I didn't add anything to the feed. The feed store told me that Purina and most feed manufacturers automatically add something to the feed in the summer, a preservative.
I"ve been using plastic for years. No problems. The trouble with the tin cans is that they tend to rust on the inside and that bothers me when the feed is in there rubbing on rusty aluminum.
I think you got your weevils in the feed way before you brought it home. I'll bet you anything, It was already in there especially if your feed dealer is not turing over a lot of feed, thus keeping a fresh supply in there. If you are using Purina products, complain by filling out the form on their website. Include the letters and number that you find on your bag so they can trace what mill it was made at.