Pesticides linked to the deaths of bees, butterflies and pollinators nationwide are found in the organs of far more wild Minnesota deer and in higher concentrations than previously thought.
State biologists have found neonicotinoids in nearly all – 94% – of the deer spleens collected from road killings and sent by hunters last fall. Alarmingly, about two-thirds of those deer had concentrations of chemicals higher than a threshold found to potentially reduce fawn survival and cause bone and genital deformities in a captive deer study.
It’s too early to tell whether pesticides are harming wild deer, causing fawn deaths or affecting survival rates, the scientists said. But they say it’s a possibility and more research is needed.
“What this is telling us is that exposure is ubiquitous,” said Michelle Carstensen, leader of the state’s Department of Natural Resources (DNR) wildlife health group.
Neonicotinoids began to dominate the insecticide market in the early 2000s after appearing to be safer for humans and mammals than previous insecticides. The chemicals, made up of a synthetic nicotine, act as a neurotoxin on insects.
After growing evidence that neonicotinoids contributed to the massive deaths of honey bees and other pollinators, the European Union quickly banned them.
North America, however, has embraced them. They are now used on 98 percent of the corn, soy, wheat and cotton growing on the continent, according to the DNR. They are also used in lawn care and common household products such as flea and tick prevention collars for pets.
As their use has grown, so have concerns about potential harm to mammals, birds and other wildlife.
Jonathan Jenks, a biologist and professor at South Dakota State University, has been conducting studies on 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 the spleen – at 0.33 nanograms per gram – were much more likely to die than those with lower levels.
About 64% of the Minnesota wild deer spleens collected by the DNR had concentrations above that.
The South Dakota study also found that pheasants that were fed high levels of chemical treated seeds hatched half as many chicks as those who didn’t eat those seeds. The chicks the infected birds managed to hatch were also 20 percent less likely to survive, Jenks found.
He warned that the sample size was relatively small and that there are a number of variables that could lead to morality in a constrained study.
“But it is safe to say that the hatching rate was low, the survival of the chicks was low, and the number of hatched chicks was low,” Jenks said.
The discovery of the pheasant is alarming because it “echoes the historical impacts of DDT,” said state representative Rick Hansen, DFL-South St. Paul, who leads the House’s environmental committees.
DDT is a now banned pesticide that has mutilated 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 in ignoring this. At some point the policy makers have to act.”
The DNR results on the spleen of wild Minnesota deer surprised the researchers because deer caught in the thick woods of northern Minnesota were as likely to have neonicotinoids in their systems as those caught in the vast corn and soybean fields of southern Minnesota. It is unclear exactly how chemicals get into animals, whether it is through the water they drink or by directly eating treated seeds or plants.
“There was a little ‘wow’ factor when we found deer in Boundary Waters with neon,” Carstensen said. “How is that possible? It’s moving in ways we don’t understand.”
A number of basic questions about how chemicals behave in the environment are still unanswered, said Eric Michel, a DNR researcher.
“We don’t necessarily know how long they last in the deer system, if they metabolize it quickly or how it is stored or where it is stored in the body,” he said.
All DNR spleen samples were taken in the fall during the hunting season. Exposure to neonicotinoids would theoretically be much higher in spring and summer when crops and back gardens are planted or sprayed. Michel and the DNR would like to do more investigations collecting samples of spleen and other tissues year round to get a better understanding of when neonicontinoid levels peak and how fast they move through an animal’s system.
Minnesota soybean growers have moved away from neonicotinoids in recent years, said Joe Smentek, executive director of the Minnesota Soybean Growers Association.
Research from the University of Minnesota showed that, in many cases, the chemicals weren’t effective enough against soy parasites to be worth the cost.
“We had some farms where farmers had to buy seeds treated with neonic, but they stopped doing it at our insistence because farmers don’t want to pay for something they won’t use,” Smentek said.
The state’s deer population as a whole appears to be healthy – the estimated number of deer has been stable for years. But if fewer fawns survive, it could change the way the DNR calculates its population estimates and crop quotas.
Work needs to be done to find out if the chemicals directly impact deer and pheasants in the wild, Jenks said.
“We need to evaluate the wise use of these chemicals to make sure they don’t have a significant impact on wild populations,” he said.