans within soybean pods.

Grasshopper on soybean pod.

Many field observers tend to overlook stink bugs and the potential injury they can cause, even though they may be the most important pod feeders in Illinois. 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 (Figure 3). There is usually only one generation of green stink bugs per year in Illinois.

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 also is reduced by stink bug feeding, and beans are more likely to deteriorate in storage.

Stink bug eggs on soybean leaf.

Stink bug nymph on soybean leaf.

Stink bug nymph on soybean pod.

Adult stink bug on soybean leaf.

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 Penncap-M* at 1 to 3 pints of product per acre and Warrior T* or 1E* at 3.2 to 3.84 ounces of product per acre. (Both Penncap-M and Warrior are restricted-use products and may be applied by certified applicators only.)--Mike Gray

Author: Mike Gray