By Gil Gullickson
Plenty of predators prepare to pounce on corn and soybean seeds once they’re planted into a sea of soil.
Besides diseases, subterranean insects such as wireworms and white grubs can ravage corn seed and seedlings. Meanwhile, aboveground insects, such as bean leaf beetles, can shred soybean plants.
Enter insecticidal seed treatments (ISTs). Farmers team these with fungicide seed treatments to help ensure excellent stands and early-season growth.
Some ISTs belong to the neonicotinoid family, which includes imidacloprid (Gaucho, Bayer Crop Science), clothianidin (Poncho, BASF), and thiamethoxam (Cruiser, Syngenta).
Neonicotinoid insecticides are highly water soluble, which helps trigger excellent early-season insect protection, says Seth Naeve, a University of Minnesota Extension soybean agronomist.
“They’re highly systemic,” he says. “When plants take up water, they also pull up the insecticide.”
Seed treatment growth has partially been fueled by earlier planting, says Shawn Potter, head of seedcare product marketing for Syngenta.
“Planting earlier into cool, wet, or cool and wet soils exposes seed and seedlings to different pests, whether it be diseases or insects,” he says. “Without seed treatments, you can lose stand count and early-season growth.”
“Operationally, they’re easy to use from the farmer’s standpoint,” adds Nick Tinsley, a BASF technical field representative. Almost all corn is treated with a neonicotinoid insecticide and fungicide seed treatment by companies. Meanwhile, retailers handle the seed treatment on the soybean side.
Seed companies have treated corn with fungicides for decades. ISTs, though, just started being added to fungicide seed treatments in soybeans in the early 2000s.
Industry studies typically find that a fungicide/insecticide seed treatment combo increases soybean yields 2 bushels per acre. Generally, a soybean fungicide/insecticide treatment will cost $10 to $12 per 140,000 seed unit, says Potter. (Soybean seeding rates vary but can range between 80,000 and 140,000 seeds per acre.)
Because 70% of the treated cost is due to the insecticide, Potter pegs a $16 to $17 per acre return just from the insecticide application. This assumes soybeans at a $12 per bushel price and the seed treatment insecticide portion expense of $7 to $7.50 per unit.
Entomologists in the Mid-South and South have also observed an average yield spike of 2 bushels per acre from the combination in soybeans.
“Some years are higher, some years are lower,” says Sebe Brown, a Louisiana State University Extension entomologist. Responses tend to be higher in cases when soybeans are planted early, double-cropped, or planted after a cover crop, he says.
“They provide good insurance to the grower to maintain a good stand,” adds Gus Lorenz, University of Arkansas Extension entomologist. “With today’s seed prices, the cost of an insecticidal seed treatment is a good investment.”
A compilation of studies in a 2019 paper representing agronomists and entomologists from 14 land-grant universities found an average soybean yield benefit of 2 bushels per acre occurred from teaming a neonicotinoid IST with a fungicide seed treatment. At best, a yield bump of 3.3 bushels per acre occurred. In some cases, the benefit was nearly nil, bottoming out with a slim 0.2 bushels per acre.
However, a partial economic analysis showed inconsistent evidence of a break-even cost of a fungicide-only or a fungicide/insecticidal seed treatment. Thus, widespread prophylactic use is not recommended on soybeans, according to these university and industry agronomists.
Naeve doesn’t discourage Minnesota farmers from using seed treatments. However, he advises farmers to apply them selectively.
“If you’re going to treat, give the soybeans you plant early more priority,” he says. “This improves the chances of a positive return on investment.”
Neonicotinoid insecticides have been linked to detrimental impacts on pollinators. However, long-term Syngenta studies have shown a negligible impact on bee health through use of neonicotinoid insecticidal seed treatments, says Potter.
“What most experts agree on is that bee health depends on an interaction of factors like parasites, disease, nutritional health, and weather events,” he says.
A study reported in 2019 in Scientific Reports detailed the impacts that imidacloprid concentrations had upon white-tailed deer. As imidacloprid increased in the animal’s spleen, factors such as fawn survival, jawbone lengths, body weight, and organ weights decreased. Subsequent examinations made by the Ecdysis Foundation at Blue Dasher Farm, Estelline, South Dakota, show at least 60% of hunter-killed white-tailed deer show presence of imidacloprid. Such concentrations can lead to increased mortality rates and other negative impacts, according to Jonathan Lundgren, director of the Ecdysis Foundation and CEO of Blue Dasher Farm.
“All insecticides can negatively impact mammals and bees,” says Lundgren. “It’s just that the use of these insecticides is so pervasive. They’re on most field cropping systems throughout the United States, and they do stick around. These insecticides are affecting ecosystems in ways that we just did not predict.”
Companies that sell neonicotinoid insecticidal seed treatments disagree. Syngenta officials, for example, say the studies examine just imidacloprid and no other neonicotinoid insecticides. They add that the study detailed in Scientific Reports shows a correlation between imidacloprid concentration in spleens of fawns and survival, but little else. Syngenta officials say the study in Scientific Reports has flaws, such as higher imidacloprid exposure rate concentrations than white-tailed deer would likely encounter in the wild.
Neonicotinoid insecticide manufacturers say the products are safe if used according to label instructions. Compounds, such as polymers that coat the seed, team with products like Bayer’s Fluency Agent Advanced to help prevent environmental escapes, says Chip Graham, Bayer Crop Science North America seed growth technical manager.
“We have really low levels of dust to begin with, but these products ensure that the insecticide stays on the seed and reduce dust levels during planting,” he says.
Instead of neonicotinoid insecticidal seed treatments, Lundgren calls for regenerative practices, such as slashing tillage and increasing crop diversity, which work better and also reduce insect pressure. “Farmers need to understand that these seed treatments really aren’t helping them,” says Lundgren.
Banning them, though, would disadvantage farmers, says Graham. “There are no foliar sprays to treat them [early-season insects] as a rescue application,” he says. Seed treatments are the most effective and efficient way to provide activity against
early-season insects in corn and soybeans.”
“They’re a classic example in that the best tools we have in agriculture are the ones that we have to manage the most carefully,” Naeve says. “Treating all soybean acres with them is just not a good idea. They can be a double-edged sword.”