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Ambrosia beetles nurture their gardens of fungus with alcohol

No one likes a moldy peach, so some farmers stop fungus from growing by dipping their produce in alcohol. But that trick doesn’t work on ambrosia fungus, which fungus-eating beetles raise in “gardens” that have a ready supply of ethanol. A new study suggests the alcohol not only helps the fungus grow, but it also inhibits microbial “weeds” that would otherwise crowd it out.

Ambrosia beetles survive by boring into trees and growing fungi inside. They prefer stressed or dying trees, which have more ethanol—an alcohol that’s produced naturally by the plant—flowing through their tissues. To find out why, researchers took a closer to look at the black stem borer (pictured), an ambrosia beetle native to Asia that has become a tree-boring pest in North America.

The researchers collected fungus from black stem borers in an Ohio woodland. Then they grew the fungus, a species called Ambrosiella grosmanniae, on laboratory plates that contained food with different concentrations of ethanol. They found that the fungus grew best when there was some ethanol—about 1% or 2%—but did worse when there were higher or lower concentrations, the team reports today in the Proceedings of the National Academy of Sciences. What’s more, even small amounts of ethanol stalled the growth of microbial “weeds” that crowd out the beetles’ food source, like the fungus Penicillium.

So for ambrosia beetles, the smell of ethanol isn’t just a sign that a tree is stressed or dying. It’s a sign that the conditions are ripe for the perfect garden.

*Correction, 10 April, 9:55 a.m.: An earlier version of this story misstated that Penicillium is a bacterium.