The whole trick to keeping the critters out is to shrink wrap it real good. Leave no entry points and there is no problem. My boat has never had any critter in it. Nor my last 3 boats either and where I live, theres critters a plenty.
After I wrap the boat, I go around an look for possible entry points. A little piec of wrap and some shrink tape and voila, critter proof.
I also leave a fan running in the boat pointing up all winter. The air flow stops inside rain, helps in the ventilation process and keeps the boat smelling fresh.
No, the steps that lead up to wrapping it are very important too IMHO.
The boat gets backed into my shop on a Friday night. Everything is opened up, the furnace gets cranked up to 80 and the fans go in and on and around. Saturday the frame goes on the boat, the boat gets emptied of all accessories and junk, cleaned, waxed and winterized. Sunday, the chemical dehydrator buckets go in, one in the cabin, one in the cockpit and one in the engine bay. I leave one 10 inch fan in the boat, run a small extension cord out over the transom for later plug in at home. Then it gets wrapped up clean, dry and tight for the winter. Several plastic vents that I glued screen material into to stop the fall insects and spiders from finding a home get installed along the length of the boat at the peak of the wrapped frame. The afore mentioned entry way seal ups and shes ready for bed.
Never had mold or mildew or critters EVER in the boat come spring time.
The boat still smells of fiberglass 9 seasons later.