A study conducted by scientists in South Dakota has found that the world’s most widely used family of pesticides — neonicotinoids — is likely causing serious birth defects in white-tailed deer, deepening concerns over the chemical’s potential to harm large mammals, including humans.

A subsequent study by the wildlife scientists and ecologists at South Dakota State University in Brookings will examine whether ring-necked pheasants also could be harmed by neonicotinoid pesticides, which are used heavily in agriculture across the state.

The first study, with results published in March, showed that white-tailed deer with high levels of neonicotinoid pesticide in their spleens developed defects such as smaller reproductive organs, pronounced overbites and declined thyroid function. Fawns with elevated levels of the pesticide in their spleens were found to be generally smaller and less healthy than deer with less of the chemical in their organs. The study marks the first time neonicotinoid pesticide consumption has been linked to birth defects in large mammals.

“These (neonicotinoids) were deemed to be safe for higher organisms, and the fact that we saw so many diverse impacts on white-tailed deer, that was a big thing,” said Dr. Jonathan Lundgren, an ecologist from Estelline and independent scientist who co-authored the study. “And then, the fact that whitetail deer are not that far off from our livestock or even humans suggests that maybe we need to be examining these insecticides’ risks a little bit more closely.”

Neonicotinoid pesticides entered wide commercial use in the late 1990s and within a few years became the world’s most popular family of insecticides, often used to coat and protect the seeds of crops because they kill a wide range of insects, remain effective for weeks, and were believed safe to mammals and birds.

The pesticide is often sold commercially as Imidacloprid and is sold for home use in landscaping treatments such as Bayer Advanced Tree and Shrub Insect Control.

Until recently, scientists hadn’t been able to effectively monitor for the presence of neonicotinoids in the human population. Neonicotinoids make their way into the human food chain when unabsorbed pesticides from farm operations become airborne or are carried into waterways and onto other crops by rain and run-off.

According to a paper published in September 2019, Centers for Disease Control and Prevention researchers found traces of the chemical in 49.1% of the urine samples collected from people during the 2015-16 National Health and Nutrition Examination Survey. Based on the survey data, CDC researchers estimated that roughly half the U.S. population had recently been exposed to neonicotinoids.

A study published in January 2019, called “Trends in neonicotinoid pesticide residues in food and water in the United States, 1999–2015”, found low levels of neonicotinoid pesticides on nearly 60% of cauliflower, 45.6% of spinach and 29.5% of the apples intended for human consumption in the U.S.

The study Lundgren co-authored was called “Effects of Neonicotinoid Insecticides on Physiology and Reproductive Characteristics of Captive Female and Fawn White-tailed Deer,” and its results were published in the March 2019 edition of the journal Scientific Reports. The experiment evaluated in the SDSU study was designed to expose captive deer to levels of neonicotinoid pesticide that scientists thought wild deer could encounter in their natural habitats.

The idea was to try to confirm or disprove an idea posed in a 2002 paper written by Montana wildlife rehabilitation expert Judy Hoy.

Hoy, who often dissected deer killed by cars, had seen the prevalence of birth defects such as overbite in white-tailed deer explode over the course of several years in the late 1990s. She theorized that neonicotinoid pesticides might be causing the defects.

More than a decade later, over the summers of 2015 and 2016, Lundgren and other SDSU researchers gave several groups of female deer and their fawns water laced with imidacloprid, one of the most popular types of neonicotinoid pesticide used in the U.S.

Two groups were given water with levels of imidacloprid similar to those observed in Canadian wetlands. Another group was given water with a much higher dose of the pesticide. The final group was not intentionally given the pesticide.

The adult deer in the study were artificially inseminated and the fawns born to the adults were included in the study. At the end of each summer, the fawns were euthanized and dissected so researchers could get a look at the physiological effects of exposure to the pesticide. At the end of 2016, all of the deer in the study were euthanized and dissected.

Imidacloprid was found to be building up in several organs, including the thyroid, liver and spleen. As it turned out, deer with higher levels of imidacloprid in their spleens had pronounced overbites, smaller bodies, smaller organs and were more likely to die early.

The neonicotinoid was also found in the reproductive organs of adult deer, meaning fawns were probably being exposed to the pesticide before they were born, said Dr. Jonathan Jenks, a wildlife ecologist and professor at SDSU who co-authored the study.

“It is potentially having an effect on survival of fawns in a number of different ways that could decrease the number of young available or that get recruited into deer populations,” Jenks said.

One of the study’s important findings was that imidacloprid was concentrating in deer spleens. The discovery gives future researchers a better idea of which organs to monitor for neonicotinoid exposure in all mammals, Jenks said. Prior to the SDSU study, scientists had usually looked for the pesticide in livers, he said.

Another reason the spleen discovery is significant is that the organ is an important piece of the immune system. When spleen function is impaired, Jenks said, young deer are likely to be more susceptible to disease.

Lundgren said the SDSU study is further evidence that scientists and farmers need to take a deeper look at pesticide use on mammals, including humans.

“I don’t think we entirely know what the implications are or what the mechanisms are for how these insecticides are affecting (vertebrates). I think we need to be looking at things like hormones and things like other elements of biology that we formerly understood were affected by these neurotoxins,” Lundgren said.

Imidacloprid was the first neonicotinoid introduced to the commercial market and has become the most widely used insecticide in the world. But there are six other types of neonicotinoid in use today, including Clothianidin, Dinotefuran, Nitenpyram, Thiacloprid, Thiamethoxam and Acetamiprid.

All of the deer used in the SDSU study, even those not intentionally given imidacloprid, were found to have the chemical in their internal organs when they were dissected.

Both Jenks and Lundgren said they weren’t able to pinpoint exactly how the control group — those deer not given imidacloprid in their water — were exposed to the pesticide.

“What we found … is that these ‘neonics’ are everywhere. They’re all over the place,” Jenks said.

In bugs, neonicotinoids bind to certain chemicals used to transmit messages through the nervous system and cause neurons to fire uncontrollably. They are extremely lethal to a wide range of insects.

Mammals and birds don’t have the same reaction to neonicotinoids, which is a big reason that the pesticide family rocketed to popularity over the years following their introduction. Other popular pesticides, such as organophosphates, can be highly toxic to mammals and birds.

Neonicotinoids had another advantage. They can be absorbed into plants and provide protection against harmful insects for up to 10 weeks. Farmers can plant seeds coated in a neonicotinoid pesticide and not have to spray their crop for pests for more than two months. Because neonicotinoids also happen to be water soluble, farmers can even lace their irrigation water with the chemical.

The problem is that most of a neonicotinoid seed treatment, from 80% to 98%, doesn’t actually get absorbed into the intended plant, Lundgren said. The unabsorbed treatment often gets flushed out of farm fields as runoff after rainfall or can be blown around in dust from dry, freshly planted fields. Often, non-targeted plants absorb neonicotinoids and become deadly to helpful insects such as bees.

Jenks said the SDSU study’s findings highlight the need for more research and monitoring of wild deer populations. In fact, he’s helping on a study that will monitor wild white-tailed deer populations in Minnesota for the types of defects the SDSU study found.

Lundgren and Jenks are part of a team of scientists preparing to publish results of a study on ring-necked pheasants that used the same methodology as their deer study. Jenks said he hopes to have the pheasant study submitted for publication in December 2019 or early in 2020.

For Lundgren, the SDSU deer study and its results are further evidence that farming practices need to change. Crop rotations should be more diverse and livestock should be integrated into more farming operations. Such practices would decrease the need for pesticides, make farms more resilient to extreme weather and have been shown to increase profitability, Lundgren said.

“Our farming decisions are having broad scale implications for the environment in ways that we didn’t predict, in ways that we can’t predict. And so while I’m not an advocate for banning all pesticides, I do think that we need to use them with a whole lot more respect than we are right now,” Lundgren said.

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