More than half of all U.S. soybean farmers use seeds coated with an insecticide meant to protect the plant from its ­biggest foes.

But a new report from a dozen public universities, including the University of Minnesota, says the coatings are providing few if any benefits in most cases, while raising expenses for farmers and affecting the surrounding environment in negative ways. A 2014 Environmental Protection Agency report also raised similar questions about the economic benefits of the coated seeds.

The insecticides in question are neonicotinoids, a family of compounds sold during the past decade that have been implicated in disrupting honeybee and other pollinator populations and are now being reassessed by the EPA.

"There's a lot more of this neonicotinoid treated seed going out there than would be justified by pests," said Christian Krupke, a Purdue University entomologist and one of the publication's co-authors. "The use rates are not at all aligned with the pest threat level."

Krupke said that he and colleagues at other Midwest universities have Extension appointments, and speak with thousands of farmers each year who are confused about whether the seed treatments are effective against the crop's key pest, the soybean aphid. Last year, growers planted 82.6 million acres of soybeans — 7.6 million in Minnesota, where it is the second largest crop, according to the U. S. Department of Agriculture.

Industry officials defended the performance of insecticide coatings, and strongly object to the report's conclusions, as well as the EPA guidance.

"There are many things in the paper that I disagree with or are misquoted and are not accurate," said Palle Pedersen, head of product marketing for Syngenta's Seedcare business. He said the Extension publication is also incomplete because it does not acknowledge other research that shows how seed treatments have increased yields and profits, compared to untreated seeds.

The USDA estimates that sales of neonicotinoid seed coatings for soybean crops exceeded $400 million in 2013 and the coated-seed market for all crops is more than $1 billion.

The coatings are sprayed onto soybean seeds, often with a fungicide and sometimes with other coatings, shortly before growers pick up the seeds each spring. They're a convenient way for producers to plant seeds and protect a crop from its earliest days as the soybeans germinate, take root and grow.

The insecticide coating is water soluble, so its active ingredient is taken up through the roots, making the plant effectively toxic to pests like aphids or beetles that munch on its leaves.

The problem, according to the report, is that the insecticide levels in the soybean plant drop off quickly to ineffective levels, between two and three weeks after spring planting. That's far too early to protect the crop against its worst enemy — the soybean aphid — that typically doesn't arrive in large numbers until midsummer and peaks in late July and August, researchers said.

Because of that time gap, Krupke said, a grower's chances to control soybean aphids with seed coatings "are slim to none in most ­situations."

Syngenta's Pedersen said other research has shown the coated soybean seeds to be effective against aphids and other insect populations for up to 60 days. He also said the seed coatings are more economical and far less risky to the environment than spraying with foliar insecticides, which can contaminate nearby water and kill "non-target" beneficial insects and pollinators.

"There's a reason why these products have been so successful and why farmers like them so much," Pedersen said.

A co-author, University of Minnesota entomologist Bob Koch, said neonicotinoid coatings are effective in suppressing a few insects other than soybean aphids, but those are rarely a problem for farmers in northern states where most of the nation's soybeans are grown.

The insecticide is an important tool for high-risk pest situations, he said, but it doesn't need to be used on millions of acres. "When we're putting a treatment down and there's not a pest there, it doesn't make sense and it's a wasted investment," Koch said.

The report is a summary of more than two dozen peer-reviewed studies and academic publications, and is co-authored by 17 scientists from public universities in Iowa, Kansas, Nebraska, Michigan, Minnesota, Missouri, Indiana, Ohio, Pennsylvania, Wisconsin, North Dakota and South Dakota. It focuses only on seed coatings, not on other uses of neonicotinoids in granular and spray formulations.

The paper's intent is to provide growers and other ag professionals with facts to help them make decisions, Koch said, particularly since crop prices have been low and farmers are searching for ways to cut costs. Growers may decide to spend extra for the insecticide-coated seeds just in case pests might become a problem, he said, but they should also consider the unintended consequences of overusing the chemicals. Those include increasing the chances that insects will become resistant to the insecticide, Koch said, and that runoff from fields and dust from planting can spread the neonicotinoids to flowers, other insects and pollinators.

Gene Stoel, who grows corn and soybeans with his family on 1,400 acres near Lake Wilson in southwestern Minnesota, said that insecticides on seed coatings are available in his area under Monsanto, Syngenta and Bayer CropScience brand names. "Sometimes it's being sold as something it's not real effective for, and that's where the farmer has to be careful as to what pest he's targeting," Stoel said.

However, Syngenta's Pedersen said at a cost of about $8 per acre, the insecticide coatings more than pay for themselves in terms of increased yields.

But costs and benefits have become a huge point of contention in the paper and elsewhere. Different researchers have tried to study whether treated seeds provide better yields than untreated seeds, and have come up with a range of conflicting conclusions.

The EPA reviewed research and reported in October 2014 that seed treatments provide "negligible overall benefits to soybean production" in most situations and that they "likely provide $0 in benefits to growers and at most $6 per acre in benefits" compared to spraying pests later in the season.

Syngenta, Monsanto, DuPont, Bayer CropScience and Valent USA Corp. all filed comment letters with the agency, objecting to the conclusions and presenting their own evidence of improved soybean yields and other benefits based on field test results.

David Kee, research director for the Minnesota Soybean Research & Promotion Council, said the Extension paper was balanced and helps clarify confusion for farmers.

In situations with known infestations, the neonicotinoid coatings are effective, he said. But in other cases, many growers have chosen to buy them just in case of problems, sort of like insurance.

"The question is whether our guys can afford that now when the cost may be the difference between profit and loss in an operation," Kee said. "It'll cause people to think more about their decisions, but what those decisions will be, I can't say."

Tom Meersman • 612-673-7388