PITTSBURGH — The grove of hemlock trees around where United Flight 93 crashed on Sept. 11 is being attacked by an insect that wasn’t there 20 years ago, and some scientists say it’s an example of how climate change combines with other factors to cause environmental damage.

The problem at the Flight 93 National Memorial in southwestern Pennsylvania doesn’t involve superstorms or melting polar icecaps, but rather hemlocks battling the slow, deadly spread of a tiny creature that has only one natural predator in eastern forests — extremely cold winters.