Really hard to get an idea from just the one photo/angle. Any to get at it from under the floor? If it were me (after solving the ants problem), I might cut a piece of wood to fill the gap, and if not other way to attach, use construction glue to give a flat, solid surface. After that's set, you could cut a small piece of molding and nail it to the floor and filler piece.

It'd be a lot easier to say if several pix from different angles.