‘Reduced Risk’ Pesticides Are Widespread in California Streams

Insect and weed killers designed to replace compounds that cause neurodevelopmental problems in animals and people pose their own set of risks.

A worker on a farm wears a Tyvek chemical protective suit as he sprays a field with a herbicide after the broccoli harvest. Credit: Andrew Holbrooke/Corbis via Getty Images
A worker on a farm wears a Tyvek chemical protective suit as he sprays a field with a herbicide after the broccoli harvest. Credit: Andrew Holbrooke/Corbis via Getty Images

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A new generation of pesticides promoted as safe alternatives to compounds shown to endanger the environment and public health are turning up in California streams at toxic levels, researchers with the U.S. Geological Survey report. 

Insect populations are plummeting around the world, leaving scientists scrambling to understand the relative contributions of factors as diverse as habitat loss, parasites, climate change and pesticides in a race to reverse the declines. Teasing out pesticides’ role in this biodiversity crisis has proven a daunting task, challenging scientists to keep pace with the steady stream of chemicals released to replace those found to be harmful.

In the new study, USGS scientists developed tools specially designed to detect newer classes of pesticides including neonicotinoid insecticides, best known for their suspected role in driving bee declines. To understand how these so-called “new generation” pesticides are affecting aquatic ecosystems, the scientists sampled scores of small streams along California’s Central Coast, from Sonoma County to Santa Barbara. The region is an “ideal setting” to get a broad sense of how pesticides are affecting streams, the scientists said, because it contains both intensive agriculture and large urban areas where pesticides are used.


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The team found pesticides in all but two of 85 streams sampled. When they found a specific pesticide, they found it nearly every time they sampled a stream, which they did once a week for six weeks. Many of the newer compounds would have been missed using older methods.           

The findings of the peer-reviewed study, published in the February issue of Science of the Total Environment after an early online release in October, underscore how difficult it is for scientists to understand how novel pesticides are harming diverse species and the ecosystems that support them. 

“It’s really important to keep our methods updated so we’re tracking the most widely used and most potentially toxic pesticides that are being applied,” said Lisa Nowell, an emeritus research chemist with the USGS who contributed to the study. “By their nature, pesticides are bioactive. We know they’re toxic. That’s why we’re using them.”

The older method would have missed some neonicotinoids, Nowell said. That’s a problem because the compounds work through a similar pathway and are usually applied as mixtures.

“Without the new method, we would probably be underestimating the impact of neonicotinoids as a group on aquatic life,” Nowell said.

Rates of pesticide application have in many cases plateaued or declined after a 20-year boom starting in the 1960s. But their potential to harm plants, pollinators, insects and other invertebrates has risen sharply, German researchers led by ecotoxicologist Ralf Schulz reported last spring in Science

Schulz, a professor at the University of Koblenz-Landau who was not involved in the new study, said it “fits nicely with our discovery.”

The USGS scientists measured pesticide concentrations, including some of the newer compounds that haven’t been detected very often, Schulz said, and frequently found them. Many were found at relatively low concentrations, he noted, but many others were detected above levels considered toxic. 

Whereas Schulz and his team used a method to predict the effects of pesticides like neonicotinoids, Nowell and her colleagues measured the chemicals in streams. “We thought that the potential toxicity for invertebrates increased in the U.S.,” Schulz said. “And here you can see now that this is indeed the case.”

Widespread Contamination

Nowell and her colleagues sampled small streams across a wide range of landscapes, including agricultural, urban, a mix of urban and agricultural use and undeveloped sites that are not normally monitored for pesticides. While many studies focus on agricultural areas, this approach promised to offer a much broader view of pesticides’ effects on stream life. 

The team found higher risks to invertebrates living in streams as land use intensified. The highest concentrations of frequently detected insecticides occurred in areas with both urban and agricultural use, followed by solely agricultural or urban sites, with the lowest concentrations in undeveloped sites. 

Many studies don’t consider small streams like those sampled in the USGS study, Schulz said. “And that’s a big problem.”

Small streams are very important ecologically because they are so abundant, Schulz said. But they’re also very susceptible to pesticides, which concentrate in the shallow water. 

These waterways are especially important in areas like California that are intensely populated, said Matthew Forister, a professor of biology at University of Nevada, Reno. “Insects don’t have that much space left,” he said. “Add in the warming conditions and the drying conditions and all that’s left is a trickle. And now it’s quite toxic. It’s pretty dire.”

Two of the three most widely occurring pesticides in the USGS study, chlorantraniliprole and methoxyfenozide, are especially toxic to imperiled butterflies. 

Chlorantraniliprole is highly toxic to monarch caterpillars. Methoxyfenozide, which kills caterpillars by targeting a specific protein, has not yet been tested on monarchs. But the butterflies have the targeted protein, so are likely susceptible.

Both compounds were detected in nearly every sample taken from streams in mixed and agricultural sites. Both were registered with the Environmental Protection Agency as reduced-risk pesticides or alternatives to classes of compounds recognized as toxic.

Researchers are seeing the same compounds on plants that are critical to butterfly reproduction, said Sarah Hoyle, pesticide program specialist with the Xerces Society, a nonprofit organization devoted to invertebrate conservation. Hoyle is especially concerned about contaminated milkweed—the only plant monarch caterpillars eat. 

Last year, Hoyle joined Forister and other scientists to document pesticide residues on milkweed throughout the Central Valley, in a study published in the journal Frontiers in Ecology and Evolution. The two most commonly detected insecticides were methoxyfenozide and chlorantraniliprole.

Western monarch populations have crashed since the 1980s, hitting a low of fewer than 2,000 overwintering butterflies last year. The population rebounded to nearly 250,000 this year, but the iconic butterfly’s future remains precarious. 

The USGS scientists did not document the plants growing along the streams they sampled. But there’s little doubt that milkweed grows along these contaminated streams, said Forister, who linked warming autumns to reported declines of hundreds of Western butterfly species in a Science study published last year. 

And it’s not just milkweed, Forister said. Streams support numerous plants that host other butterfly species, he said, “some of which are declining more severely than the monarch.” 

Toxic Treadmill

Environmental and public health scientists have been playing catchup with pesticide manufacturers ever since they discovered that DDT—promoted as safer than arsenic-based compounds—was driving rapid declines of eagles and other raptors by weakening their eggshells. 

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And now, as the USGS study reported, they must evaluate the effects of not just one compound, but chemicals in combination. The USGS study found that the vast majority of hundreds of samples contained two or more pesticides. About a third contained at least 10 compounds.

“We barely know much about even the compounds acting in isolation on your average insect,” said Forister. “We know even less, which is to say pretty much nothing about synergies among those compounds,” he said, referring to toxic effects that exceed the sum of the compounds’ individual effects.

“There’s a catastrophic gulf between what we need to know, and what we can know, using the traditional lab approaches,” Forister said. “The complexity of mixtures that insects are encountering out there is really stunning.”

Inevitably, by the time scientists begin to understand the risks even a single pesticide poses, it’s already ubiquitous in the environment. And they face an uphill battle because regulators evaluate risk based on single chemicals, even though one pesticide product contains multiple ingredients that work together.

It’s also critical to use the right species to gauge toxicity, said Nowell of the USGS. Different classes of organisms have different susceptibilities to a given pesticide, she said, so you might think toxicity has declined only to discover that you’re not testing the right species. 

“As we start to get more environmental data and more information on the impacts of these pesticides on wildlife,” said Hoyle, “we start to understand that perhaps they are not quite as safe as we thought they were.”

That presents “a fairly grave concern” about the potential for effects to ripple through the food chain, she said. “Impacts to invertebrates are often unseen, but they are really critical as the basis of food webs and keeping birds and fish and other larger wildlife populations healthy.”

U.S. environmental officials banned DDT in 1972, largely in response to public concerns about its effect on eagles. Now, pesticides’ toxicity has shifted from birds and other vertebrates to plants, insects and other invertebrates, as Schulz reported last year. And scientists fear that the public may not even notice the disappearance of species that lack the charisma of America’s national symbol.   

Yet birds and other vertebrates are still being affected by pesticide toxicity, Schulz said, because they’re losing what was once an abundant food source.

And already there’s evidence that a crustacean species favored by California’s endangered salmon is developing resistance to the newer pesticides. There are numerous fitness costs associated with expending the energy to develop resistance. If the crustaceans suffer other problems from making those adaptations, which might, for example, reduce their abundance, Schulz said, there might be “knock-on effects.” Salmon and other animals that depend on them for food could suffer.

Schulz thinks people may assume that pesticides are well tested and safe and don’t realize the ecological risks they carry. Maybe once they know, he said, they’ll see that environmental damage as an acceptable cost of growing food. “But I have the feeling that people just don’t know.”

People need the information to help them decide if they want their food grown under conditions with this kind of environmental damage, Schulz said, or if they want to invest in sustainable approaches that are not heavily reliant on pesticides.

But it’s not just agricultural pesticide use that’s threatening aquatic ecosystems, as the USGS paper makes clear. Pesticides are entering streams from parks, schools, homes and other developed areas.

“Urban areas are pretty significant contributors, and that includes everything people spray in their yards,” said Forister. “It’s just amazing the potency of what you can buy at Home Depot these days, often with very unclear directions on how to apply it.”

And with emerging evidence that climate change is having severe impacts on insects in natural areas far removed from pesticide contamination in the American West, Forister said, urban areas are becoming critical to insect survival.

Natural spaces are suffering because of climate change, so insects need urban parks and backyards, along with everything else, Forister said. “Your backyard is both part of the problem and part of the solution.”