I suggest a night-time foray with a flashlight. My guesses would be that
your pest is the invasive Asiatic garden beetle. They have only recently
reached the midwest. They feed at night and hide during the day.

http://extension.entm.purdue.edu/cap...cGardenbtl.htm



Nasty bugs. I was going to go out last night, fell asleep, woke up and it
was daylight again...I'll try it again tonight, I've been wanting to do a
slug census lately anyhow - we have the famous Oregon Slugs here, and yes
they really do grow up to six or eight inches long. And the bigger puppies
can do an amazing amount of damage in just one night!