Hi there, I'm not an entomologist so I can not identify your problem species with any accuracy. That being said I did find some info on Grain Moths who seem to have a similar apperence and cause similar issues to what you are describing. More info on a couple species here: https://cals.cornell.edu/new-york-state-integrated-pest-management/outreach-education/whats-bugging-you/pantry-moths It seems likely that these came in with the bag. Especially since it is now quite cold and most insects are now dormant in Michigan. And since you are using Sunflower seeds without shells, the seeds are more vulnerable to infestation. In terms of preventing spread or issues with new seeds, it's always a good idea to store your seeds in a pest proof container. Metal Trash cans with firmly closing lids are a popular option for storing seeds. Plastic storage containers with locking lids also work, but do note squirrels can chew through plastic often. I store mine in an old broken chest freezer. Either way getting a pest proof container is a good idea and will help make your seeds last longer. In general birds can eat these grain moths, so having the larva in the seed won't be a problem, but rotted and spoiled seed should NOT be given to birds. It sounds like your first bag was rotted so good idea to throw it away. If your other bags are infested but not rotted, freezing them for 24 hours will kill off the moths. My guess is you are getting 24 hour periods below freezing now or should be shortly. Hope that helps! Best,