I have a field corn bag I received as a gift for years and have never had a problem with any kind of bugs or anything. I would think just by heating it in the micro wave would kill any bugs.
My bag is fairly long, A bit longer than 12 inches. And it is not stuffed too full. I use it at my feet or for my shoulders,and I also take it camping just to warm me up in the camper. It is a nice size for wrapping it around a knee or where ever you need it. But YES you must be careful not to make it too hot because you can burn yourself.As for myself I really love mine and it helps a lot on my aches and pains.