Oh wow, you have a nice big garden going. You may want to look into doing a soil solarization treatment on the bed you think you have root pests in. You may already be fairly familiar with solarization, but in case you're not, essentially all you're doing is covering the bed with plastic and letting it basically cook in the sun. Should kill off most of the harmful microbes and organisms in the soil. And most of your beneficial organisms will either survive in sufficient numbers to quickly repopulate, or will move back in relatively quickly. Or you can even reintroduce them yourself. Good way to deal with soil problems without having to use chemicals.
That's actually a good idea.