Weather creating ideal conditions for insects in soybeans
The pods on soybeans are beginning to fill. With that stage in growth comes an increased concern about prominent pests.
According to Jill Scheidt, agronomy specialist with the University of Missouri Extension, weather conditions are creating situations suitable for insects not commonly seen.
“For example, two-spotted spider mites are usually not a problem this time of year, but with temperatures being hot and dry, they could pose a problem,” Scheidt said.
Spider mites are usually translucent in color and can be identified with a hand lens by the two dark spots on the sides of the abdomen. Spider mites also have sucking-piercing mouth parts and feed on the underside of leaves. If a leaf is held up to the light, it is possible to see the webs glistening on the underside of the leaf.
Threshold levels for spider mites are 20 percent yellowing before pod set and 10 percent yellowing after pod set when mites are present.
“A high rate of insecticide must be used to kill spider mites,” Schedit said. “If a high rate is not applied, the insecticide will kill beneficial insects that control spider mite populations.”
Thrips are similar in size to spider mites and vary in color from yellow to dark brown, but can be identified by horizontal stripes on the back of their lower body. Thrips are thought to transmit soybean vein necrosis. Thrips are a rare pest in soybeans, so threshold levels have not been determined yet.
Green stinkbugs, both the nymphal and adult stages, attack primarily the seeds and pods of soybean plants. They also will feed on soybean plant stems, foliage, and blooms.
Green stink bugs have piercing sucking mouthparts and punctures can be identified by the presence of small brown or black spots. Green stinkbugs are bright green and are identified by the dark bands on their antennae.
“Direct feeding damage can lead to a reduction in seed quality and quantity,” Scheidt said. “Young seeds can be deformed, undersized or even aborted. Older seeds will be discolored and shriveled. The germination rate also will be reduced for beans produced as a seed source.”
Indirectly, feeding damage by stink bugs can delay plant maturity and cause the abnormal production of leaflets and pods. Green stinkbugs can usually be found on the edges of fields first. Threshold levels for green stinkbug are 1 per foot of row during seed production.
Bean leaf beetles feed on pods once seed development begins. Bean leaf beetles are small insects about one-quarter inch long with black spots or stripes on their back. Their most identifying characteristic is the black triangle located on their forewings, just below the head.
Bean leaf beetles can clip pods and feed on developing seeds. Threshold levels for bean leaf beetle are 10 or more bean leaf beetles per foot of row and 20 percent defoliation or at least 15 bean leaf beetles per foot of row and at least 10 percent pod damage.
Pod worm moths, also known as corn earworm, like to lay eggs in an unclosed canopy in soybeans. Identify moths by the black banding on the hind wings. Pod worm eggs hatch seven to 10 days after moth flights. Six weeks after moth flights is when pod worms could stop foliage feeding and begin feeding on pods and become a problem.
“Don’t spray unless pod worms reach threshold, which is 1 pod worm per foot, because beneficial clover worms carry a fungus that kill pod worms, and may take care of the pod worms without a need to spray an insecticide,” Scheidt said.
Pod worms come in all colors, from brown with yellow spots to white with black spots and green. To distinguish pod worms from green clover worms, Scheidt says to look at their legs. Pod worms have four pairs of large abdominal pro-legs right in the center of the body while clover worm only has three pairs.
“Hero, Warrior II and Mustang Max are effective insecticides to control all of these insects in soybeans. Read the label for appropriate rates and water use for each pest” Schedit said. “Remember if multiple insecticides applications need to be made, use a different mode of action to prevent insect resistance to insecticides.”
For more information, contact any of these MU Extension agronomy specialists in southwest Missouri: Tim Schnakenberg in Stone County, 417-357-6812; Jill Scheidt in Barton County, 417-682-3579; John Hobbs in McDonald County, 417-223-4775 or Brie Menjoulet in Hickory County, 417-745-6767.