Eastern Tent Caterpillars are out in force! Every place you look, you'll see the web-like tents with hundreds of caterpillars inside. The larvae often eat all the leaves off the host trees but the damage is not permanent and does not usually kill the tree. More information is available from the MSU Extension website.
more info: http://www.msue.msu.edu/msue/imp/mod02/01500620.html