It looks like it may be the early stages of Tent catterpillars, or Web Worms. I've seen them do what looked similar to your picture under a Wild Cherry tree they seem bent on stripping along my driveway. But I've never seen them in a perfect circle like your picture. It is pretty grassy there and the drive is graveled so that may have a bearing on them not being able to form much of a perfect circle like your picture.
You've got my curiosity up and hope you post what they are. Have a Good'n.
Jesse