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By Shannon Kelleher
Rodent studies given to U.S. regulators by insecticide makers close to 20 years ago revealed the chemicals could be harmful to the animals’ brain development — data worrisome for humans exposed to the popular pesticides but not properly accounted for by regulators, according to a new research report published on Oct. 1.
The analysis examined five studies that exposed pregnant rats to various types of insecticides known as neonicotinoids (commonly called neonics). The studies found that the offspring born to the exposed rats suffered shrunken brains and other problems.
Statistically significant shrinkage of brain tissue was seen in the offspring of rats exposed to high doses of five types of neonics – acetamiprid, clothianidin, imidacloprid, thiacloprid, and thiamethoxam, the paper states.
The authors said the impacts on the brain appeared similar to the effects of nicotine, which they said is known to disrupt mammalian neurological development.
The animal studies also support the possibility of a link between neonic exposure and attention-deficit/hyperactivity disorder (ADHD), the authors said.
In most cases, the companies submitting the studies did not submit data for all dosage levels, leading the U.S. Environmental Protection Agency (EPA) to assume negative effects were only seen at the high dose, according to the study.
“We found numerous deficiencies in the Environmental Protection Agency (EPA)’s regulatory oversight and data analyses,” the authors state in the paper, published in the journal Frontiers in Toxicology.
The industry studies, which the EPA used to determine what neonic exposure levels are considered safe for humans, were not publicly available and were obtained through a Freedom of Information Act request.
“Consistently, effects were found at the high dose and EPA did not demand data for the lower doses, therefore leaving it unclear how little of a substance it takes to actually cause adverse effects such as reduced size of certain brain regions,” said Bill Freese, the science director for the environmental advocacy group Center for Food Safety and an author of the study.
The study found that the EPA consistently made determinations about what levels of neonic exposure were “safe” for humans without enough data to support its conclusions.
For example, a 2001 study submitted to the agency by the pesticide manufacturer Bayer found effects on fetal brain development when pregnant rats were exposed to high doses of the pesticide imidacloprid.
The EPA asked the company for mid- and low-dose data, then set a limit for imidacloprid at the mid-dose level even though it never received data showing that exposure at this level was safe.
“EPA needs to more rigorously demand data from registrants when they fail to submit,” said Freese. “They’re making a decision to say these lower levels cause no harm, even though they have no data to back that.”
Since two of the neonics, imidacloprid and thiacloprid, break down into so-called metabolites that are as potent as nicotine, “one might expect to see neurodevelopmental impacts of exposure to their parent chemicals at low exposure levels,” the authors wrote.
The authors also concluded that the EPA should assess the cumulative exposure and risk for neonics as a group, a measure required under the Food Quality Protection Act for pesticides that work similarly.
The findings come as the EPA proposes reevaluating occupational exposures to three neonics, clothianidin, imidacloprid and thiamethoxam, by 2025, as the agency carries out its standard registration review of neonics as a class.
The EPA identified additional risks to workers while treating the seeds with the chemicals and cleaning seed treatment equipment, “even when the use of maximum personal protective equipment is considered.”
“These compounds have been approved for decades and now EPA is saying, ‘maybe we underestimated the risk,’” says Freese.
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The European Union banned outdoor use of all three pesticides in 2018.
Neonics, like other pesticides sold or distributed in the U.S., undergo an “in-depth evaluation of potential risks … to the environment and the US population,” said the EPA in an email.
“The rodent developmental neurotoxicity studies discussed in this article have been independently reviewed by EPA and incorporated into the most current human health risk assessments for these neonicotinoid pesticides,” said the EPA.
Neonics were first introduced in the 1990s as safer replacements for older pesticides and are now the most widely used insect-killing chemicals in the U.S. and around the world.
In addition to their agricultural use as a coating on seed crops such as corn and soybeans, neonics are sprayed on lawns, gardens, parks and playgrounds, and are applied on pets in flea and tick treatments.
In recent years, neonics have been detected in surface waters and treated drinking water in the Midwest, and a 2022 study found the chemicals in the bodies of over 95% of pregnant women across the U.S.
A 2015-2016 monitoring study by the Centers for Disease Control and Prevention found the highest levels of neonics in young children.
In the most recent U.S. Food and Drug Administration pesticide residue monitoring report, imidacloprid was tied for the pesticide most frequently found in human food samples, while acetamiprid and thiamethoxam made the top 10.
Research increasingly calls the safety of neonics into question. Some scientists suspect that neonics are driving losses in populations of important pollinators, including bees. A 2023 EPA assessment found that three commonly used neonics may be driving over 200 endangered plants and animals toward extinction.
Another 2023 study found behavioral changes in zebrafish exposed to neonics, with effects persisting into adulthood, while a study published this summer found that exposure to the neonic clothianidin caused behavioral changes in developing female mice.
In contrast, a 2015 review by the companies Bayer, Syngenta and other pesticide manufacturers concluded that “the collective evidence indicates the neonicotinoid insecticides are not developmental neurotoxicants.”
Originally published by The New Lede.
Shannon Kelleher is a reporter for The New Lede.
