While it would be nice to know exactly what is causing the problem, we first need to confirm that it is some kind of insect eating the roots. I would move to another patch with affected plants and take a look at the roots on them. Anybody know of a general insecticide that can be applied to the ground? If you confirm, then there are some options you could try.
I would try putting down a layer of DE (diatomaceous earth) around the base of the plants. It's cheap and does organically fight a number of types of insects. Do it during a dry period. Be sure to wear a mask when applying as you don't want to inhale it.
You could also dig all of the surviving plants in a patch, wash them, then spray with a general insecticide and replant in a new location. Do this at the first sign of infection of a new patch as well. It's a lot of work, but you would be interrupting the bugs lifecycles. As a minimum, remove any plant showing signs of infection from healthy patches at first sign of a problem. Remove and sterilize (or seal in a bag and throw out) dirt from around infected plants.
