Ecyrus dasycerus.
Size: approx. 5 mm. Photo: 05/01/12, 10 pm.

This longhorned beetle is Ecyrus dasycerus. The second reference states: "Variable in size, and darker and lighter variants occur, but the combination of body shape and the dark, inverted arc-like marking at the elytral base is distinctive (though the mark is sometimes faint)." The small photo shows a different specimen found on 09/14/14. 

bug guide (this photo):
D. Yanega Field Guide to Northeastern Longhorned Beetles (Illinois Natural History Survey, 1996):