For this problem I am implementing two solutions next go round which probably wont work for you as I grow in coco. First is that I wont be using open sided pots for my next grow as I dont want the coco exposed on the sides which allows bugs access. Second thing I am changing is to start adding play sand over the top inch of the media which might work for you. The sand will smother the bugs and prevent anything from being able to breath or crawl out or in.