You can try other things like bird toys, cat toys(the little fuzzy mice are a favorite around here), rawhide chews for dogs, pig ears, paper towel and toilet paper tubes. You can also put a treat in some newspaper and ball it up really tight so they have to work for the treat. Pretty much anything, except for citrus fruits. Many times I have seen pieces of orange in with mice... They can't have it! Stupid pet store people... Hooray for kidney damage...

Anyway, like I said, pretty much anything is ok for them to chew on, even bakes pieces of wood from outside. I would take the bark off first just to be safe. Hope this gives you a few things to try to get the little boogers' minds off of the silicone.