For the past several years, the decline of wild bee populations in North America has been seen as a sign of the impending implosion of the natural world. Bees pollinate crops, and if bees go, then food resources could become limited. By some estimates, populations have been reduced by as much as 30 percent, with certain states like Tennessee seeing an 80 percent loss of colonies in some years.

There is no single cause for colony collapse disorder; scientists believe disease, pesticides, and low species diversity in agriculture are making an impact. While pests and chemicals that kill them have long been thought to be culprits, researchers at the University of Texas at Austin say they’ve pinpointed a specific threat—one that could be sitting in your garage.

In a study published in Proceedings of the National Academy of Sciences, blame is being assigned to glyphosate, the active ingredient in the weed killer Roundup. Glyphosate targets an enzyme found only in plants and some microorganisms and is not thought to be harmful to animals. But the paper argues that glyphosate is having an effect on the gut bacteria in bees, making them susceptible to sometimes-fatal infections.

Researchers collected hundreds of honey bees from a single hive and gave some of them a solution with either 5 mg or 10 mg of glyphosate per liter, similar to concentrations found in the environment. Bees in the control group got a harmless solution of syrup. Researcher dabbed the dosed bees with pink paint to tell them apart. After three days, researchers observed that the glyphosate-dosed bees had less beneficial gut bacteria than the controls; one microbe that protects against infections had the biggest decrease. Two days later, the bees were introduced to a common pathogen, Serratia marcescens. Half of the bees with normal gut microbiomes survived the resulting infection, while only a tenth of the glyphosate-exposed bees survived.

Reacting to the study results, Bayer—which owns Monsanto, which developed Roundup—said that bees are not necessarily absorbing the same quantities of the chemical out in the open and criticized the study’s small sample size.

Previously, glyphosate had been linked to bees having an impaired sense of direction after exposure and having greater difficulty finding their way back to the hive. The chemical has also been implicated as a possible carcinogen in humans.

[h/t NPR]