Stink bugs in soybeans - most are herbivores and can be found feeding on developing pods; however a few species are beneficial predators.
Stink bugs are often more prevalent in soybean fields around this time of year, especially in the southern half of the state. Stink bug nymphs and adults are fluid feeders with piercing-sucking mouthparts. Most species are herbivores but a few are beneficial predators. Because herbivorous stink bugs build up in soybeans after bloom, they can be found feeding on developing pods, resulting in shriveled, desiccated seeds. The species below are known to occasionally cause problems for Iowa producers.
ISU Extension entomologist Erin Hodgson and entomology graduate student Rebekah Ritson provide the following explanation and information.
Green stink bug (Acrosternum hilare)
This pest migrates up from southern states each summer and has only one generation per year in Iowa. They usually begin feeding in soybean in early August. Green stink bug nymphs are black with bright green and yellow or red markings. Adults are light green, shield shaped, and have fully developed wings. They are large, about 5/8 inches long.
Brown stink bug (Euschistus servus)
These nymphs are light greenish brown and will develop brown spots on the middle of the abdomen as they get older. As an adult, the brown stink bug is brown with slightly rounded shoulders. The underside of their abdomen is yellow to light green.
In addition to these well-known species, several economically important stink bugs from other regions are now entering the Corn Belt. None have been documented causing serious damage in Iowa, but these species are on the move. Scout carefully!
Brown marmorated stink bug (Halyomorpha halys)
Red banded stink bug (Piezodorus guildinii)
Originally from Asia, this pest was first found in Pennsylvania in 1998. Since then, it has been identified in many states in the continental U.S., including Iowa. "Marmorated" refers to their marble-like coloration. Adults have light bands on the antennae and dark bands on the membranous part of the wings. Additionally, they have small, round coppery or metallic blue depressions on the head and pronotum. Adults are about 2/3 inches long. Nymphs have a yellowish to off-white abdomen, and at the last stage before adulthood, the abdomen will have reddish spots. Their eyes are deep red.
This pest has been observed in Arkansas, Louisiana, Mississippi, Missouri and Tennessee and has spread quickly throughout these states. It has presented management challenges, especially in Louisiana, and has a lower economic threshold than the brown stink bug. Nymphs are mostly green with red and black markings. Adults are green and just under ½ inches long; they can be identified by the two strips (one yellow and one dark red, purple or black) across the "shoulders." When flipped on its back, a spine is visible on the red banded stink abdomen between the third pair of legs.
Red shouldered stink bug (Thyanta custator)
This pest has also been reported in several states but is somewhat easier to manage than the red banded stink bug. Adults are green and just under ½ inches long; they have markings that are very similar to the red banded stink bug. However, when flipped over, there is no spine on the abdomen.
Remember, not all stink bugs found in soybeans are pests. Some are beneficial and will use their mouthparts to pierce the body and suck out the internal fluids of a variety of soybean pests, particularly caterpillars, beetle larvae and adult beetles.
Spined soldier bug (Podisus maculiventris)
Scouting and Economic Thresholds for Stink Bugs
The most common beneficial stink bug in Iowa soybean is the spined soldier bug. Both the immature and adult stink bugs of this species are predatory and will kill many soybean pests, including green cloverworm. The spined soldier bug adult is easily identified by its sharply pointed "shoulders" and dark brown spot on the wing tips.
To scout for stink bugs, use sweep nets or shake plants over a drop cloth. Adults tend to aggregate, so sample several areas in each field. Combine counts of older nymphs (larger than 1/4 inch in length) and adults. Economic thresholds for each species are listed in Table 1.