When St. Louis Cardinals third baseman Matt Carpenter picked up a bat for the 2016 baseball season opener Sunday against the Pittsburgh Pirates, he selected one made of maple. These days, maple is the bat material of choice for most pros. But scientists say that if ballplayers like Carpenter ever want to go back to traditional ash bats, they might not have the chance. The supply of ash in the United States is under threat, and with it, the iconic Louisville Slugger. The culprit: an invasive beetle called the emerald ash borer.