What Makes Dime-Sized Holes in the Ground?

Q. Three years ago when we redid the back yard, they brought in "landscape sand" to make mounds.

For the past couple of years something has been digging out the sand in a couple of areas to the point it is covering the rock mulch as you can see in the photos. I am in the yard a lot and never see any activity so it must be happening at night.(tunnels are a little smaller than a dime), Any ideas on what this is?

A. I don't think the holes are due to scorpions like the bark scorpion. To my knowledge, they are not known to create holes in the ground. Holes smaller than a dime might because by several critters:

• cicada
• digger wasps, aka cicada killer
• June beetles
• Birds pecking at the ground feeding on insects
• Earthworms

I'm not an expert at this but those be some of my guesses. Maybe someone else has ideas.