Insecticide resistance in soybeans is a growing problem in the Midwest. In South Dakota, North Dakota, Iowa and Minnesota, research sponsored by state checkoffs and the North Central Soybean Research Program (NCSRP) identified soybean aphid populations that are resistant to popular insecticides. Spider mites also appear to be developing some resistance. While these issues haven’t arrived in Ohio yet, it’s something Ohio State University (OSU) Extension Entomologist Kelley Tilmon says farmers need to know about when developing crop protection strategies.
“One reason we encourage growers to limit insecticide use until scouting and thresholds indicate it’s necessary is that the more heavily we use insecticides, the more likely resistance can develop,” explained Tilmon. “With fewer new insecticides coming to market, it’s important to safeguard the tools we have.”
Since scouting is effective but time consuming, the Ohio Soybean Council is investing in new techniques to help farmers make the most of their time in the field.
“We’re looking into passive trapping devices for stink bugs that can indicate which fields are at risk so scouts can better focus their efforts,” said Tilmon. She’s also involved in multistate research to simplify and standardize defoliation scouting and threshold guidelines across the region.
Keith Kemp, NCSRP secretary-treasurer and farmer from West Manchester, has dealt with corn rootworms, stink bugs and Mexican bean beetles in the past and knows it’s only a matter of time before pests show up in Ohio fields.
“Every year, we’re getting more pests with new ones moving in our direction, so it’s important to stay on top of what’s coming into our area. We appreciate the university folks keeping us informed about how to manage them and giving us options to help avoid insecticide resistance,” Kemp stated.
As you head out to the fields to scout, Tilmon offers these tips for identifying and treating three types of Ohio’s most damaging pests.
Several species of stink bugs are found in Ohio soybeans, including green, brown, red-shouldered and the brown marmorated stink bug. Nymphs and adults feed on the developing seed by using their piercing/sucking mouthparts to poke through the pod.
Damaged seeds look flat or shriveled and can also be discolored. It’s common for farmers to have significant damage, but not realize it until harvest.
Start scouting for stink bugs when soybeans begin putting on pods. This typically goes from the time flowering ends until pods are beginning to yellow. To sample for stink bugs, take 10-sweep samples with a sweep net in multiple locations throughout the field. Average the number of stink bugs in the 10-sweep samples. The threshold to treat grain soybeans is four stink bugs and, for seed or food-grade soybeans, that threshold drops to two.
Damage can be prevented by treating with most insecticides labeled for soybeans. Keep in mind it is easier to kill nymphs than adults.
DEFOLIATORS AND POD FEEDERS
Several insect species feed on soybean foliage throughout the summer at different times, including caterpillars such as green cloverworms and soybean loopers, Japanese beetles, bean leaf beetles, Mexican bean beetles and grasshoppers.
While it might sound difficult to keep up, growers can use a generalized defoliation threshold and make treatment decisions based on the overall leaf material lost, regardless of species. In Ohio, Tilmon and team recommend treating when overall defoliation in the field averages 30% in the vegetative growth stages i.e., before bloom and 20% from bloom to full maturity.
“It’s important to make this decision based on the field as a whole, not just the edges where feeding is usually heavier. Growers should also look at the whole plant when deciding since the upper leaves are often more damaged,” explained Tilmon. “Once the defoliation threshold is reached, scout to identify the most common defoliators and choose a product based on the pest.”
Slugs do most of their damage in late spring/early summer when plants are small and are detrimental to both corn and soybean. They tend to build up in a field over time and feed on plant leaves or seeds and sprouts when the furrow is left open.
According to Tilmon, there’s no treatment threshold for slugs so it’s up to the grower to determine when and how to treat.
“Baited pellets formulated with metaldehyde or iron phosphate are one way to help, but they don’t provide a total knockdown,” said Tilmon. “Studies have shown that ground beetles, a type of predatory insect common in agricultural fields, are effective slug predators.”