Darlene,
I've no clue about earwigs eating seeds, but them wanting to be in there is not a good sign. I do know storing seeds in a shed is not the best place for them. Seeds need cool, dark, and dry(not humid.) The shed covers the dark, but at certain times of the year, it would be neither cool or dry. A closet of a room that stays cool like on the north side of your home in new earwig free box would be best. : )
Remy

I would say move the seeds too... earwigs like dark, damp places. I googled "earwig food" and copied this for you.

The common earwig is one of the few insects that actively hunt for food and are omnivorous, eating arthropods, plants, and ripe fruit. To a large extent, this species is also a scavenger, feeding on decaying plant and animal matter if given the chance.