Dawn Georgieff
Every spring, mayflies rise from the Mississippi River in the billions, a swarm so large it once showed up on weather radar. That swarm has shrunk by more than half since 2012. The mayflies aren't just disappearing on their own. This is an early signal of a much bigger problem moving through Minnesota's water, soil, and food chains: neonicotinoid pesticides.
In February 2020, researchers led by Phillip Stepanian published a study based on radar-detected mayfly emergence flights in the Upper Mississippi River and the Western Lake Erie Basin. Combined with river bottom sampling from 1992–2002, the study found that mayfly production declined by over 50 percent between 2012 and 2019.
Several human activities contribute to this decline. But the study's authors point to one factor in particular: pesticide concentrations are rising in freshwater ecosystems, and Hexagenia mayflies are among the aquatic insects most sensitive to neonicotinoids.
Even at low levels, these pesticides make mayflies more vulnerable to low-oxygen conditions, reduce their overall fitness, and leave them more exposed to predators. Because pesticide residue settles into sediment at even higher concentrations than in the water itself, mayfly nymphs, which feed on detritus, absorb the chemicals from both the water and the sediment they live in.
The stakes go well beyond mayflies. This annual event transfers thousands of tons of biomass into the surrounding landscape and insect-eating birds that rely on this food to raise their young.
What are neonicotinoids?
Neonicotinoids (neonics) are pesticides developed as neurotoxins for insects. They are now the most widely used pesticides in the world, with their use expanding rapidly since the early 2000s. Once absorbed by a plant, every part of that plant becomes toxic to insects.
In 2020, the Minnesota Department of Agriculture (MDA) estimated that 40,000 kilograms of neonics were sold for agricultural use in Minnesota and were subsequently applied to 12 million acres of land across the state. Of the neonics used, 90% comes from coated seeds used in row crops. Nearly all Minnesota corn seed, and about 45% of soy seed, is coated with neonics.
Yet only 5% of the neonic pesticide on coated seeds is absorbed into the plant itself. The other 95% goes into the soil, surface water, and groundwater. In fact, the MDA found neonics in 95% of the surface waters that it sampled regularly between 2011 and 2022. Neonics are also found in 10–41% of the springs and wells in Minnesota (i.e., our drinking water). In Dakota County, for example, they’re found in 25% of wells. Because they are water-soluble and persistent in soil, they simply wait for the next heavy rain to wash into our waterways and aquifers.
A major cause of biodiversity decline
Put bluntly, neonics are a leading cause of species decline. The Environmental Protection Agency (EPA) has found that neonics potentially jeopardize 200 endangered species, and they’re a primary source of harm to pollinators like bees, butterflies, and beetles.
Numerous studies link neonics to weakened hive reproduction and survival in honeybees, along with impaired worker memory and foraging ability. The decline of aquatic insects exposed to neonics also has a cascading effect on the fish that depend on them for food (including trout, an important sport fish in Minnesota).
Birds are affected too. Pesticide use is one of the major causes of the 29% plunge in the US bird population since 1970. Neonics fuel a decrease in terrestrial insects, which in turn lessens the food supply for insect-eating birds. In addition, neonic-treated seeds are toxic to birds that eat spilled seed piles from planted fields.
Toxic to humans, too
Neonics were originally believed to be harmless to mammals. Recent research tells a different story.
Neonics are toxic to insects by binding to a neuroreceptor (nAChR) present in all insects, causing stimulation of the nerve cell and ultimately death. Unfortunately, neonics and some of their metabolites also bind to the nAChR receptor in people. Studies have linked neonic exposure to reproductive, metabolic, and endocrine harm.
Pregnant women exposed to neonics show significantly higher rates of low birth weight, preterm delivery, reduced infant head size, anencephaly (a fatal neural-tube disorder), and a heart defect called Tetralogy of Fallot. Exposure during pregnancy has also been linked to disrupted brain development. Lab studies on the offspring of pregnant rodents exposed to neonics show thinning of brain tissue at multiple sites, along with learning and behavioral deficits. In humans, prenatal neonic exposure has been associated with a higher risk of autism, lower neurodevelopmental scores, hyperactivity, and emotional symptoms in children.
Do neonic-coated seeds even work?
For fruit and vegetable crops, neonics have generally proven effective as a pesticide. But the case for coating all corn and soy seed is much weaker.
Field trials comparing neonic-coated corn seed to untreated seed or other pest-control methods found no significant yield increase in 83–97 percent of cases. Comparable trials on soy seed found no yield benefit in 82–95 percent of cases.
What should be done?
Neonic-coated seed should be limited to situations where it's actually needed. Right now, it’s difficult for farmers to even find untreated seed if they want it.
Several states have adopted "verification of need" programs, where a state agency determines whether neonics are necessary for a given field before they can be used. Paired with access to untreated seed, this approach would benefit both farmers and the environment. A bill requiring the MDA to implement such a program was introduced in the last Minnesota legislative session.
Just as important, neonic-coated seeds need real regulatory oversight. The Minnesota Pesticide Control Act tasks the MDA with regulating pesticides in the state. But because the EPA exempts itself from regulating "treated articles"—a category that includes pesticide-coated seeds—the MDA has used that same loophole to avoid regulating neonics, too. In doing so, the agency is shirking its responsibility to Minnesotans, who have effectively become unwitting participants in a study they did not consent to.
Spread the word!
Share this article with everyone you know who cares about people and the planet. Download and print this 2-page neonic info sheet to share with your legislators.