It’s mostly harmless wax, says Kay Bogart, a winemaker who works in outreach for the University of California at Davis’s viticulture program. The grape plant produces it to protect the berries from moisture loss. It’s also often just plain old dust, adds Jim LaMar, a professor of wine sensory evaluation at California State University, Fresno. Until recently, winemakers believed the white stuff was yeasts, responsible for wine fermentation. Now they believe such yeasts are airborne. In any case, that white stuff isn’t pesticide residue. Which is not to say that there isn’t pesticide residue on the grape, so wash it anyway before you pop it into your mouth.