I think that beetles will be more trouble than they're worth for only the occasional skull. Boiling ang bleaching should work just as well. What kind of peroxide are you using? The standard peroxide you get in a bottle is not what you want. You will need to go to a hair salon and get some 40% peroxide. It is a two part mixture. Mix into a paste, and paint on the skull. Wrap the skull in plastic wrap to keep the moisture in. Let it set for a day or two. Afterwards, rinse off the peroxide and set it in the sun for a week or more. This really brings the white out. A light coat of white paint will also cover the yellow spots.