Data show increasing insecticide levels in Minnesota deer

Two deer.
Two deer near Duluth, Minn.
Andrew Krueger | MPR News

Updated: Aug. 30, 10:01 a.m. | Posted: Aug. 23, 1:24 p.m.

A second year of testing for neonicotinoid insecticides in white-tailed deer in Minnesota found the chemical more widespread and at higher levels than in previous sampling.

Researchers found the insecticide in 61 percent of 799 deer spleens tested in 2019. But the chemical was found in 94 percent of 496 samples collected in 2021.

The 2019 samples were collected from hunter-killed deer across the state.

Last year the DNR narrowed the focus to 12 deer permit areas located in southwestern agricultural lands, the area of transition from farming to forest in central Minnesota and in northern forested areas.

"We're not exactly sure why we saw that increase," said Department of Natural Resources Ungulate Research Scientist Eric Michel. "But regardless, the two years of data are showing us that neonics are being detected pretty much across the state. When we look for them we find them in deer spleens. So that's kind of the big takeaway from what we're seeing right now."

The prevalence of detections increased in last years samples, and so did the average level of the insecticide in the spleens.

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The deer sampled in 2021 were 61 percent adults, 32 percent yearling and seven percent fawns.

A measurement of .33 parts per billion of neonicotinoid insecticide is considered a risk threshold, based on a 2019 study on captive deer in South Dakota which found increased fawn mortality at that level. Michel participated in that study while at South Dakota State University.

Researchers found 29 percent of the samples were at or above that benchmark level in 2019, while 64 percent of neonicotinoid exposures reached or exceeded the benchmark in spleens collected last year.

"So we had quite a big increase," said Michel. "We went from about a third of the samples to two-thirds of the samples."

There is no evidence those levels are causing fawn mortality in free ranging wild white-tailed deer.

The only evidence of increased mortality is from the South Dakota study.

"Does that translate to a wild free range population?" said Michel. "We just don't know if it's impacting them or not."

Deer spleen testing kit
The DNR asked roughly 2000 hunters to extract the spleens from any deer they might shoot this fall for a study on pesticides in deer. Reporter John Enger shot this doe on opening morning in northwestern Minnesota and sent the spleen to the DNR for testing.
John Enger | MPR News

While neonicotinoid insecticides are widely used in agricultural production, they're also used in a range of home and garden products. The insecticide works by being taken up by plants as they grow, along with water and nutrients. When insects eat the plant, they get a dose of the chemical.

A University of Minnesota study published in 2020 found the chemical widespread in Minnesota surface water.

The DNR data on exposure in white-tailed deer also points to widespread contamination.

"We've found some evidence of neonics up around the Boundary Waters. So how it's getting there is a really good question to ask,” said Michel. “At this point, we really aren't sure.”

"We're finding neonics statewide, this isn't just in our agricultural regions. This isn't just an agricultural issue, if anything, our data showing that,” he said. “So I think it's just really important to remember that this is something we're finding across the state. And we're just trying to better understand it."

A proposed study of fawn mortality in wild white-tailed deer was not funded by the legislature this year. Michel said based on the 2021 spleen testing results, such a study would need to be redesigned.

The DNR plans to do more intensive sampling this fall in two deer hunting permit areas, but hasn't yet identified the specific areas of the state.

Michel also hopes to collect more samples at other times of the year, to better understand if there are seasonal variations in insecticide exposure to the deer.

Correction (Aug. 30, 2022): An earlier version of this story misstated the risk threshold for neonicotinoid insecticide. The story has been updated.