It's hard to tell from the pic above but in my yard "perfectly round holes" (in the ground, not the stem?) are made by skunks at night time digging for grubs or worms.

oops, I just spotted the small hole you are referring too. Not a skunk. Could be an emerging larval form of a cicada, June bug, Japanese beetle, etc.

Keep an eye out for bees, hornets or wasps going in. It appears as though the mud around the hole was carefully placed there and not as a result of something emerging. Just my two cents.

That's a really nice vole hole. He has his nest in the roots of your plant.

Dave