I have some very old ones I made with rice, and have never had a problem with bugs or mice. And I live in Florida, so that is saying something. I always travel with at least one or two, because if you need, you can pop them in the microwaves at the convenience store (I would put a napkin down first, lol).
I recently went to a family reunion, where we all took something to auction off to raise money to defray the costs of the park, food, etc. I took along four of the rice and lavender pillows...and ended up buying one back, lol...because I forgot to bring one of my own along.
I use plain old white rice, and it gives off a moist heat every time I heat it. I am surprised that not everyone finds this to be true of theirs.
I would ask before making them with lavender for friend,...my cousin is allergic.