Scouting vital to managing soybean pests
Soybeans are the top crop in Virginia and the second-largest crop, after corn, in Delaware and Maryland.
Pest management is critical to the soybean crop in all three states and regional soybean and pest experts say routine crop scouting and targeted treatment are key components.
Corn earworms, stink bugs, and soybean loopers are among the pests that soybean farmers typically need to deal with in Delmarva.
“The corn earworm is the number one pest year-to-year for the soybean plant in the Mid-Atlantic states,” said Dr. David Holshouser, Extension soybean agronomist at Virginia Tech. Corn earworms feed on the pods of soybean plants, while stink bugs pierce the pods to eat the seeds produced by the soybean plants. Soybean loopers eat the leaves of soybean plants.
“There are days you can walk into a field of soybeans and it will be abuzz with noise,” said Dr. Holshouser. “Many of the insects are beneficial to soybean plants. Spiders, for example.”
One method to determine whether insects and pests are a substantial enough problem to consider using insecticide is to sweep the fields. “Usually, farmers will find both corn earworms and stink bugs in these sweeps,” said Dr. Holshouser. Decision making should be based on multiple sweeps of the soybean fields. The threshold on whether to use insecticides is generally dependent on the number of pests found in the sweeps.
“Don’t spray because of marketing from insecticide businesses,” Holshouser said. “Indiscriminate use of an insecticide can cause targeted insects to become resistant to the insecticide. It is critical to only use insecticides when necessary to maintain a crop in an economic manner.
“We’re not saying not to spray insecticide, but just do so in a careful fashion … broad-spectrum pesticides can present major problems.”
“David Owens, Extension entomology specialist for Delaware Cooperative Extension breaks down the soybean growing season into three time periods; early, mid- and late season, “each with their own suite of pests.”
In the early season, growers contended with seedcorn maggots which are attracted to organic matter in the fields — manure, or a previous crop residue.
“We had fields with heavy seedcorn maggot and slug damage this year,” Owens said. “The primary guidance is to wait at least three weeks from incorporation to planting. Seedcorn maggot is usually only a potential threat to early planted full season beans. Insecticidal seed treatments are available, but can be overwhelmed if pressure is high enough, and are probably not necessary at all for double crop soybean.”
As for slugs, Dr. Kelly Hamby, Assistant Professor at the College of Agriculture and Natural Resources of the University of Maryland said older plants are better able to withstand the pests, so employing cultural practices for speedy growth is encouraged.
“Consider hybrids with excellent emergence and early season vigor ratings. Delay planting until soil temperatures favor rapid germination,” Hamby said. “Use surface residue management devices (trash wheels, row cleaners) on the planter to form a 10-12-inch band of relatively clean soil in the planting row and consider using starter fertilizer. Get good seed-to-soil contact by not planting into wet soil where the seed slot may not close; this restricts slug feeding to above ground and reduces damage.”
Owens said mid-season pest levels were “fairly quiet” this year, noting some spider mite pressure around field edges, but several types of pests can affect soybean plants on Delmarva during the late season.
“Corn earworm typically affects Sussex County and a little bit into Kent [County],” he said. “It can be sporadic, not every field and not every year is going to have treatable numbers. Double crop soybeans are at the greatest risk….We had some fields with higher than normal populations of bean leaf beetle. It is primarily a defoliator but will scar pods, allowing pathogens to enter. We have two generations per year, the last of which typically peaks in early September.”
While an individual farmer may strive to keep their fields cleared of most pests, “farmers realize that some of the pests fly into fields of soybeans,” said Dr. Holshouser.
One example of such a pest is the Dectes stem borer. This beetle “cannot be controlled with foliar insecticide applications because adults emerge over a relatively long period of time and can fly into the field from surrounding areas,” said Hamby. “Fields with heavy pressure should be harvested as soon as possible to avoid losses due to lodging.”
(The Delmarva Jordbruk Chronicles is a news column that details agriculture in Delaware and the Eastern Shore of Maryland and Virginia. “Jordbruk” is Swedish for “Agriculture.” Please contact Richard McDonough at firstname.lastname@example.org. © 2021 Richard McDonough.)