There are soil drenches that can be used to kill off bugs, but I've never used them and don't know anything about them, except they exist. Beneficial nematodes control a lot of soil borne pests. I know they work, but if your problem is root aphids, they might not be enough.
Look up root aphids and learn what they look like. Also look up fungus gnats, which are a nuisance, but really don't bother the plants (except they get themselves stuck on trichs and die). I actually use nematodes regularly for fungus gnats.