"Late-season pests of soybeans are very dangerous, because they feed directly on the yield, and because they are often overlooked," he said.
The large number of stink bug populations could partially be caused by the drought in the Purchase Area and other parts of Western Kentucky. Typically, stink bugs feed on a variety of plants, but this year's drought may have limited their feeding options, causing them to concentrate on remaining crops. In addition to soybeans, this includes many grass crops, especially newly seeded ones.
Three or four stink bug species are common in Kentucky, and they are typically brown or green. Adults are about one-half- to three-quarter-inch in length and shield shaped. Stink bug nymphs are smaller, have colored spots on their backs and do not have wings.
Stink bug populations are normally higher in later-maturing fields, especially those with flowering plants. Adults tend to clump together. Early infestations may be found along the edge of a field.
The pests normally feed directly on the pod or seed of soybeans. This results in discolored or shriveled beans. The pod wall may have a black spot where the stink bug penetrated the crop with its mouth, but no other symptoms are usually visible until the beans are exposed. Stink bugs tend to avoid eating plant leaves, which makes it difficult for producers to find them.
"Unless a producer goes out into the field and looks directly at the pods or sweeps or shakes these pests from the plant, they are unlikely to see them until it is too late," Johnson said. "Treating the crop after the damage is done or after the plants mature will not produce a favorable economic return."
Producers may want to use a sweep net to determine whether an insecticide treatment is needed in a field. Pest thresholds vary depending on the crop's maturity stage. Producers should treat fields in the beginning bloom or R1 stage to about the beginning seed or R5 stage when they find an average of 12 or more stink bugs per 100 sweeps. They should treat fields in the R5 to R7 stages when pest populations are 36 or more bugs per 100 sweeps. Treatment after the R7 stage is likely not economical, Johnson said.
Producers should not confuse the damage-causing stink bugs with the spined soldier stink bug, which is a beneficial insect and does not damage soybeans. It has a very pronounced spine on its shoulders. Johnson said it closely resembles the rice stink bug, but rice stink bugs usually are not found in soybeans.
Purchase Area producers should be on alert for the red-banded stink bug in their fields. This pest has yet to be found in the state, but it is migrating north from Louisiana and is found in Tennessee and the Missouri counties directly across the Mississippi River from Kentucky. It is green with a red band across its back. If producers find what they suspect is a red-banded stink bug in their fields, they should send the bug to their county agent for agriculture and natural resources or to Johnson at the UK Research and Education Center, 1205 S. Hopkinsville St., Princeton, KY 42445 to confirm the bug's identity.