A 3-ounce nocturnal bat is shaping up to be a monster for Pennsylvania highway planners.

The endangered Myotis sodalis, better known as the Indiana bat, is threatening to bottleneck a $500 million extension of Interstate 99.Biologists want to make sure the elusive bats won't be harmed by the planned route along a heavily forested ridge in Blair and Centre counties.

"The thing that probably bothers me most is that people don't know if it's up there," said Kim D.F. Bartoo, an environmental manager with the state Department of Transportation. "Nobody's actually seen a bat up there, an Indiana bat, anyway."

Pennsylvania has only five caves that contain the Indiana bat, a species considered to be an important enemy of mosquitoes and agricultural pests. One cave, an old limestone mine where 200 to 300 bats hibernate in Blair County, is only 20 miles from the planned four-lane highway that would replace outmoded Route 220.

Although the bats spend October through April in the caves, they are believed to range some 100 miles during the summer. Biologists believe some bats from the Blair County cave may fly to Bald Eagle Mountain in the summer and roost in trees slated to be chopped down.

Those concerns, as well as lawsuits and scientific data, led David Densmore of the U.S. Fish and Wildlife Service to reconsider the bat issue this summer.

He is now recommending that crews cut down trees only during the winter, when bats would be hibernating.

"When you whack down a forest, yeah, you're going to affect bats. You're going to affect a lot of things. But there's very few projects that are going to take the bats' entire foraging area," he said.