There’s been a lot of buzz about the disappearance of honeybees, but explanations for why their numbers have shrunk by more than one-third nearly every year since 2007 have been slow in coming.

Bees may be annoying when they sting, but they also pollinate a third of all food crops, including almonds, strawberries, and pears. Honeybees contribute $12 billion to agriculture income each year.

Pesticides have emerged as one major suspect in the mystery of the missing bees. But of the many in use, only neocontinoids, nicotine-like chemicals, have been somewhat conclusively linked to bee colony collapse.

A recent study published in the Journal of Economic Entomology sought to provide a more complete answer by testing a host of common pesticides to see what effect they had on honeybees.

The work identified 26 highly toxic chemicals, including not just neocontinoids but also organophosphates, which are often used when cities spray for mosquitoes, and pyrethroids, which are the active ingredient in many household insecticides.

The Dose Makes the Poison

Researchers Yu Cheng Zhu, John Adamczyk, and Jeff Gore looked at toxicity in a more nuanced way than previous studies have.

They considered how deadly a chemical was to bees at the average concentration used in agriculture.

That turned up some new suspects, including carbaryl, a pesticide that is favored for food crops in the United States because it isn’t concentrated in fat or secreted in milk.

Dennis vanEngelsdorp, Ph.D., a honeybee expert at the University of Maryland who was not involved in the study, said it offered “an evenhanded approach that considers real-world exposures.”

The long list of chemicals analyzed — and some of the surprising findings — “suggests we need to think broadly, beyond one pesticide class, when we consider real-world threats to honeybees,” vanEngelsdorp said.

But pesticides aren’t the only threat to honeybees, according to Jennifer Sass, Ph.D., a senior scientist at the Natural Resources Defense Council.

Their numbers have been falling for several decades as agriculture has expanded its land base and ramped up the chemicals it uses. While herbicides, or weed killers, aren’t directly toxic to bees, they eliminate their habitat.

“It also has a lot to do with habitat — the expansion of farmland and the increased use of pesticides and herbicides on their lawns and gardens. People didn’t used to do that,” Sass said.

The Usual Suspect

RoundUp, Monsanto’s controversial herbicide used primarily on genetically modified (GM) crops, is often at the heart of critiques of industrial farming. But the new study ranks RoundUp among the least toxic chemicals to bees.

Sass was not surprised, though. RoundUp is not designed to kill insects. It’s designed to kill noncrop plants that bees might otherwise enjoy.

“We can have agricultural land that provides bee habitat, but it would involve letting a few weeds grow,” Sass said.

The researchers pointed to another way that RoundUp may indirectly threaten bee populations.

As GM crops have become more widespread, the types of pests have shifted. Insecticides that coat leaves are more effective on these pests.

But when leaves are coated, the chance of “foraging honeybees coming into direct contact with insecticides” goes up, the study said.