With a shield on their back, a stinky odor and mouth parts that pierce and suck the life from developing soybean seeds -- this real image of stink bugs seems straight out of a sci-fi horror film.
But this insect is all too real for soybean farmers.
"Stink bugs are one of the few pest groups that are actually increasing in Midwest field crops," says Christian Krupke, Purdue University Extension entomologist. "They are vulnerable to very cold temperatures, but more stink bug adults survive mild winters." In turn, this has allowed populations to increase.
Further south, stink bugs consistently rank as the most prevalent soybean pest, says Mississippi State University Extension entomologist Whitney Crow. "Most states in the Mid-South have been dealing with numerous species of stink bugs for many years," she says.
CROP AND SEED DAMAGE
Stink bugs can be detrimental to soybean yield, seed quality, germination rates and delayed plant maturity. They also can transmit fungi and yeast inside the pod. The severity of damage depends on the density of stink bugs, the duration of feeding and the soybean pod and seed developmental stage.
Early-stage feeding during pod and seed development (R4 to R6) leads to pod loss and seed abortion; middle-stage stink bug feeding causes deformed, shriveled and small seeds; late-stage feeding on maturing seed causes slight deformation and discolored puncture marks.
Various stink bug species are primarily attracted to soybean pods and seeds -- mostly the native green, southern green, brown and redbanded stink bugs in the nymph and adult stages. However, they also can feed on soybean plant stems, foliage and blooms. "We also see green and brown stink bugs feed on young corn plants following a cover crop producing a seed," Krupke says. "They target the protein and fats in seeds that help them produce offspring."
LEARN INSECT'S LIFE CYCLE
Like any pest, knowing the stink bug's life cycle aids scouting and control practices. Crow says Mississippi State begins its ditch bank monitoring program in April and May, checking flowering crimson clover for redbanded stink bugs.
"By counting the number of stink bugs by location, it helps us correlate the presence of the more destructive redbanded species to what we may expect in the growing season," Crow says. "We report the numbers on our Mississippi Crop Situation blog and are working on a predictive model for the future."
Overwintering adults begin feeding on whatever plant is in the flowering or seed-producing stage during warm spring days. Mating occurs, and females lay barrel-shaped eggs in clusters on the underside of leaves in early summer. Upon hatching in one to three weeks, stink bugs develop through five instars before adulthood. One to three generations will occur per season, depending on the weather and the environment.
Both nymphs (immature adults/fifth instar) and adult stink bugs feed on soybean pods and seeds. Begin scouting at full bloom (R2) and continue weekly through full seed set (R6 growth stage). "Generally, stink bugs aren't really an issue until reproductive stages, anywhere from the R3 to R6 stage," Crow says.
It can be daunting to learn to identify the numerous stink bug species and how their markings and colors change by life cycle. However, many land-grant universities offer great resources, and some entomologists publish weekly pest email alerts.
FMC Technical Services Manager Eric Rebek, a former Oklahoma State University entomologist, says there is significant economic value in learning major groups of problem pests for any crop. "Also knowing beneficial and pollinator insects are critical to optimizing spray timing."
STINK BUG THRESHOLDS
In the Midwest, Krupke recommends using a sweep net to take 20 sweeps in five different field locations beginning at the edge of a field. "If you reach a threshold of 40 green or brown stink bugs after these 100 sweeps, it's time to spray," he says.
Crow says thresholds vary by stink bug species and crop stage in the Mid-South. "If you have green, southern green and brown stink bugs, the threshold is nine stink bugs per 25 sweeps until R6. Then, at R6, the threshold doubles to 20 per 25 sweeps until R6.5," she says. "If we're dealing with the more damaging redbanded stink bugs, the threshold is four bugs per 25 sweeps until R6, then it's 10 per 25 sweeps until R7 unless there are adverse environmental conditions."
Fortunately, the non-native brown marmorated stink bug currently isn't a significant pest in field crops. "While some of us entomologists feared that it would become a large issue, so far, the native stink bugs are currently the larger problem," Krupke adds.
Mid-South growers who can plant soybeans early might miss late-season pests. "For soybeans planted in April, those fields usually avoid late-season insecticide applications. So, if you can avoid the window of susceptibility in mid-August and September, you'll decrease the need to apply," Crow says.
The only other control method is timely contact insecticides when nymphs and adults are active. "We advise growers and consultants that they will not control late-season stink bugs by relying on the common spray applications in soybeans that occur at R3," Krupke says. "R3 soybeans don't interest stink bugs, and they may not show up until R5 or later, and you will need to contact them with a foliar application for effective control."
Mississippi State insecticide recommendations call for a full-rate pyrethroid application, like bifenthrin, to control green and southern green stink bugs. "Brown and redbanded stink bugs are more difficult to control, so we recommend adding acephate with bifenthrin, making sure there's no rain in the forecast for a minimum of eight hours," Crow adds.
Rebek says that FMC and university research show good control of the toughest brown and redbanded stink bugs with Elevest(R) insect control. The premix contains the leading pyrethroid bifenthrin and the systemic ingredient chlorantraniliprole (Rynaxypyr(R) active). "We also have data that shows good stink bug control at the lower rate of Elevest insect control (6.7 ounces per acre instead of 9.6 ounces) when tank-mixed with acephate," he says.
"This also gives you three active ingredients to reduce resistance issues, and the systemic benefit with Rynaxypyr active adds long residual control of Lepidopteran species as needed," Rebek says.
-- To learn more about Elevest insect control for stink bug management, visit https://ag.fmc.com/…
Elevest insect control is a Restricted Use Pesticide. Always read and follow all label directions, precautions and restrictions for use. Some products may not be registered for sale or use in all states. Elevest insect control may not be registered for sale or use in all states. Contact your local FMC retailer or representative for details and availability in your state. FMC, the FMC logo, Elevest and Rynaxypyr are trademarks of FMC Corporation or an affiliate. (C) 2023 FMC Corporation. All rights reserved. 05/23
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