Cabbage first takes on a sickly gray-blue color. Other infested crucifers may appear stunted or pale in color. If severe damage has been done, plants may wilt and die during the heat of the day. Infestations are difficult to detect in radishes and turnips because the tunneling of maggots in these large-rooted crops does not cause the foliage to wilt. Damage to these crops can be determined only by pulling some plants early and inspecting them.
Cabbage maggots are usually most severe when the weather is cool and wet for a long period of time.
Life History - Cabbage maggots overwinter as pupae 2 to 13 cm deep in the soil. As the soil warms in spring, adult flies emerge from cocoons, feed on the nectar of flowers, and mate. Appearing as early as April, females soon begin depositing eggs in the soil at or near the base of host plants. Three to 7 days later, young maggots emerge from the eggs and move into the soil searching for roots upon which they feed for 3 to 4 weeks before pupating. Pupation may take place within root burrows or out in the soil and usually lasts 2 or 3 weeks.