Nearly all Minnesota deer exposed to pesticides linked to pollinator die-off

The pesticides linked to bee, butterfly and pollinator deaths across the nation are being found in the organs of far more of Minnesota's wild deer, and in higher concentrations, than previously thought.

State biologists found neonicotinoids in nearly all — 94% — of deer spleens collected from road kill and sent in by hunters last fall. Alarmingly, roughly two-thirds of those deer had higher concentrations of the chemicals than a threshold found to potentially lower fawn survival and cause bone and genital deformities in a captive deer study.

It's too early to tell if the pesticides are harming wild deer, causing fawn fatalities or affecting survival rates, scientists said. But they say it is a possibility and more research is needed.

"What this is telling us is that exposure is ubiquitous," said Michelle Carstensen, wildlife health group leader of the state Department of Natural Resources (DNR).

Neonicotinoids began to dominate the insecticide market in the early 2000s after they appeared safer for humans and mammals than previous insecticides. The chemicals, made of a synthetic nicotine, act as a neurotoxin on insects.

After growing evidence that neonicotinoids contributed to massive die-offs of honey bees and other pollinators, the European Union quickly banned them.

North America, however, embraced them. They're now used on 98% of the corn, soybean, wheat and cotton growing on the continent, according to the DNR. They're also used in lawn care and common household products such as flea and tick prevention collars for pets.

As their use has grown, so to have concerns about potential harm to mammals, birds and other wildlife.

Jonathan Jenks, a biologist and professor with South Dakota State University, has been leading studies of their effects on captive deer and pheasants for several years. In 2019, he led a project that found that fawns with higher concentrations of neonicotinoids in their spleens — at .33 nanograms per gram — were far more likely to die than those with lower levels.

About 64% of Minnesota wild deer spleens collected by the DNR had concentrations higher than that.

The South Dakota study also found that pheasants that were fed high levels of seeds treated with the chemicals hatched half the number of chicks as those that did not eat those seeds. The chicks that the infected birds did manage to hatch were also 20% less likely to survive, Jenks found.

He cautioned that the sample size was relatively small and that there are a host of variables that could lead to mortalities in a captive study.

"But it's safe to say that the hatch percentage was low, chick survival was low and the number of chicks hatched was low," Jenks said.

The pheasant finding is alarming because it "echoes the historical impacts of DDT," said state Rep. Rick Hansen, DFL-South St. Paul, who leads House environmental committees.

DDT is a now-banned pesticide that mangled the reproduction of bald eagles, osprey and other raptors, helping to push them to the brink of extinction.

"We know there are devastating impacts of neonicotinoids on the environment and wildlife," Hansen said. "But there has been a willful ignorance to ignore this. At some point policy makers need to take action."

The DNR's findings on Minnesota's wild deer spleens surprised researchers because deer taken in the thick woods of northern Minnesota were just as likely to have neonicotinoids in their systems as those taken among the vast corn and soybean fields of southern Minnesota. It isn't clear exactly how the chemicals are getting into the animals, whether it's through the water they drink or from directly eating treated seeds or plants.

"There was a little bit of a 'wow' factor when we found deer in the Boundary Waters with neonics," Carstensen said. "How does that happen? It's moving in ways we don't understand."

A number of basic questions about how the chemicals behave in the environment are still unanswered, said Eric Michel, DNR research scientist.

"We don't know necessarily how long they last in the deer's system, if they metabolize it quickly or how it's being stored or where it's being stored in the body," he said.

All the DNR's spleen samples were taken in the fall, during hunting season. Neonicotinoid exposure would theoretically be much higher in the spring and summer, when crops and backyard gardens are being planted or sprayed. Michel and the DNR would like to do more surveys that collect spleen and other tissue samples year-round to get a better understanding of when neonicontinoid levels peak and how quickly they move through an animal's system.

Minnesota soybean growers have been moving away from neonicotinoids in recent years, said Joe Smentek, executive director of the Minnesota Soybean Growers Association.

Research out of the University of Minnesota showed that, in many cases, the chemicals weren't effective enough against soybean pests to be worth the cost.

"We had some companies where farmers had to buy seeds treated with neonics, but they stopped that at our insistence because farmers don't want to pay for something that they're not going to use," Smentek said.

The state's deer population as a whole seems healthy — the estimated number of deer has been stable for years. But if fewer fawns are surviving, it could change the way the DNR calculates its population estimates and harvest quotas.

Work needs to be done to find out if the chemicals are directly impacting deer and pheasants in the wild, Jenks said.

"We need to evaluate wise use of these chemicals to make sure they're not having a significant impact on wild populations," he said.