A recent investigation into the death of thousands of bees last month in Minnesota revealed that fipronil, a widely used insecticide, was to blame.

In mid-September, three colonies of bees in Minneapolis were found twitching and dying on the ground. Local apiarist Mark Lucas paints a grim picture of the poisoning event, which he witnessed, recalling that bees inside the hive came “spilling out of the hive as if they were drunk.”

University of Minnesota Bee Lab, the University’s Bee Squad, and the Minnesota Department of Agriculture (MDA) carried out the investigation, taking samples from hives to confirm pesticide poisoning. Indeed, MDA tests found that all three of the affected hives tested positive for the presence of fipronil.

Although neonicotinoid pesticides such as clothianidin, thiamethoxam, and imidacloprid have been widely implicated in the phenomenon of colony collapse disorder (CCD), other pesticides are known to adversely affect honey bee health. Fipronil has also been heavily implicated in elevated bee toxicity and decline. Indeed, the European Union (EU) recently put forth a proposal to restrict the use of the pesticide in recognition of the high acute risks it poses to bees. The chemical is widely used for indoor and turf pest control in the U.S., incorporated in more than 50 pest-killing products, is highly toxic. Fipronil has been shown to reduce behavioral function and learning performances in honeybees. A 2011 French study reported that newly emerged honey bees exposed to low doses of fipronil and thiacloprid succumbed more readily to the parasite Nosema ceranae compared to healthy bees, supporting the hypothesis that the synergistic combination of parasitic infection and high pesticide exposures in beehives may contribute to colony decline. An extensive overview of the major studies showing the effects of pesticides on pollinator health can be found on Beyond Pesticides’ What the Science Shows webpage.

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