mud wasps in the mill

Couldn't start the Lucas mill this morning, turned the key and all I got was a loud clicking noise. Oh oh the starter motor's packed it in, or so I thought, but after closer inspection I discovered a few mud wasp nests on the fly wheel (the one with the teeth). There were enough nests to stop the motor turning over.

Has anyone had this problem before and did you discover a way to prevent future problems ?