AMES, Iowa, Sept. 11 (UPI) -- Thousands of ash tree seeds must be saved before the emerald ash borer completes its destructive drive across North America, biologists said.

The ash borer so far has killed an estimated 70 million trees -- the devastation spreading from Michigan, where the Asian pest was discovered in 2002, said Mark Widrlechner, who heads a nationwide effort to collect seed from stands of healthy green, white, blue, and black ash.

Widrlechner estimates he's collected about 10 percent of the seed needed to reintroduce ash trees in the future, he said a release from the North Central Regional Plant Introduction Station in Ames, Iowa.

"It doesn't just attack sick trees," Widrlechner said the borer. "It attacks healthy trees. It attacks small trees. So you don't have just big, old trees falling to this, you've got 2 to 3 inch saplings falling to this."

Seeds collected from healthy ash trees could be used to make a more resistant variety of ash tree, Widrlechner said.