Pesticide Causes Massive Bumble Bee Massacre

By Dewey M. Caron

In a suburban shopping mall in Wilsonville, OR, south of Portland, a massive loss of bumble bees has been attributed to use of the neonictinoid pesticide Safari.  It is an unusual pesticide poisoning incident highlighting that suburban locations, not merely agricultural fields, can sometimes be hazardous to foraging bees.

Early Saturday morning shoppers to the Argyle Square Target store June 15th in Wilsonville where concerned when they saw “tens of thousands” of dead and apparently dying bees. Bodies littered the ground and sidewalk beneath flowering trees at the store entrance and in the parking lot. Still living bees were acting like they were drugged, spinning on the asphalt while others clung and buzzed crazily among the flowers and foliage on the same trees. Ironically this gruesome discovery marked  the beginning of the week designated as Pollinator Week in the U.S.

Rich Hatfield of Xerces Society was called to come investigate the kill. Xerces Society is a world-renowned, non-profit invertebrate and habitat conservation organization based in Portland. Rich, a bumble bee specialist, found large numbers of dead and dying yellow-faced bumble bees Bombus  vosnesenskii. B. vosnesenskii is a distinctive and common Oregon bumble bee, easily recognized by it’s bright yellow markings on head and top of the first thoracic segment and a single yellow stripe on the lower abdomen. It is otherwise densely black and hairy with blackish wings. It is a small, round-shaped, slow flyer. He estimated over 25,000 bumble bees were involved likely representing more than 300 colonies.

Rich also noted a smaller number of B. mixtus and B. melanopygus bumble bees among the bodies. He found a smaller number of dead honey bees and lady beetles as well. NOTE: Bombus vosenesenskii, represented with over 95% of the observable kill, should not be confused with the yellow-banded bumble bee Bombus terricola, a bumble bee species of great concern and considered threatened in the eastern U.S.

Surveying the extreme losses  “We immediately contacted the Oregon Department of Agriculture (ODA) and asked them to test the bees for pesticide poisoning,” said Mace Vaughan, the Xerces Society’s Pollinator Conservation Director and colleague of Rich Hatfield. “They were literally falling out of the trees.”  ODA investigators from pesticide safety, contacted the landscaping firm responsible for maintenance of the parking area and learned that the pesticide Dinotefuran ( Safari) had been applied to the trees of the parking lot that very morning.

Safari is a neonicotinoid sold by Valent, a division of Syngenta. It is readily available to homeowners and can be applied without a pesticide license.  In fact the very same Target store sells it to homeowners for insect control on roses.  As listed in our soon to be released update of  PNW Extension Publication How to Reduce Bee Poisoning from Pesticides (PNW Publication No 591), Safari is  classified as  “highly toxic” to bees and buried in the product label is the caution to avoid application “if bees are visiting the area.” How long the material remains toxic following application is unknown.

Suspecting the bee deaths were directly related to application of Safari, ODA took samples from the dead and dying bees and from flowers and foliage of the trees. By Friday, analysis confirmed residues of the chemical in bees and on the trees.  With the early Saturday spraying and residue analysis it was concluded that pesticides, specifically Safari,  was the culprit and responsible for the massive bumble bee losses, since revised upward to over 59,000 individuals.

Safari had been applied to the trees to control aphids, which secrete a dark sooty honeydew. Aphids are indicted as about the only insect pest of silver lindens. Lindens, including silver linden, are known for heavy honeydew secretion and are viewed by some as a “problem” when the honeydew falls on car windshields creating a sticky nuisance.  Apparently the mall management firm had authorized the application of the pesticide simply to avoid customer complaints. A “cosmetic” application situation. Unfortunately at application, the silver linden, 55 trees in all, were in full bloom.

Could there be syngerism?

But the story gets complicated. The dead and dying bumble bees were beneath silver linden trees Tilia tomentosa. This non-native, heavily fragrant tree is described as ideal for shade and its attractive form and fragment bloom in large parking lot islands, as found in the Target shopping mall. Lindens are also known as lime trees (in Europe) or basswood to honey connoisseurs. Basswood honey is a distinctive, water-white honey, often with slight greenish tint. Some consider it one of the finest honeys for direct consumption. Basswood timber was also once used extensively for comb honey section boxes.

Silver leafed lindens are not common in the U.S. Beekeepers might better know the little-leaf linden tree, Tilia cordata, another non-native, more widely planted especially as a hearty street tree or the native American linden, Tilia americana. Still rarer is the white basswood tree, Tilia heterophyllia,  also known as “bee tree linden.”

The unusual twist to this story is that linden is one of a small number of plants known to be toxic to bees. Among lindens, the silver linden tree has been shown to secrete a toxic nectar, perhaps due to the sugar mannose, that is apparently particularly toxic to bumble bees. The abstract of a 2007 study in Germany noted “dying of bumblebees under the late flowering lime tree Tilia tomentosa has been described by several authors” and that “numerous dead bumble bees are found under silver linden trees.” The study found that “Honey bees foraged on T. tomentosa only at times of high nectar availability…. Bumblebees … visited the trees throughout the flowering season.” The different foraging strategies of the two bees was surmised by the authors Ingrid Illies and Werner Mühlen as the reason why bumble bees, more so than honey bees, fall prey to the toxic nectar. (published in Entomologia Generalis Volume 30 Number 2 (2007), p. 155 – 165)

The website ehow states “Linden tree flower nectar is safe for honey bees in low amounts, but poisonous in excess amounts. Honey bees are most likely to become poisoned under certain environmental stressors, such as abnormally low soil moisture in the bee’s foraging area, which will force the bee to take in too much nectar from the linden tree’s flowers.”

Why so many bumble bees?

This shopping square is in a suburban area with some remnant farms. Directly behind the store is a 32 acre field now of mixed flowers. Last year it was a crimson clover seed field. The large numbers of bumble bee nests might be attributed to the rich foraging opportunity in this field. Honey bee colonies, rented the previous season for seed pollination, were not on site this year as the property is being sold and no longer actively farmed as in previous years. There was no indication of dead bees in this field nor reports of massive bee kills in previous seasons. Bee colonies located in the area are being checked for evidence of pesticide damage.

Whether the massive bumble bee kill is a highly unusual case of an erroneous application of pesticide leading to the poisoning of native bumble bees or a still rarer instance of massive toxic nectar poisoning, or a synergistic effect of the two, may never be determined. Further analysis will be needed. It is possible, as we are finding with CCD in honey bees, that multiple factors may have been at play creating another instance of unfortunate and inadvertent heavy losses to our pollinators in this suburban mall. The bumble bees are the clear losers.

Finding a temporary solution

With the large kill it was obvious a solution was needed. In an effort to mitigate further damage to foraging bees, several options were considered. One option discussed was to immediately strip off flowers and leaves from the trees. Another was to apply a non-toxic repellent to keep bees and insects from visiting the blossoms.  Both were rejected in favor of covering the linden trees with large nets to prevent bumble bees and other pollinators from reaching the flowers. Plastic shade netting was used to cover the 55 trees. Netting of the trees was completed finally on Friday, nearly a week after the first kills were reported.

And in yet another twist, a well- known phenomena, a “copycat” story, appeared in the local media. In another Portland suburb, Hillsboro, about a 100 dead bees were discovered under a tree maintained by the city. These trees too had been sprayed with Safari, but legally as a systemic application, several months earlier. The city was planning to cover the tree with a net as was done in Wilsonville as a preventive measure.

In the meantime specialists were trying to inform the public of the nature of the event. The Xerces facebook and website ( has been keeping the public well informed as well as trying to respond to the numerous phone and media inquiries. Mace Vaughan, the Xerces Society’s Pollinator Conservation Director  stated “To our knowledge, this incident is the largest mass poisoning of bumble bees ever documented. “

Could some good come from this unfortunate pesticide kill of bumble bees? Scott Hoffman Black, Executive Director at the Xerces Society put the loss in perspective as quoted in the The Oregonian newspaper lead story  “If the trees are indeed toxic they should be cut down and replaced by something that will provide non-toxic pollen and nectar for bees… On the other hand, if pesticides are the cause, we need to spotlight this as a real-world lesson in the harm … toxic chemicals are causing to beneficial insects.”

Mace Vaughn of Xerces and others hope this story, before it is replaced by something else in the news, will spur an effort to limit pesticide sprays for “cosmetic” (i.e non-essential)  use, especially since this case highlights the unintended consequence such spraying may have. The ODA is considering the possible fining of the applicator but the fine would only be in the $1000 to $10,000 range. Someone has to speak for the bumble bees. Could this bumble bee massacre spur Oregon, perhaps like Ontario, to enact legislation to speak for them and against careless and inconsiderate (and unnecessary) use of pesticides?


  • Dewey M. Caron says:

    CORRECTION: ID as Silver Linden is incorrect – trees are European Linden and consensus is that they are little-leaf linden Tilia cordata. OR Dept of Ag has banned pesticides for ornamental uses that contain dinotefuran for 180 day period as their analysis of bees from Wilsonville did reveal high levels of the chemicals in bees and linden foliage. Results of analysis of bees from Hillsboro “Copycat” not yet available from ODA. Dewey M. Caron 7-2-13

  • Cristobal Corredor Rincón says:

    Did Oregon State Scientific Staff cheked the honey and pollen at the honeybee combs

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