The Cabbage Maggot (Delia radicum) is a very destructive fly pest, attacking plants of the genus Brassica: cabbage, broccoli, cauliflower, brussels sprouts, radishes, rutabagas, turnips, etc., –all cool-season plants. Wilting leaves and a delay in plant growth are usually a sign of maggot damage and indicate that roots need to be checked for maggots. Sometimes as many as 300 larvae can be found on a single plant! The maggots damage the inner parts of the main root, interfering with the transport of water and nutrients to the stem and leaves, often leading to the death of the plant.
The adult flies [cabbage maggot adult fly photo at wikimedia], which hatch in the spring (month of May) from overwintering pupae in the soil, are grey in color and resemble the common house fly. They feed on nectar, mate, and lay their eggs at the base of whatever Brassica plants they can find.
Additional Information: Cabbage Maggot