Firstly, I assume you are talking about the ventilation cones on the roofs of the outer lifts of your WBC hives ?
If the plastic has been destroyed by mice then on close inspection you will see the teeth marks (like tiny scooped chisel marks) in the plastic - if they are bigger scoop marks it could be rat damage as they are looking for winter quarters at the moment and the space between the lifts and the hive could be their ideal home - rats are inquisitive creatures.
Generally rodents will not expend energy unnecessarily so something is appealing to them if it is rodent damage - and at the top of the hive it is hardly the easiest place to get at.
So a few questions:
1. Have you got mouseguards on the entrances of the hives ?
If you have then there should be no way that a mouse could get into your hives.
2. Have you got open holes in the crown board ?
If there are then close them down with a piece of tile or timber (I can't think you would have but we hear some very odd ideas on here !).
3. Have you lifted the roofs off and looked to see if there are any mouse droppings/nesting material on the crown boards or between the lifts and the hive ?
If you do find evidence of rodents then I would not risk dismantling the hive but there is nothing to stop you taking the lifts off carefully (might need someone to help you as you want to leave the hive intact) and then tilting the hive a little off its floor when, if a mouse has got in there, it should run out.
Whatever - I would get rid of the cones and tack a piece of stainless insect mesh over the holes - plenty of suitable stuff on ebay:
http://www.ebay.co.uk/itm/5pc-5x5cm...Equipment_Lab_Supplies_ET&hash=item2a46e2517a