If you want to see what a live, healthy ash tree looks like, either go online or consult a guide book.

If you want to see what a dead ash tree looks like, just drive around Knoxville.

Those bare-branched specimens along roadsides and in yards? Those gray skeletons on otherwise lush, green forested hillsides? Chances are they're ashes.

Or were.

For this tragic loss, we can blame (1) the emerald ash borer, an Asian insect and (2) human beings, exotic or otherwise, who have helped spread the infestation by moving firewood from one location to another.

Doesn't matter if the firewood is dead. It still could contain larvae of the borers. The only way to be certain you aren't aiding and abetting the ash die-off is to use kiln-dried wood. This is now required on many government lands, including the Great Smoky Mountains National Park.

Kasey Krouse, Knoxville's urban forester, says the borer infestation in North America is believed to have originated shortly after the turn of the 21st century. Apparently the first larvae arrived in this country via wooden packing crates from China. The problem stayed in the northern U.S. for a few years. Then it took off like a shot.

"Borers were detected in the Knoxville area in 2011," he said. "Affected trees were found near a gas station on I-40 near Watt Road. We suspect they came in on firewood."

Since then, the insects have devastated trees throughout this region.

Krause is treating a cluster of 25 ashes with pesticide in hopes of saving them. But such gentle care, while effective in small areas, simply isn't feasible on a grand scale.

University of Tennessee forestry professor Scott Schlarbaum believes the genie is already out of the bottle. The infestation is so widespread, it has taken on a life of its own.

"It's just another example of how a native species is negatively affected by a nonnative pest," he said, likening the loss of ashes to earlier purges of the American elm and American chestnut.

From an economic standpoint, ash is important in the manufacture of tool handles, flooring, furniture and, of course, baseball bats. A few days ago, National Public Radio aired a report describing the borer's impact on the bat industry, noting this wood has no rivals for strength and durability. In Tennessee alone, the value of ash is estimated at $11 billion.

Think this picture is bleak? Hang on. Things could get much worse. Schlarbaum cited the potential for even greater disaster via "sudden oak death."

"It came from Europe," he said. "Already there are infestations in California and Oregon. If this gets into the Southern Appalachians, the results would be horrible."