Although insect pest problems have been relatively minor this year, farmers should stay vigilant and continue to scout corn and soybean.
Planting dates were late in many areas and cool weather has slowed plant growth, leading to delayed maturity in many areas. These late-maturing crops may remain vulnerable to pest injury longer than usual.
Stink bugs are significant economic pests to a variety of crops in the southern United States. Over the past 15 years there appears to be a general trend of increasing stink bug populations in more northern states, including Nebraska.
Nebraska Stink Bugs
Figure 1. Green stink bug
Figure 2. Brown stink bug
Figure 3. Red-shouldered stink bug
(Photo by Russ Ottens, University of Georgia, Bugwood.org)
Based on stink bug surveys made in Nebraska in 2009-2011, four main species can be found in soybeans and corn:
green stink bug
brown stink bug
onespotted stink bug
red-shouldered stink bug
All are shield-shaped as nymphs and adults. In general, adult green stink bugs are bright green, and adult brown stink bugs are brown with a yellow or light green underside. Green stink bug nymphs change color and pattern as they grow, but brown stink bug nymphs are yellow to tan with brown spots down the center of the abdomen. The onespotted stink bug looks similar to the brown stink bug, except it has a small spot on the underside of the abdomen. The red-shouldered stink bug is green and has a red stripe across the base of the wings. Another brown stink bug, the spined soldier bug, is a beneficial predatory insect.
General Stink Bug Biology
Adult stink bugs overwinter primarily in leaf litter, under bark, or in wood piles. We believe that the green stink bug does not overwinter in Nebraska, but migrates north in late spring to early summer. We typically begin to find the green stink bug in July. Brown, one-spotted, and red-shouldered stinkbugs, as well as a few other recently collected species, appear to overwinter in Nebraska.
In the spring, adult stink bugs leave the overwintering sites and feed on a variety of wild and cultivated hosts. In Nebraska crops, the first significant numbers of bugs appear in the spring in wheat, followed by alfalfa. After feeding a few days, stink bugs mate and lay clusters of eggs. As the season progresses, female stink bugs are attracted to a variety of flowering plants, including corn and soybean. Populations peak during the pod-filling stages of soybean and ear-filling stages of corn. There are likely one to two generations in Nebraska, depending on species.
Stink Bug Injury to Corn and Soybean
Stink bugs have piercing and sucking mouthparts and feed by piercing a plant part (or another insect in the case of the spined soldier bug), injecting digestive enzymes and removing fluids.
Corn. Nymphs and adult stink bugs injure vegetative stage corn by feeding at the base of the seedling corn plant. Feeding results in irregular shaped, oblong holes with yellow margins in the leaves as they emerge from the whorl, twisting of the whorl, and in some cases, death of the growing point. Nymphs and adult stink bugs damage reproductive stage corn by piercing the husk and feeding on the developing kernels from the beginning of kernel formation through milk stage, although they can feed through the hard dough stage. Damage appears as missing or shrunken kernels. Severe damage causes ears to curve (banana ears).
Soybeans. Nymphs and adult stink bugs injure soybeans by puncturing various soybean plant parts and extracting plant fluids. They prefer young tender growth and developing seeds. As they feed they inject digestive enzymes, which cause deformation and abortion of seeds and pods, and predispose the feeding site to various pathogens. In addition, stink bugs can cause delayed maturity and deformed leaf growth. Yield and quality losses depend on when the bugs injure soybean, and can be severe.
Injury often appears first on field borders as the stink bugs move into the field for both corn and soybean. With time, the stink bugs can move throughout the field.
Management of Stink Bug in Corn and Soybean
In general, thresholds are based on counts of large nymphs (1/4 inch or greater) and adults, as those are the most damaging stages. Green stink bugs are more numerous in soybeans and brown stink bugs more numerous in corn, but don't be surprised to find a mix of species.
Note that the following thresholds are single values and do not explicitly consider changing crop prices and treatment costs. This is because there is not a comprehensive database from which to develop these types of thresholds. These thresholds are set low enough to account for price and cost fluctuations. However, if you believe you have a robust and rapidly increasing population, you may want to lower the thresholds slightly (for example, to one bug per 5 plants instead of one bug per 4 plants for the early reproductive stage of corn).
Research is ongoing in Nebraska, but until we have more definitive information, we recommend using the following treatment threshold:
• 1 stink bug per 4 plants (25% infested plants) prior to pollination, and
• 1 stink bug per 2 plants (50% infested plants) after pollination up to early dough stage.
Thresholds for stink bugs on soybeans vary considerably by state, and also do not explicitly consider variable costs of control or market value. Again, while we don't have definitive research data specific to Nebraska, we believe the following treatment thresholds are appropriate:
1 stink bug per row-foot during the reproductive stages, and
If using a sweep net, 3.6 stink bugs per 15 sweeps (i.e., 0.25 bugs per sweep).
If thresholds are met, the standard insecticides registered for corn or soybean should be effective.