There has been a lot of buzz in recent years about a group of chemicals known as "neonicotinoids." These pesticides affect the central nervous systems of insects, and are a suspected link to colony collapse disorder in domesticated honeybees as well as the rapid decline of many wild pollinator species.
About 85 percent of Earth's flowering plants rely on pollination by bees and other pollinators, according to the Xerces Society, a nonprofit that protects wildlife through invertebrate conservation. Bees also pollinate more than 30 percent of all plants that produce foods and beverages consumed by humans around the world.
"Neonicotinoids are one of the most serious causes of downward negative pressure on pollinators," according to Keith Delaplane, a professor of entomology and director of the Honey Bee Program at the University of Georgia. In fact, he rates neonicotinoids as the second-leading cause of decline in the nation's honeybees, reserving the top spot for the parasitic varroa destructor mite.
What are neonicotinoids?
"Neonicotinoids are a broad-spectrum pesticide that get their name from their basic chemistry, because it is close to that of nicotine," said Delaplane, emphasizing that "neonics," as they are often called, are not the same as nicotine. The neonicotinoid family includes specific pesticides such as acetamaprid, imidacloprid, dinotefuran, clothianidin and thiamethoxam. They gained popularity in agricultural and commercial ornamental production because they are effective against a wide range of insect pests, and are considered less hazardous to humans and other vertebrates than many insecticides.
"The hallmark of neonicotinoids is that they are systemic," Delaplane added. That means they travel throughout a plant via its vascular system and distribute the chemical to all parts of the plant tissue 24/7, including its nectar and pollen.
"Neonicotinoids just hammer insects," Delaplane said. While there are many target insects, such as whitefly, Japanese beetles, emerald ash borer and others, neonicotinoids are used in general to control sucking and chewing insects and beetles. But some of the insects they "hammer" are important pollinators such as honeybees, bumblebees and solitary bees.
How neonicotinoids became a cause of concern
In a 2014 report, David Smitley — a professor of entomology at Michigan State University who works with horticulture industries on solving insect pest problems — included neonics in a timeline tracing the decline of honeybees.
According to Smitley, honeybee decline began in the 1950s and sharply increased when parasitic mites were introduced into the United States around 1987. The neonicotinoid class of pesticides were introduced in 1994, but the rate of honeybee decline, while continuing, did not immediately get worse.
A turning point for neonicotinoid awareness occurred in June 2013, when 50,000 bees died in the parking lot of a Target store in Wilsonville, Oregon, near the Xerces Society headquarters. Scott Hoffman Black, executive director of the Xerces Society, said he confirmed the bees died from being sprayed with an insecticide that contained the neonicotinoid dinotefuran. He claimed the label instructions weren't followed.
In 2014, a Harvard School of Public Health study linked low doses of neonicotinoids to colony collapse disorder. Additional studies produced mixed results regarding pesticides' impact on bee declines, and also pointed to other factors such as the varroa mite and insufficient food sources.
In 2016, the U.S. Environmental Protection Agency (EPA) issued a "preliminary risk assessment" warning that bee colonies could be in danger from imidacloprid, a pesticide the agency had approved 22 years earlier. In hives exposed to more than 25 parts per billion of imidacloprid, the EPA reported a higher chance of "decreases in pollinators as well as less honey produced." A few months later, a study in the journal Nature reported that bees who frequent neonicotinoid-treated crops have suffered worse population declines than species who forage on other plants.
And while domesticated honeybees tend to get more attention, an array of wild native bees may also be at risk from neonics. In a 2017 study, for example, researchers found thiamethoxam dramatically reduces egg-laying by queen bumblebees, which were 26 percent less likely to lay eggs after being exposed to it.
As lead researcher Nigel Raine told the Guardian, this could have a disastrous effect on the formation of new bumblebee colonies — and thus on bumblebee populations overall. "A reduction this big in the ability of queens to start new colonies significantly increases the chances that wild populations could go extinct," said Raine, a professor of environmental sciences at the University of Guelph in Ontario, Canada.
As dangerous as neonics can be for bees, some species do seem to have natural defenses against certain types of the insecticide. In one study, researchers from the University of Exeter, Rothamsted Research and Bayer AG — a multinational corporation that produces neonicotinoids — reported that enzymes in honeybees and bumblebees buffer them against thiacloprid, a neonic that's less toxic to bees than others, like imidacloprid. This may shed light on new ways to protect bees from the insecticides, the study's authors say, although more research will be needed.
How do pollinators absorb neonicotinoids?
Bees can absorb neonics in several ways, such as by drinking nectar or transferring pollen. Another is a process called guttation, or the act of a plant sweating.
Corn, for example, sweats during the night. Bees can obtain water from guttation droplets, especially during dry weather.
Aphids, one of the real targets of neonicotinoids, insert their needle-like mouthparts into plant tissue and suck plant juice all day long rather than imbibing guttation droplets. The neonicotinoids are also in the sweet excrement, or honey dew, from the aphids, which honeybees collect. So it's possible for the honeybees to absorb neonicotinoids indirectly from a treated plant without ever visiting that plant.
How are neonicotinoids applied?
The most common form of applying neonicotinoids to agricultural crops is to treat seeds before they are sown rather than treating plants. The goal is to eliminate application issues such as drift that can cause collateral damage.
That doesn't always work out as planned, Delaplane said. There was a case in the Midwest, he pointed out, involving spring planting of neonicotinoid-coated corn seed. As the seed was being poured into the hoppers and run through the planters, insecticide-coated dust was released into the air.
There was so much dust that it formed a pink cloud, which drifted off-target onto nearby bee hives. Manufacturers have since tried to improve the formulation to prevent airborne drift, Delaplane said.
Also in 2014, Michigan State University conducted specific research about the use of neonicotinoids and made recommendations about their use for greenhouse growers that produce flowering annuals. In 2013, the EPA produced a strengthened bee advisory label. The agency required registrants of commercial pesticides that could be harmful to pollinators to include the label on packaging beginning in 2014.
A gardening center in a Home Depot store. (Photo: RustyClark/Flickr)
Neonicotinoids in the retail trade
Perhaps the best way for home gardeners to know whether ornamental plants they purchase at retail garden centers or big box stores have been treated with neonicotinoids is to ask the staff or look at the plant labels. Smitley's PowerPoint, for example, points out that Home Depot, one of the large retail chains that controls a lion's share of the flower and nursery market, is requiring a label in each pot of plants treated with a neonicotinoid insecticide. (Home Depot did not respond to a request for information for this story, but the company has announced plans to phase out the use of neonics on its plants "by the end of 2018.")
Lowe's, another major retail home garden plant source, is working with growers and suppliers of live plants to eliminate the use of neonics on plants that attract bees and other pollinators. It has pledged to phase out the pesticides by 2019, and to make brochures and fact sheets about pollinator health available in stores.
"Lowe's is also encouraging growers to use biological pest control methods when practical," said Steve Salazar, manager of Lowe's corporate communications. Neither seeds nor seedlings at Lowe's stores are treated with neonicotinoids, he added.
In the meantime, "Lowe's will be tagging plants and nursery products with information highlighting bee health and encouraging customers to be mindful of pollinator health when using pesticides," Salazar said.
Broad-spectrum insecticides are not necessary to grow a successful garden. (Photo: Parker Knight/Flickr)
What can home gardeners do?
Because neonicotinoids have been in the news, the public eye has been focused on plants at garden centers. Smitley says warnings about these plants harming pollinators have been exaggerated. In fact, he believes that purchasing flowering annuals, perennials and trees is beneficial for bees and other insects. "The discovery of neonicotinoid insecticide in the leaves and flowers of some garden center plants should not stop [home gardeners] from buying and planting flowers, because the benefit to bees far outweighs the potential risk," Smitley wrote in a 2014 paper.
Home gardens are not a primary food source for most bees, and even if neonics are present in some plants from retail centers, those plants will not necessarily harm bees, according to Smitley. Here are some reasons why:
- Many bedding flowers — such as petunias, impatiens and marigolds — are not typically treated with neonicotinoids.
- Many trees and shrubs (including all types of conifers) are pollinated by wind, and therefore not visited by bees.
- Perennial flowers, roses, flowering shrubs and flowering trees will only have neonics in their pollen and nectar for the first year or two after they are planted. However, these plants will be a valuable resource for bees and other pollinators for many years to come.
- Bees feed on a large variety of flowering plants within a mile of their colony home. The presence of a neonicotinoid in one plant will be diluted when the bees feed on untreated plants.
- Flowers in flats should be completely safe to bees.
Still, Smitley said in the paper that homeowners can take steps to help ensure bee safety with purchased perennial flowers and flowering trees.
These steps include:
- Removing the flowers in their first year in your garden or plant trees after they have finished flowering.
- Avoid spraying plants in your garden with insecticides, and never spray the flowers.
If holes that insects chew in leaves become unsightly, bee-friendly insecticides include products containing Bacillus thuringiensis (B.t.) and horticultural oils and soaps, according to Smitley's paper. B.t. can be used any time for caterpillars, and soaps and oils are safe to bees if sprayed early in the morning before bees are present. Be careful not to exceed the application rate on the product label, because at higher concentrations soaps and oils can cause plant injury.
Safe for humans
Neonicotinoids should not pose any threat to humans if they are used according the product label and stored in places not accessible to children. They have a low toxicity for all mammals, said Delaplane.
In fact, according to Smitley, the most widely used neonicotinoid, imidacloprid, is less toxic to people than caffeine, and about twice as toxic as ibuprofen.
Smitley offered a calculation that puts the toxicity of neonicotinoids for humans into perspective. Based on the required studies with laboratory rats, he has concluded that once garden-center products containing imidacloprid are mixed into a bucket of water for use as a drench around the base of a tree, the toxicity of that solution to people is about the same as the toxicity of wine.
Editor's note: This article has been updated since it was originally published in April 2015.
Photo of man spraying pesticides: Iakov Filimonov/Shutterstock