Apparently this bug came into the US from Asia and, based on their name, does a very good job of boring into ash trees (the larvae) which kills them. Surprisingly enough (for me, at least) the State of Illinois has a pretty comprehensive web site detailing the Ash Borer threat right here. From the site:
Native to Asia, the Emerald Ash Borer is an exotic beetle that was unknown in North America until June 2002 when it was discovered as the cause for the decline of many ash trees in southeast Michigan and neighboring Windsor, Ontario, Canada... No bigger than a penny, this green menace has wreacked havoc on millions of ash trees in the Midwest and if not controlled it could wipe out the ash tree species in North America. The adult beetles nibble on ash foliage but cause little damage. The larvae (the immature stage) feed on the inner bark or cambium layer which is the crucial layer between the bark and wood of ash trees, disrupting the tree's ability to transport water and nutrients. Emerald ash borer probably arrived in the United States on solid wood packing material carried in cargo ships or airplanes originating in its native Asia.From looking around the site a bit, there apparently isn't too much that you can do about the nasty critters. You can pre-inject insecticide into your ash tree to try to prevent them from infesting the tree in the first place but that appears to be about it. I am far from an arborist, however, so don't take my advice.
While based on my super-simple knowledge of trees the "Ash" tree isn't viewed as a particularly valuable type of tree for your yard, it is sad to me to see the trees I grew with all coming down so fast.