New research from the University of Minnesota found low levels of neonicotinoid insecticides in lakes and streams across Minnesota.
The widely used insecticides are well known for their effects on bees and other pollinating insects.
Neonicotinoid insecticides are widely used in agriculture; the chemical is applied to many of the seeds farmers plant. It's known as a systemic insecticide because as the plant grows, it takes up the insecticide with water and nutrients.
If an insect tries to eat the plant, it gets a lethal dose of insecticide. Concerns about the impact on pollinators is based on the fact the insecticide is also found in the pollen of flowering plants near farm fields where bees feed.
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But the U of M study raises questions about potential environmental effects in lakes and rivers from persistent low levels of insecticides, and about the level of the chemicals in urban waterways.
"They are ubiquitous at low levels at least, with higher levels in certain environments, especially the urban areas, and wastewater treatment effluents," said researcher Matthew Berens, a recent U of M Ph.D. graduate.
The researchers tested hundreds of samples from streams across the state, lakes in urban and agricultural areas and groundwater wells in several counties.
"We wanted to see if there was a difference in neonicotinoid use between urban and agricultural environments, both in terms of the types of pesticides used, as well as the amounts that were in the environment," said professor Bill Arnold.
They found the insecticide clothianidin was most common in agricultural areas of the state where it's widely used to treat seeds that farmers plant.
But water in urban areas had insecticide levels that were similar or even higher in some cases. The most common neonicotinoid detected in urban water was imidacloprid, which is widely applied on urban landscapes, used in flea and tick control products for pets, and used to control the emerald ash borer among other uses.
While water in agricultural areas is more likely to experience a neonicotinoid spike after spring planting, urban areas are more likely to have persistent levels throughout the year. For example, the researchers did extensive sampling for imidacloprid on Lake Como in St. Paul in 2019.
"And it was always there, which means there is a continual source in the watershed to the lake," said associate professor Paul Capel. "And so what is that source? And how does it get there on that kind of continuous basis? I think that is another really important question."
While the researchers focused on surface and ground water, they did test tap water in Minneapolis and found no neonicotinoids.
Arnold wants to be clear that they found no evidence of levels that pose a human health risk. "The risks I think, are largely environmental," he said.
The effects happening below the surface of lakes and rivers are complex and challenging to unravel. In most cases the insecticide levels in water are too low to kill aquatic life. But other researchers have found negative long-term effects on aquatic life even at very low levels.
"It's much harder to characterize any kind of long-term effects that there might be at the low levels, and how that might affect the aquatic insects or the algae or other pieces of the food chain. And that's the other aspect of this project that we're still investigating," said Arnold.
"One of the major theories is that it's affecting the algal grazers, the small insects that are feeding on the algal blooms," said Berens. “The neonicotinoids are very specific and they target the aquatic insects very well."
The next phase of the research will dive more deeply into the question of how neonicotinoid insecticide in the water is affecting the tiny animals at the bottom of the aquatic food chain, and if that's a factor in the growing problem of large summer algae blooms on lakes.
They also want to better understand the sources of persistent neonicotinoid contamination of water in the urban environment.
"They're potent chemicals," said Arnold. "And especially in the urban environments, the personal choices that we're making about the chemicals we purchase and use for various household reasons have a potential downstream environmental impact. And so your personal choices matter."
The research was published Dec. 10 in the journal Environmental Toxicology and Chemistry.