The Philadelphia Department of Parks and Recreations is starting to chop down the city's ash trees  in preparation for the emerald ash borer's arrival in the county.

The invasive beetle from Asia has been marching east after landing in the U.S. Midwest more than a decade ago.

"We have confirmed positives in Montgomery and Bucks counties, so at this point, it's most likely (in Philadelphia), we just haven't confirmed it yet," said Lawrence Barringer, an entomologist with the Pennsylvania Department of Agriculture.

"There's no stopping it, sadly," said Joan Blaustein, director of urban forestry and ecosystem management at the Philadelphia parks department.

Blaustein's division is removing trees close to roads, hiking paths and power lines that could hurt people if they were allowed to fall naturally.

About 5 percent of Philadelphia's tree canopy is made up of ash trees susceptible to the invasive beetle, Blaustein said. She projects most of the trees will be dead within five years.

"Other cities have documented that it kills about 95 percent of the trees," Blaustein said. "Once (the borer) has infested, it takes about three years for it to kill the tree, and then they just fall."

The city has targeted 1,000 healthy trees to save by treating them with insecticides over the next 10 years.