While state highway crews and utility companies remove thousands of ash trees killed by a voracious little bug, a few researchers in central Ohio hope to save lingering ash trees that show signs of resistance to the emerald ash borer.

December is the biggest month of the year for tree trimming and removal along Ohio's roads, and ash trees remain a prime target for cutting because so many have died and become potential hazards.

This year, the Ohio Department of Transportation has invested almost 51,000 man hours and more than $5 million to trim and remove trees, said ODOT spokesman Matt Bruning, who noted that the cost includes work by both ODOT employees and contractors.

American Electric Power, one of several big utility companies in Ohio, spends $50 million a year on tree trimming and removal in the state, and ash trees remain a focus in that work, said AEP Ohio spokesman Scott Blake.

The larvae of the emerald ash borer, an iridescent green insect smaller than a dime, first were detected in North America in 2002 when they started chewing their way through street trees in Detroit and Windsor, Ontario. They had arrived in those port cities in the wood of shipping containers from China. The larvae kill trees by eating the life-giving, nutrient-carrying layer beneath the bark of ash trees.

From those cities, the insects have spread through dozens of states, killing most ash trees in their path.

In earlier years of infestation and with the deaths of so many ash trees, AEP Ohio was cutting more than 10,000 trees a year. Because ash trees were long considered hearty, fast-growing and able to stand up to the harsh conditions in cities, they were popular street trees. That created a lot of work for AEP, which has power lines running along many of those same streets. So AEP approached leaders in cities across Ohio with a one-time offer to remove all ash trees — living or dead — in the public rights of way in one fell swoop at no cost to the municipalities.

"It's certainly cheaper than sending people to replace lines and poles — and the inconvenience to customers," Blake said. "We've been aggressive about this, and outages by trees in rights of way are down 83 percent in the last three years."

The rest are tackled in December when ODOT crews aren't paving or plowing — and when endangered species such as the Indiana bat are hibernating in caves rather than nesting in the dead and dying trees they like.

And while chain saws are busy along the roads, U.S. Forest Service researchers Kathleen Knight and Jennifer Koch and their colleagues working north of Columbus in Delaware are hoping that their findings might result in some ash trees surviving the ash borers' assault.

Knight, a research ecologist, said that before the ash borer, one in 10 trees in Ohio was an ash. Ash trees are now considered endangered for extinction. Their loss leaves a gaping hole in the ecosystem because they played important roles as water filters, shade providers and habitats for many species, particularly along waterways.

Knight had been tracking the path of the emerald ash borer and noticed that among some large stands of green ash killed by the bug, a few ash trees had survived even after being attacked by the insects. She mentioned that to Koch, a research biologist, who is working with others to understand why those "lingering ash trees" survived.

They get excited when they slice into the branch of an ash that clearly has been attacked and they find ash-borer larvae that were killed by "host defense mechanisms," which means that something in that tree's system is toxic to the bugs.

So the researchers are cross-breeding the healthiest lingering ash trees and growing saplings in hopes of generating seeds for trees that could have a natural ability to ward off attacks by emerald ash borers.