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ash tree

An emerald ash borer
User: USDAgov / flickr

Researchers with the U.S. Forest Service are looking for ash trees that survived the attack of the emerald ash borer.

The invasive insect has been spreading across the Midwest and beyond since 2002 - killing millions of ash trees in its wake.

Here's an animation showing the spread of the emerald ash borer from 2002 to 2014: