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New Study Revisits 2013 Pesticide Bee Kill in Oregon

Vosnesensky bumble bee (Bombus vosnesenskii)

In June 2013, a pesticide application on ornamental trees in a shopping-center parking lot in Wilsonville, Oregon, led to the largest documented mass fatality of bumble bees in North America. A new analysis of the incident estimates more than 100,000 bees—primarily Vosnesensky bumble bees (Bombus vosnesenskii)—from nearly 600 colonies were killed, which researchers cite as a cautionary tale about the dangers of pesticides to native bee populations. (Photo via Flickr/USGS Bee Inventory and Monitoring Lab, public domain)

By Ed Ricciuti

Ed Ricciuti

Ed Ricciuti

By a rather unfortunate coincidence, the largest documented pesticide die-off of bumble bees in North America, which killed as many as 100,000 Vosnesensky bumble bees (Bombus vosnesenskii) in Oregon eight years ago, occurred during National Pollinator Week.

Bad as it seemed at the time, the kill portends a more troubling long-term threat to bees from the insecticide, dinotefuran, and its family of widely used neonicotinoid pesticides, according to a retrospective analysis of the kill published in June in the journal Environmental Entomology. “Our study underscores the lethal impact of the neonicotinoid pesticide dinotefuran on pollinating insect populations in a suburban environment,” write the authors.

By extrapolating the number of dead bees photographed in a given area, the researchers estimated the death toll was perhaps twice the original figure of 50,000. Beyond that, DNA fingerprinting of the bees employed by the scientists indicated that they belonged to almost 600 colonies, whose well-being may have been greatly harmed as a result. Even if exposure to the pesticide is not lethal, bees that take a hit from it can suffer biological and behavioral damage that reduces the reproduction levels of their colony.

The incident occurred at a Target store parking lot in Wilsonville, Oregon, in June 2013, after 55 blooming littleleaf linden trees, known for abundant flowering that draws bees, were treated with the pesticide to combat an aphid infestation. Another touch of irony about the incident was that the aphids posed no threat to the trees but rather to vehicles parked under them, which were spattered with the aphids’ honeydew waste.

Wilsonville, Oregon, bee kill map

A satellite view and diagram depicts the pesticide application that took place at a shopping center in Wilsonville, Oregon, in 2013 that resulted in the fatality of an estimated 100,000 bumble bees. White and gray circles represent the ornamental trees to which a foliar and soil-drench applications of dinotefuran were made. (Image originally published in Hatfield et al 2012, Environmental Entomology)

As far as the bees were concerned, the event was a perfect storm. The application occurred on a warm day in late spring when the linden trees, noted for abundant flowering, were in full bloom and bees were out in full force. The timing could not have been worse.

“They just happened to do it when there were a lot of bees,” says James P. Strange, Ph.D., chairman of the entomology department at Ohio State University and one of the authors of the new analysis. Strange served as a research entomologist at the U.S. Department of Agriculture-Agricultural Research Service (USDA-ARS) Pollinating Insect Biology, Management and Systematics Research Unit at the time of the incident. Other researchers on the new report are from the USDA-ARS, the Oregon Department of Agriculture, and the Xerces Society, a conservation organization that focuses on invertebrates.

The Oregon agriculture department came down hard on the applicator after ruling the pesticide was improperly applied. The label on the pesticide, approved by the federal Environmental Protection Agency (EPA), states that the product is toxic to bees exposed to it for more than 38 hours after treatment. Pointedly, it prohibits application to flowering parts of plants.

However, even with precautions, according to the paper, “Recent research suggests that there may be no safe time of year to apply systemic neonicotinoid insecticides to trees and shrubs to avoid sublethal/lethal effects on bees, even if label directions and bee precaution language are followed.”

After the kill, Oregon prohibited the use of neonicotinoid pesticides on linden trees, which are commonly used as ornamentals in urban areas, especially near parking lots and transportation corridors. Neonicotinoids are long-lasting and systemic, absorbed in plant tissue, so that the plant itself, including pollen and nectar, is toxic to insects. Flowers to which dinotefuran had been applied contained much, much more than the concentration (LC50) scientists measure as enough to kill half of an exposed population, in this case if it consisted of honey bees. Dead bumble bees from the kill had more than the maximum LC50 for honey bees.

The Vosnesensky bumble bees killed in the Wilsonville event are common along the West Coast of North America, their home range, but the researchers warned that “there is no way of telling if colonies of rare or at-risk species of bumble bees … were affected. Given the scale and scope of this event, it is likely that if any colonies were nearby, they may have been severely affected, potentially disrupting conservation and recovery efforts.”

While the decline of honey bees, which are considered domesticated, has received a huge amount of ink, the impact of pesticides on wild groups such as bumble bees has been understudied, according to the authors. Bumble bees’ long tongues, toleration of bad weather, and ability to shake loose pollen via muscle vibrations—”buzz pollination,” which honey bees cannot manage—makes them second only to honey bees as pollinators of agricultural crops. As well, they are key pollinators of wild plants throughout temperate regions.

The analysis presented in the paper does not look rosy as far as the continuing impact of pesticides such as neonicotinoids. “The lethal effects of pesticide poisoning on non-target beneficial insects continues to occur today,” the authors say. Adds Strange, explaining why the team conducted and published the analysis, “We wanted to alert people and make them more aware of the dangers.”

Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.


  1. The article left out most important information a reader would want to know – did the bumblebee population in that shopping center parking lot recover from the pesticide kill? I went to see for myself a year later in mid-June and also the June after that one. I am happy to report that yes, bumblebees were once again in abundance drinking nectar from the Linden trees in that parking lot.

  2. Has anyone conducted research about the potential toxicity of neonicotinoids to bees with regard to foliar applications only on trees/shrubs that have already flowered out or are not used by bees for pollination?.

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