Researchers have identified a culprit for the dramatic honey bee die offs among the almond groves of California's Central Valley.

Experiments showed a mixture of insecticide and fungicides, harmless in isolation, combined to form a deadly chemical cocktail.

"Fungicides, often needed for crop protection, are routinely used during almond bloom, but in many cases growers were also adding insecticides to the mix," Reed Johnson, bee expert at Ohio State University, said in a news release. "Our research shows that some combinations are deadly to the bees, and the simplest thing is to just take the insecticide out of the equation during almond bloom."

There aren't nearly enough native bees to pollinate California's almond trees, so every spring, growers pay to truck in 1.5 million colonies. The seasonal bee rental costs $300 million. In 2014, 80,000 colonies experienced adult bee deaths or dead and deformed broods. Many colonies were lost entirely.

In an effort to understand what went wrong, scientists at Ohio State compiled a list of all the fungicides and insecticides deployed by Central Valley growers. Back in the lab, researchers combined the chemicals. Tests showed bees exposed to the cocktail of insecticides and fungicides were more likely to die or produce deformed offspring.

All of the insecticides tested by Johnson and his colleagues were legal and considered safe for bees. But the new research, published in the journal Insects, suggests insecticides safe in isolation can prove deadly when combined with other pesticides and fungicides.

Researchers want almond growers to stop spraying insecticides of any kind during the almond bloom -- when 80 percent of the nation's honey bee population is concentrated in a single region.

"I was surprised -- even the experts in California were surprised -- that they were using insecticides during pollination," Johnson said. "I think it was a situation where it wasn't disallowed. The products were thought to be bee-safe and you've got to spray a fungicide during bloom anyway, so why not put an insecticide in the tank, too?"

The growers weren't spraying just to spray, they were trying to curb populations of the peach twig borer, which can damage trees and suppress yields. Scientists argue growers can spray for the peach twig borer earlier in the year, when billions of honey bees aren't around.

"They have other opportunities to do that before the bees enter the almond orchards or after they are gone," Johnson said.

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