Weevils don't get into flour, corn meal, etc., they're already in there when you buy it. The eggs of the beasties can't be sifted out by the manufacturer, so no matter how you pack it, left unused, they will hatch, mate, and reproduce. This happens also to celo wrapped dried peppers, packaged spices, and just about any other dried produce or condiments.
It's best to use the flour as soon as you can and reprovision regularly. I don't know about the microwave thing, I guess you'll know if it's working if you hear the eggs popping.