New research finds sky-high insecticide levels in Colorado water

Adding to evidence about the pervasiveness of pesticides that endanger human and environmental health, new research has found widely used insecticides in Colorado waterways at levels 100 times higher than what researchers say is needed to protect aquatic life.

Neonicotinoids (neonics), the most widely used types of insecticides in the nation, were present at sky-high levels in both surface and groundwater samples, according to a report published this week by the nonprofit Natural Resources Defense Council (NRDC). The analysis, which relied on federal and state water testing data, concluded that agricultural  seeds coated in neonics are likely to blame for the heavily contaminated water.

This contamination “is likely causing significant and widespread damage to aquatic ecosystems and an increase in human exposure from groundwater,” the report states.

The author used the federal benchmark for one neonic, imidacloprid, to derive benchmarks for how chronic exposure to other neonics affects aquatic life, according to the report.

Some levels detected in the water samples surpassed federal regulators’ worst-case scenario estimates for how much of two neonics, clothianidin and thiamethoxam, may be present in Colorado groundwater.

“The contamination levels in Colorado are alarming, but not surprising,” said Pierre Mineau, author of the report and a professor at Carleton University in Canada who has studied the environmental risk of pesticides for over 40 years.

“The US EPA (Environmental Protection Agency) isn’t doing enough to protect water and ecosystems from these harmful chemicals,” he said.

Indiscriminate killers

Neonics are considered some of the deadliest insecticides ever made, targeting insects’ nervous systems to paralyze and kill them. While they are intended for pests that damage crops, such as aphids and beetles, neonics also kill butterflies, bees and earthworms essential for healthy ecosystems. The insecticides, which are applied to over 150 million acres of cropland each year, are primarily used as seed coatings and are taken up by plants’ leaves, stems and pollen as they grow, poisoning the insects that make contact with them.

The European Union has banned outdoor uses of some neonics due to evidence that they cans harm honeybees and wild pollinators. Environmental advocates for years have called for similar bans in the US. In 2018, over 200,000 public comments on EPA ecological and human health assessments urged the agency to ban the insecticides, citing pollinator decline.

Colorado’s neonic pollution reflects a national problem, said Allison Johnson, a senior attorney at NRDC.

“We’re planting toxic pesticides in soil across the country, and these chemicals persist for a long time,” said Johnson. “They are building up in our water, our soil, our air – and in our bodies. Fortunately, there’s an easy first step to solving this problem: stop using neonics where they aren’t needed.”

A 2015 study by the US Geological Survey identified at least one neonic in nearly two-thirds of the 48 streams it sampled across the country

The US EPA in 2023 found that three neonics – clothianidin, imidacloprid and thiamethoxam – each may jeopardize 9-11% of species listed as endangered or threatened.

Studies have linked neonics to the decline of birds, butterflies, aquatic invertebrates and deer.

Neonics have been infamously linked to mass bee die-offs, a problem that continues to intensify. From April 2023 to April 2024, US beekeepers lost about 55% of their honeybee colonies – well above the average yearly loss of about 40% since tallying began in 2010, according to a US beekeeping survey. So far, the losses for 2025 are even more devastating, with about 60% of honeybee colonies lost – a financial hit of at least $139 million, according to a survey of 234 beekeepers from across the US.

Other factors also present widespread threats to honey bee colonies, including parasitic mites that feed on the bees and their larvae and spread bee-killing viruses. Pesticide makers deny that neonics are behind pollinator die-offs at all.

“Comprehensive studies conducted under realistic field conditions have shown that residues of neonicotinoids in the flowers of seed-treated crops are clearly below the levels that could cause adverse effects on honeybee colonies,” Bayer, which manufactures neonics and other pesticides, wrote in an April 2025 report.

Neonics are highly regulated, wrote Bayer, and the company’s insecticides are extensively tested “to ensure they don’t have unacceptable adverse effects on non-target insects and the environment.”

Health dangers

A survey from the Centers for Disease Control and Prevention using 2015-2016 data found that about half of the US population three years of age and older had been recently exposed to neonics, with young children at risk for higher levels of exposure.

The findings suggest a large chunk of the population is exposed to the chemicals on a regular basis. A 2022 study found neonics in the bodies of more than 95% of pregnant women tested across the US. The pesticides have been found in a variety of popular and otherwise healthy foods.

The most recent pesticide residue report issued by the US Department of Agriculture reported finding neonics in various baby foods, including those made with pears, peaches, apples and peas, as well as in blackberries and potatoes. Research has linked neonic exposure to birth defects in newborns’ brains and hearts, as well as symptoms associated with autism.

Despite research on the health and environmental dangers of neonics, the EPA has continued to allow their use. The agency is set to decide in 2025 whether to reapprove five neonics – acetamiprid, clothianidin, dinotefuran, imidacloprid, and thiamethoxam. In the meantime, a dozen states so far have implemented their own outdoor restrictions on neonic use, although only New York and Vermont have restricted agricultural uses of neonic-treated seeds.

Last December Minnesota regulators rejected a legal petition alleging the state’s agriculture department was violating Minnesotans’ rights by failing to regulate the use of neonic-treated seeds.

(Featured image by jabberwocky28 on Unsplash.)