We looked at your photos and we cannot say for sure what is causing the holes. Some possibilities include squirrels, voles, or chipmunks. You will have to go out and look at the hole sizes and see if you notice any sightings of animals in the area. You can also sprinkle some flour in the area and look for tracks. Depending upon identification no treatment may be necessary.
Squirrels dig in various places to find food such as buried acorns. There is no control for this. https://extension.umd.edu/hgic/topics/squirrels
If it is a vole, you would also see "runways" under the surface of the lawn, silver dollar sized entrance holes, etc. Look at the webpage for more clues and photos. They feed on the roots of trees and shrubs and cause plant decline.
Chipmunks - Holes are about 2 inches in diameter and cleanly dug. You should be able to see them during the day. http://extension.umd.edu/hgic/chipmunks
Also, see the link from the Internet Center for Wildlife Damage Mgmt which has information on ground hole identification http://icwdm.org/inspection/outdoors/groundholes/