Most root maggots overwinter as pupae and, at least according to most of the universities, seem to prefer soils rich in organic matter and yet most often any damage to plants done by them is in soils with too little organic matter in them. Frequent tilling can expose the pupae to predators, but is also harmful to your soil. Floating Row Covers, if placed early in the growing season, can help keep Ma Fly from laying the eggs that will hatch into these buggers. Diamotaceous Earth might help control them as they hatch and start to move into the soil. But the best means of control is to not plant susceptible crops in the same area until the wee buggers are no longer there.

Here is a link that might be useful: Root Maggots

Thanks for the info!