The crack of bats this time of year means two things: Baseball is back and winter is over. 

But Major League Baseball's spring soundtrack relies heavily on wood from white ash trees treasured for strength and flexibility. And it may soon sound different as a tiny beetle threatens the northern Pennsylvania stands that for more than a century have supplied wood for bats. 

Experts say climate change may alter just how far north and south the tree-killing pest will spread. Northern Pennsylvania is right in the emerald ash borer's sweet spot.

"If climate change reduces those extreme cold temperatures in northern states, which can kill them, it may allow it to spread north faster," said Dan Herms, a professor and entomologist at Ohio State University. "But surely they'll get to where Louisville Slugger is." 

'Knocking on the doorstep'
Half of Major League players use a Louisville Slugger bat, according to the company. White ash makes up 45 percent of their wood bats, and all of the company's white ash comes from forests in northern Pennsylvania and New York. 

Hillerich & Bradsby Co., which has been making Louisville Slugger bats for more than a hundred years, turns about 12,000 to 15,000 white ash trees into bats every year. 

"We haven't seen it affect our ability to get logs yet, but it's knocking on the doorstep," said Brian Boltz, a general manager at Hillerich & Bradsby Co., the parent company of Louisville Slugger. "It's pretty established both 50 miles north and south of our main harvesting areas."  

The pest was first found in Western Pennsylvania in 2007. Today 47 out of 67 counties have emerald ash borer problems, said Sven-Erik Spichiger, an entomologist with the Pennsylvania Department of Agriculture.  

A buffet line
Native to Asia, the insects were first found in metro Detroit in 2002. They have spent the last decade expanding their range, and the outlook is grim. 

The beetles first eat ash tree leaves. Then the females lay eggs in the bark. 

When these eggs hatch in summertime, larvae chew through the outer bark. The inner, nutrient-carrying tissue of the tree becomes a buffet line. The tree, starved of nutrients and water, eventually dies.

 "Given enough time nearly 100 percent of all green, white and black ash" – the three most important and abundant North American ash species – "will be killed" throughout their North American range, according to a 2013 U.S. Forest Service study.

Ash grows primarily in the Midwest and East, with an estimated 8 billion ash trees in the United States. Of the 32 states with sizable stands, researchers estimate that about 20 are battling the pests. 

Hastening the decline
And climate change – particularly more frequent or harsher drought – could hasten the decline. 

"When ash trees are stressed by something like drought, you'd probably see a slightly faster growth of the emerald ash borer population," said Deborah McCullough, a professor in Michigan State University's department of entomology. 

A healthy tree, she added, might take two years to succumb to a borer infestation. "In a stressed tree, it'd be more like one year."

Very cold days – specifically those lower than 30 degrees Fahrenheit – can kill emerald ash borer larvae that overwinter in tree bark, said Louis Iverson, a landscape ecologist with the U.S. Department of Agriculture Forest Service. 

"More areas experiencing the deep colds might slow the spread," Iverson said. 

But it's unlikely that cold snaps – even ones as deep as this year's – will get rid of entire populations, said Ryan DeSantis, a natural resource advisor with the University of California's Cooperative Extension. For instance, snow can act as insulation and keep trees warm during a cold snap, he added. 

Helping the spread
The Northeast had about 30 percent fewer extreme-cold days over the past 30 years compared to the century before, according to the National Oceanic and Atmospheric Administration. Annual precipitation in the region is trending upward, but drought trends are unclear, according to the agency. 

But climate change isn't the main driver in the spread of the borer. People are.

"To be perfectly blunt, by the time serious climate change impacts kick in, place like Pennsylvania might not have any ash trees left," McCullough said. 

Added Iverson: "When you look at spread potential, these things move quickly and take over places that are near highways, heavy traffic zones, places where there might be camping."

Boltz said the forests that Louisville Slugger taps for bats are largely isolated. 

Reluctantly diversifying
Still, the company is reluctantly diversifying.  Within the past couple of decades, the company has started using yellow birch and maple for bats as well [Sidebar: With shift to maple, more broken bats]. 

But many of Major League's stars – including retiring Yankees great Derek Jeter and Cincinnati Reds first baseman Joey Votto – remain "ash guys," Boltz said.

"With white ash you have the rare combination of strength and flexibility," Boltz said. "And the white ash trees in northern Pennsylvania are perfectly suited for those characteristics."

This article originally appeared at The Daily Climate, the climate change news source published by Environmental Health Sciences, a nonprofit media company.