Critters under the hood

Any suggestions on preventing mice or whatever(I never see them), from nesting under the engine cover on my Kia Rio? The car was recently running poorly, and when I took the cover off to check the plugs, the plug wires had bee chewed up by varmints. After replacing the wires, I recently rechecked and found another nest.in the same place.

I had the same problem and my mechanic told me to put moth balls under the hood.