these are fly larea. They live in your fertilizer most likely. They are hamfull to the extend that birds love them and will drop by to look for them. This results in little holes dug in your pot.
If you come to my garden today, you can fill a bucket with substrate that the birds have tossed out of my pots. For the shalower pots with better developed trees in them I make small nets that lay on top of the pots and the birds can't dig anymore.
The larvea itself are not really a concern.
Well, that explains it! I have had the holes dug and thought it was squirrels. I could not figure out why all my squirrel prevention didn't stop it.
Since I switched from garden dirt to the standard mix it has stopped. Now I know why.