These widely used insecticides may be a threat to mammals too

Neonicotinoids are already accused of contributing to widespread insect declines. But there’s evidence they can also harm rabbits, birds, and deer.

These corn seeds are treated with the neonicotinoid pesticide clothianidin. Neonics, linked to insect declines, are also being found in larger animals, like deer and birds.
Photograph by Anand Varma, Nat Geo Image Collection
This story was produced in collaboration with the Food & Environment Reporting Network, a nonprofit investigative news organization.

On an overcast January day in Estelline, South Dakota, Jonathan Lundgren zips his quilted jacket over a fleece, pulls down a wool cap, and crunches through the snow on Blue Dasher Farm to his barn, a milking parlor that he has kitted out as a biochemical laboratory.

Lundgren is an unusual hybrid: a working farmer interested in reforming that profession, and a working scientist, a former U.S. Department of Agriculture entomologist who still does chemical analysis. Surrounded by the usual lab paraphernalia—a spectrophotometer, a PCR machine, a centrifuge—Lundgren glances out the window at the sheep huddled in his pasture and a large flock of geese, chickens, turkeys, and ducks. Then he turns to the deer spleens in front of him. For months, he’s been analyzing them for traces of insecticides called neonicotinoids.

Chemically related to nicotine, neonics, as they’re known, were developed in the 1990s as a safer alternative to more toxic, longer-lasting farm chemicals. They’re now the most widely used pesticides in the world, effective against aphids and leafhoppers and a wide range of worms, beetles, and borers. Deployed as coatings on seeds for crops that cover more than 150 million acres in the United States, neonics are taken up by all plant parts: roots, stems, leaves, fruit, pollen, and nectar. Insects chew or suck on their preferred portion, then curl up and die.

History tells us that such broad-spectrum pesticides may have unintended consequences, and scores of studies suggest that neonics, along with climate change and habitat destruction, are contributing to the steady decline of insects across North America and Europe. Bees, essential for crop pollination, have been especially hard hit.

(Read a National Geographic cover story on insect declines.)

The evidence of harm is strong enough that the European Union has banned outdoor use of three popular neonics. And while the U.S. hasn’t yet taken such decisive action, it’s becoming increasingly clear that bees and other beneficial insects aren’t the only animals at risk.

Over the past several years, scientists have found that only about 5 percent of neonic seed coatings are taken up by crop plants. The rest washes or wears off seeds. The chemicals accumulate in soils and waterways, where a wide range of wildlife is exposed to them. Evidence is growing that compounds tailored to take out invertebrates can also harm mammals, birds, and fish.

In his barn this winter, Lundgren has been compiling some of the newest evidence—data suggesting that a significant number of wild deer in the upper Midwest have neonics in their spleens.

A singular experiment

One of the first signs that neonics can affect large animals came from another study Lundgren worked on, also involving deer—but captive ones this time.

In 2015, a team of scientists at South Dakota State University set out to determine how a neonic called imidacloprid—which is used on corn, soy, wheat, and cotton—might affect large herbivores. The scientists ran a first-of-its-kind experiment on a captive herd of white-tailed deer, consisting of 21 adult females and 63 fawns born to those females during the course of the experiment. Graduate student Elise Hughes Berheim and wildlife ecologist Jonathan Jenks mixed imidacloprid at a range of doses into the animals’ water.

When they euthanized the herd after two years, the researchers found that animals with higher levels of the pesticide in their spleens had shorter jawbones, decreased body weight, and undersized organs, including genitals. More than a third of the fawns died prematurely, and those fawns had much higher spleen levels of imidacloprid than the survivors. Both fawns and adults with higher levels had been less active while alive—which in the wild would have made them more vulnerable to predators.

Some of the deer had gotten doses of imidacloprid far higher than any yet reported in natural streams or wetlands. But the team also examined the spleens of wild deer collected, over an eight-year period, by North Dakota game officials. Jenks was surprised to find they contained imidacloprid at levels more than three times higher than those that produced abnormalities in his captive herd. He surmised the wild animals had been contaminated by their forage plants or water.

Published in Scientific Reports in March of 2019, the results were big news for anyone who managed or hunted game around farmland, and for anyone concerned about the impacts of farm chemicals on wildlife. After all, animals with malformed jaws and undersized reproductive organs may have trouble eating or breeding. “Neonics could have a catastrophic effect on white-tailed deer populations,” says Jennifer Sass, a senior scientist with the Natural Resources Defense Council, an environmental group.

Five manufacturers in Europe, the U.S., and Japan dominate the neonic market. Bayer CropScience, formed when the German pharmaceutical company took over Monsanto, is one of the world’s largest makers of neonicotinoids, and it is the primary manufacturer of imidacloprid. Bayer spokesman Alexander Hennig dismissed the South Dakota white-tailed deer study as “unreliable.”

“None of the effects mentioned have been reported in deer populations in the wild,” Hennig wrote in an email. “Many veterinary uses are approved which allow the direct application of neonicotinoids to pets and livestock to protect them from fleas and ticks. This would not be possible if we, or regulators, determined there was a risk to vertebrates.”

According to Hennig, one of the reasons neonics were developed as insecticides is precisely because they don’t affect vertebrates in the same way—they attach to cell-surface receptors that are much less prevalent in vertebrates.

Growing evidence

Deer aren’t the only species inadvertently consuming neonics. Charlotte Roy, a biologist with the Minnesota Department of Natural Resources, has found that many kinds of animals will gladly consume neonic-treated seeds when they get a chance—as they do during spring planting.

In a 2019 study, Roy set up camera traps in agricultural fields where she had deliberately spilled treated seed. Her motion-triggered cameras recorded more than a dozen bird species (including ring-necked pheasants, geese, and turkeys), plus bears, raccoons, rodents, rabbits, foxes, and skunks, all feeding on the treated seed.

Accidental spills of seeds from mechanized planters happen frequently, according to Roy. Farmers are instructed by seed companies to clean them up, but small mounds of grain, containing thousands of seeds, are common. Roy and her colleagues estimate that tens of thousands of spills occur annually across Minnesota.

Exactly how neonic-coated seeds affect growth, development, and organ function of vertebrates remains an open question. But evidence of harm is accumulating.

Researchers in Canada have shown that consuming as few as four imidacloprid-treated canola seeds over three days can interfere with a sparrow’s ability to migrate. A graduate student at South Dakota State last year demonstrated that ring-necked pheasants—the number-one game animal in the Dakotas—became more underweight, weak, and lethargic the more treated corn seeds they consumed. (According to the researcher, the birds were fed fewer treated seeds than they’ve been observed to eat in the wild.) Higher-dosed birds also laid fewer eggs, started their nests a week later, and suffered a 20 percent decline in chick survival.

Lab studies have reported a slew of evidence that exposure to neonics is harmful to vertebrate animals. It reduces sperm production and increases abortions and skeletal abnormalities in rats; suppresses the immune response of mice and the sexual function of Italian male wall lizards; impairs mobility of tadpoles; increases miscarriage and premature birth in rabbits; and reduces survival of red-legged partridges, both adults and chicks.

In Japan, scientists linked the collapse of a lucrative fishery to the widespread adoption of imidacloprid on nearby paddies and farm fields.

Last year, Eric Michel, an ungulate research scientist at the Minnesota Department of Natural Resources (and a coauthor of the white-tailed deer paper), put out a call for the spleens of hunter-killed deer. His aim was to learn more about the presence or absence of neonics in Minnesota animals, to help set limits on doe-hunting permits. “Anything that affects population dynamics, we want to know,” he says.

Nearly 800 spleens eventually were delivered to Lundgren for chemical analysis. Preliminary results suggested that more than 50 percent of the spleens were positive for neonics; Lundgren is currently rerunning those samples to double-check his work.

As a companion study, Lundgren is also analyzing the spleens of 100 river otters, bobcats, and fishers—top predators that had been trapped, legally, in North Dakota. His preliminary results suggest neonics contaminated between 15 and 30 percent of the samples. The animals could have consumed the pesticides in contaminated plants, prey, or water, he says.

Lundgren’s findings don’t surprise him at all; he’s convinced that pesticides are having a significant effect on biodiversity globally. “We’ve been seeing the deterioration of biological communities for quite some time. Clearly we are not fully understanding the implications of these pesticides.”

Asked to respond to studies that suggest neonics can harm vertebrates, CropLife America, a trade association representing the makers and distributors of pesticides, stated: “Based on various conclusive studies performed around the world, neonicotinoids are proven to be effective in controlling harmful insects in agricultural and non-agricultural settings with no unreasonable adverse effects on nontarget organisms when used according to label instructions.”

What about humans?

Humans are exposed to neonics too, of course. We accidentally inhale the stuff or touch treated surfaces on farms, in gardens, and when applying flea-and-tick treatments to our pets. During the past decade, the U.S. Environmental Protection Agency has recorded more than 1,600 cases of human imidacloprid poisoning. Symptoms ranged from rashes, headaches, and wheezing to memory loss and renal failure.

But people also consume neonics in their food. The pesticides are routinely applied—often as leaf sprays or soil treatments—to cauliflower, spinach, apples, grapes, squash, melons, tomatoes, and other produce and grains. Almost 100 percent of corn in the U.S. is treated with neonics. A 2015 study by the American Bird Conservancy and Harvard University’s T.H. Chan School of Public Health found neonic residues—albeit at levels the EPA deems acceptable—in almost every dish served at cafeterias in U.S. Congressional buildings. A 2019 National Institutes of Health study found neonics in 49.1 percent of 3,038 human urine samples.

There is no direct evidence so far that dietary exposure to neonics causes harm to humans.

The EPA is currently reviewing the registrations of five neonics, including imidacloprid. Environmental organizations and human-health experts claim the agency’s ongoing analyses have consistently underestimated the costs of neonic use and overestimated the benefits. These groups have called for EPA to cancel or severely restrict many neonic uses and disallow their presence in food, which effectively would prevent their use on food crops. (Organic growers don’t use neonics.) After further study, EPA could come up with new, more stringent tolerance levels, which would allow some agricultural use.

Clearly, more research on how neonics potentially affect vertebrates is needed. But field studies of animals are vanishingly rare because they take a great deal of time, effort, and money. Few states support such research the way Minnesota and South Dakota have. Empirical data is hard to come by. Wild animals that show signs of poisoning, writes environmental toxicologist Pierre Mineau, a former senior scientist with Environment Canada, “run a high risk of predation or demise”—that is, they die without a trace. Wildlife rehabilitators and game wardens often do encounter malformed animals, but they lack the resources to study them scientifically.

Meanwhile, neonic-treated seeds are a $1.5-billion global market that the industry has a strong interest in protecting. After the peer-reviewed white-tailed deer paper was published, says Lundgren, an anonymous (to him) seed-company accused the research team of misconduct and falsifying data. An investigation by the university, South Dakota State, found the complaint had no merit.

“I think they just wanted to harass us,” Lundgren says. “It was disruptive to our work, though, and our credibility is very important to us.”

Neonics are very good at killing crop pests, but studies have shown they don’t necessarily increase yields of soybeans or corn, and they may reduce farmers’ profits by increasing their costs. The European Union has banned all outdoor use of three major neonics, including imidacloprid, to protect pollinators (although farmers continue to apply for “emergency exemptions” during pest outbreaks). Canada is considering a similar ban, and scores of bills to restrict or ban neonic use have been introduced in U.S. statehouses in the last two years alone.

Federal bills to limit these compounds have stalled in recent years, and environmental advocates doubt the Biden Administration will prioritize neonic regulation (although it does plan to reexamine the approval of chlorpyrifos, a highly toxic, non-neonicotinoid pesticide). The incoming Secretary of Agriculture, Tom Vilsack, also led the USDA during the Obama Administration; during that time neonic use by farmers increased.

If the USDA continues to promote the current system of coating genetically engineered seeds with herbicides, says Willa Childress, an organizer with the Pesticide Action Network North America, “it will continue to drive neonic use up, unless EPA intervenes and says no.”

A more holistic approach

Lundgren, in South Dakota, avoids the weediness of the regulatory landscape to focus on larger issues. Besides studying animal tissue and soil chemistry, his Blue Dasher Farm also develops, evaluates, and teaches ecologically based—and economically profitable—agricultural practices to farmers and ranchers across the nation.

Those practices fall under the heading of “regenerative agriculture,” because they’re aimed at restoring degraded soil to a natural, healthy, uncontaminated state. For Lundgren, neonics are a symptom of a bigger problem: industrial agriculture’s general reliance on chemical inputs that contaminate waterways and reduce the health and biodiversity of the soil.

“Banning neonics isn’t going to solve the underlying issues with our food production system,” he says. “Our work on regenerative cropping and livestock systems”—which includes tilling less, planting cover crops, and promoting beneficial insects and more diverse crop rotations—“is showing that insecticides are really not needed.”

“Change isn’t coming from the government,” Lundgren continues, “but from the grassroots. Regenerative ag is gaining momentum at an astounding rate. I look to that as a real sign of hope.”

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