From Field to Stream

Pesticide seed coatings threaten invertebrates and have contaminated streams and rivers. Sarah Derouin unearths the problem. Illustrated by Miranda Zimmerman.

Illustration: Miranda Zimmerman

Rummaging through a closet in her lab, Michelle Hladik shoves aside plastic bags to pull out jars of flowers and tubes of bees. Researchers from around the U.S. have sent these plants and animals to her lab, where workers check them for signs of pesticide poisoning. It can be a messy job: “Where are the crayfish vials?” she asks her team. There’s a collective groan as they recall an unfortunate crustacean, loaded with parasites, that was mailed to the lab. Pesticides weren’t the cause of death that time; the crayfish was ravaged by ramen-shaped worms. “I’m no biologist,” says Hladik, laughing, “but there were a lot of parasites.”

Hladik, an environmental organic chemist, solves mysteries about pesticides. By collecting clues from water, dirt, plants and animals near agricultural fields, she investigates what happens to pesticides after they are applied to crops and enter nearby waterways, potentially harming wildlife.

Lately, Hladik’s lab at the U.S. Geological Survey (USGS) in Sacramento, California, has been examining neonicotinoids: a popular group of pesticides, called neonics for short, that farmers and homeowners use widely. After the pesticide is applied, some of it stays on the plants, but much of the chemical dissolves into water. Water is forever moving, and as it flows, it carries the pesticide along, eventually dumping the chemical into streams and wetlands.

But how much pesticide is flowing away from fields? A lot, as it turns out. Over the past few years Hladik and her colleagues found the number of neonics in surface water far exceeded anyone’s expectations. This was especially surprising because pesticide companies have developed new methods to try to limit the spread of the chemicals.

Hladik wants to know how pesticides migrate away from the fields where they are applied. Her work shows that neonics are appearing in streams around the country. Hladik is now collaborating with scientists to determine what might happen if pesticides move out of the water and into the bodies of living creatures.

Pesticide prevalence

The use of synthetic pesticides exploded after World War II. DDT was the first mass-use pesticide, efficiently killing mosquitoes and stopping malaria. It was applied at public pools and picnics; photos from the time show children dancing through clouds of DDT. But in the 1960s, things began to change. Rachel Carson wrote Silent Spring, exposing the human and ecological harm caused by DDT. Scientists realized that DDT could linger in the environment, causing unforeseen collateral damage, such as wiping out the iconic bald eagle. Agricultural use of DDT was banned, and the Environmental Protection Agency was created to regulate the use of pesticides and other chemicals.

In the 1990s, organophosphates (OPs) were used widely by farmers. But once scientists found that the chemicals were leaching into water, sickening farm workers and decimating fish and birds, regulators banned their use on crops. “OPs are extremely toxic to animals, and people were afraid of them,” says Aimee Code, a pesticide program manager at Xerces Society, a nonprofit organization in Portland, Oregon, that advocates for invertebrate conservation.

Next came the pyrethroid pesticides, which didn’t leach into water, but did contaminate soil near agricultural fields. Chemical companies started looking for a pesticide that loved water, and thus was easy to apply to crops, but wouldn’t harm people and animals. They began designing neonics, which they hoped would only target pest insects.

Graphic: Sarah Derouin

Prolific neonics

Neonicotinoids are a class of insecticides closely related to the chemical nicotine. The insecticides, like nicotine, affect the nervous system of an organism. But unlike organophosphates, neonics were engineered to target the nervous system of insects and were thought to be less toxic to birds and mammals. In 1985, Bayer developed the first commercial neonicotinoid. During the 1990s, neonics steadily gained in popularity. They are now the most widely used class of insecticides worldwide.

Farmers apply neonics to both crops and farm animals to eliminate biting and sucking pests. They are also applied in urban settings, fighting the invasion of cockroaches, ants and termites in homes, or helping Fido remain flea-free.

Because neonics dissolve and move in water, they are easy to apply. Farmers can spray them, spread them in granules, drench the soil, paint them on tree trunks, and—in the most popular method of all—coat a seed in them. A seed coating is like a chocolate-covered nut; the seed is like the nut inside, and the pesticide is the outer chocolate shell. Seed coatings have skyrocketed over the past decade. They now account for about 60% of global neonic use.

The water-loving nature of neonics is key to the idea behind seed coatings. After the treated seed is planted, the plant grows, and the seed’s shell of insecticide dissolves in water and is carried up through the plant, spreading from root to petal. Picture putting daisies in a vase of water that is dyed pink. The flower stems suck up the pink water, and eventually the petals turn pink as well. “[The pesticide] can go into the leaves, the pollen and the nectar,” Hladik says. All parts of the plant become a pesticide buffet, toxic to biting and sucking pests—and incidentally, to bees.

In theory, coating seeds is a precise way to apply pesticides. Before seed coatings were used, farmers would apply pesticides by donning head-to-toe protective gear, hooking up a chemical-filled tank to a tractor, and driving through a field, spraying clouds of pesticide. Wind carried the pesticide in all directions.

Seed coatings eliminate the need for spraying. But precision application has unintended consequences. Coating every seed is preventative, meaning that it introduces pesticides to crops before pests appear. With sprays, farmers apply pesticides only when they see a problem, not before. And although each seed is tiny, acres of crops with millions and billions of coated seeds start to add up.

Moreover, plants take up only about 10% of the prophylactic pesticide. The other 90% of the dose is released into the soil. When it rains or the crop is irrigated, that water-loving chemical gets carried along with the moving water, ending up at the edge of the field. That’s where Hladik and her team scoop it up for measurement.

In this podcast, Sarah Derouin examines why scientists are finding surprisingly high doses of pesticides in streams and rivers, and what this means for ecosystem health. Illustration by Miranda Zimmerman.

Pesticide puzzles

To get a better idea of how widespread pesticides are in streams, Hladik and Dana Kolpin of the USGS in Iowa City, Iowa, tested for six different neonics in a nationwide study. Their results, published in 2015, showed that more than half of the 38 waterways they tested around the U.S. had at least one neonic present, and a handful of streams had three. But in California, streams carried a much higher number of neonics.

At one site on the Central Coast near Salinas, for example, Hladik found five neonics in a single sample, the highest number she saw anywhere in the nation. The site likely bore so many different chemicals because it was situated in a diverse agricultural area. Farmers favor different neonics for different crops, unlike farmers in Iowa, who might grow either corn or soy, and not much else.

“They grow so much there: berries, Brussels sprouts. It’s not a monoculture,” Hladik says.

But the sheer number of neonics surprised Hladik: “When I found five, I was like ‘Ooo, I found five! Wait… I don’t know if I want to be finding five,’” she recalls. Other sites around California were similar: Almost one-quarter of the streams sampled in the state had at least four neonics present.

We’ve been screaming for years about native bees. I think people are listening.

Even more puzzling, there were no signs of the origins of these neonics. California keeps a database of pesticide use, noting the date, time and amount of chemical applied to a crop. But the records didn’t show that farmers had applied such a diversity of neonics to their crops.

After poring through the data, Hladik had a thought: What about seed coatings? She called the California Department of Pesticide Regulation to ask whether such coatings were included in the database. The regulators replied that since farmers don’t apply seed coatings directly, they’re not required to report how many coated seeds they plant. Suddenly, seeing five neonics at one site made sense. The pesticides were arriving on the backs of the seeds, then flowing from fields to streams in agricultural runoff.

Hladik’s lab has now moved on to sampling plants and animals around the country for pesticide exposure. For example, she’s studying how much pesticide is contained in soil, plants and bees surrounding agricultural fields. The team wants to see whether planting rows of vegetation around crops helps reduce neonics escaping from fields.

Hladik lights up when she talks about the projects. When scientists in her lab process bees or other organisms, they first grind them up for analysis. The process releases the odor of the flowers on whose nectar the bees had dined: “They smell delightful,” Hladik says. While her staff usually takes on the grinding task, Hladik plans to process the bees herself. In a lab that might be testing mucky sediments or fish livers, working with floral, honey-scented bees is a welcome reprieve.

Beyond the honeybee

The precarious state of bees has garnered international attention over the past decade. From 2006 to 2008, 30% to 90% of honeybee colonies died worldwide. Scientists searched for reasons why these mass deaths occurred and determined that neonics played a role, both by killing bees and by weakening them enough that they succumbed to mites and disease. In 2013, the European Union banned three neonicotinoids—thiamethoxam, clothianidin and imidacloprid—that were deemed especially harmful to bees.

“Ten years ago, people’s reactions would have been, ‘Ew, bees… all they do is sting you,’” Hladik says. “But now, people care about bees. They really care about bees. A lot.” That rise in bee love has spurred researchers to ask other questions about how the pesticides act in nature—especially when it comes to other insects.

Once pesticides leave farm fields, their original intent—to help protect crops from pests—is thrown out with the proverbial bathwater. “Pesticides cannot differentiate between pests and helpers,” says Code. She’s particularly concerned about foundational species at what amounts to the bottom of the food chain. Code admits invertebrate insects are easy to ignore: “It’s hard to get people excited about cabbage flies or mayflies,” she says.

In this animation, illustrator Miranda Zimmerman depicts how pesticides from a plant’s seeds can suffuse the plant and the soil around it, ultimately draining into nearby waterways.

But invertebrates have key ecological functions, such as breaking down plant matter and keeping other insects in check. They also serve as food for birds, amphibians, bats and fish. Johanna Kraus, a research ecologist at the USGS in Fort Collins, Colorado, studies the effects of pesticides on insects in North Dakota wetlands. Her study area is a critical habitat for waterfowl that pass through North Dakota while migrating. “Adult insects are very important to insectivorous birds like songbirds, and for juvenile fowl, like ducks,” says Kraus.

Her preliminary work found five different pesticides, including one neonic, in the bodies of adult insects. She’s finding that wetlands with higher concentrations of pesticides have fewer insects, and thus less food for everything from spiders to birds.

Kraus also wants to know how insects with pesticides might affect food webs in these systems. For example, will pesticides bioaccumulate in the predators who eat the contaminated insects, just as mercury levels build up in some predatory ocean fish? She and her team are just beginning to suss out the effects of pesticides on these wetland ecosystems.

Native bees also are under severe threat from neonics. “There are 3,600 species of native bees in the U.S.,” says Code. Their life cycles and behaviors are distinct from those of honeybees. These pollinators are often solitary ground nesters and fly during even the cool parts of the day, unlike honeybees, which take flight in warm sunlight. Bee species like this face double jeopardy. Their ground nests are vulnerable to spraying and they are susceptible to “bloom sprays,” which farmers aim at flowers at times when honeybees aren’t flying.

Native bees are ubiquitous, but it has been hard for scientists to call attention to their plight until recently. Native bees are active pollinators, visiting more than 90% of crops. In early 2017, the rusty-patched bumblebee was added to the endangered species list after its population crashed by 87% over the past 20 years. “We’ve been screaming for three to four years about native bees,” says Code. “I think people are listening.”

Reduction or elimination

With neonics spreading into waters around the country, scientists are considering how to reduce or eliminate them. One solution is simply to reduce their use—not an easy task, especially when it comes to seeds. More than 90% of corn seeds are coated with pesticides, and Hladik notes that some of her colleagues in Iowa say they cannot even find untreated seeds. Worse yet, insurance claims on a bad yield year can be denied if a farmer plants a non-treated seed; planting coated seeds is viewed as “accepted standard practice” to prevent crop losses.

Farmers are becoming more aware of the environmental ramifications of using pesticides. Kim and Pat Gallagher of Erdman Farms in California’s Central Valley are third-generation almond farmers. When they took over their farm, they were using neonics, but they stopped in 2010 when the UC Davis agricultural extension office started distributing information about bee health and pesticides.

On almond farms, it was common practice to combine bloom sprays—a fungicide application to the almond flowers—and neonics, just because it was easy to apply two sprays at the same time. But neonics and blooming trees are a bad mixture for bees busily collecting pollen and nectar. The Gallaghers stopped using neonics and switched to soft sprays: less-potent pesticides that have been tested and are considered safe for bees. Moreover, the Gallaghers spray in the evenings, after the bees have flown for the day.

Results have been positive, they say: “There was no real noticeable change in yields,” says Pat Gallagher. Just as important, he says, “There was no increase in [pest] problems either. We’re happy to do something to help the bees.”

There is also evidence that pesticides may suppress some crop yields. Scientists at Pennsylvania State University found that neonic-coated soybean seeds killed insects that feed on slugs. The slugs multiplied, feasting on soybean plants and reducing soy yields by 5%. In addition, the neonic did not kill the slugs, but the slugs carried the pesticide in their body. Any animal or insect that ate the squishy critters got a dose of pesticide as well.

While conservationists like Code would like to see neonics out of the environment, she knows what might happen in the resulting void. “All banning does is create a space for a new chemical,” she says. “And every time we find a new chemical, we get a new problem.” Many of her fellow researchers refer to this cycle as the “pesticide treadmill.”

Hladik agrees. “It’s the devil you know versus the devil you don’t,” she says. If regulators took neonics off the market today, Hladik isn’t sure what would replace them; farmers might return to using older chemicals. “It’s not like they’re going to stop using pesticides,” she notes.

Hladik knows the pesticide treadmill might never turn off. But she also knows agencies like the EPA use her work to protect the environment by creating regulations and testing programs. As new pesticides hit the market, Hladik will keep tracking their fates.

© 2017 Sarah Derouin / UC Santa Cruz Science Communication Program

Sarah Derouin

Sarah Derouin


Sarah Derouin

B.S. (geology) St. Norbert College

M.S. (geology) Fort Hays State University

Ph.D. (geology) University of Cincinnati

Internship: EARTH Magazine

I grew up on a farm on Michigan’s Upper Peninsula. My days of quiet wonder growing tomatoes, uncovering crawfish, and collecting the prettiest rocks were the beginnings of my development as a scientist, although I didn’t realize it. In college and grad school, geology felt like the perfect fit. For my Ph.D. research I discovered new details about the glacial history of my home state, and I thought back to my childhood explorations through the lens of geologic time.

My work as a professional geologist, with its dry site characterizations and seismic safety analyses, dulled those passions. Only when I started teaching at a community college and writing about current geology topics did I realize that science reporting, not technical reports, fulfilled me. Now all the world, both past and future, is open to explore.

Sarah’s website

Miranda Zimmerman

Miranda Zimmerman


B.A. (biology; minor in visual art) California State University, Monterey Bay

Internship: Field Museum of Natural History, Chicago

From an early age, Miranda was an art and nature enthusiast. Growing up in Fresno, California, she passed the time by illustrating digitally with the peaceful sounds of David Attenborough in the background. Her work is influenced by both the real and fictional—often drawing inspiration from mythology and animated stories. Unable to choose between her two loves of art and science, she studied biology (with a concentration in ecology and evolutionary biology) at CSU Monterey Bay. It was there that she discovered the field of Science Illustration, and subsequently, CSUMB’s renowned graduate program. She also manages her own personal brand.

Miranda’s website