I have always used deer corn in my bags. They hold heat well. They can also be placed into a zip lock bag and stored in the freezer to be used for cold packs/ice packs. You can also make a smaller size, freeze and use as boo-boo bags for childrens injuries.
Insects and or rodents can be controlled by proper storage. If microwaved ever few weeks or stored in an air tight container like a zip lock bag or plastic or metal containor, you will not get insects and/or rodents. They can also be placed into a zip lock bag and stored in the freezer to be used later for a cold a pack/ice pack.