Planting Date - Plant early, or plant fall crop only. You can sidestep much of the maggot threat simply by not sowing or transplanting the plants during the pest's egg-laying peaks. Maggot populations are usually highest in May and early June and again during late summer. Mature plants are less susceptible to maggot damage. An early June sowing and July transplanting, after the spring maggot population has peaked, will help to reduce your losses.
The first defense then, is the careful timing of plantings. Hold off transplanting your seedings as long as you can, and then transplant into the garden so the plants will be large enough to resist second and third generation maggot infestations later in season.
Predator Birds And Animals - Sparrows, starlings, wrens, and toads all feast on root maggots when they can find them.
Predator Insects Found In Garden - The following beneficial insects are commonly found in and around healthy, well-balanced gardens in many parts of the country. See page xxx for more information about each one.
Rove beetles feast on the maggot pupae in soil. Beetles are one of the most important natural enemies of cabbage maggot. It has been estimated that as many as 80% of cabbage maggots in the field may be destroyed by rove beetles, assuming you have rove beetles in your ecosystem.