Delia radicum

Delia radicum, known variously as the cabbage fly, cabbage root fly, root fly or turnip fly, is a pest of crops. The larvae of the cabbage root fly are sometimes known as the cabbage maggot or root maggot. Delia brassicae (or Hylemya brassicae) was the most common name in literature prior to 1981. The adult flies are about 1 cm long and are grey in colour, but otherwise resemble the common house fly. The flies can be found all over Europe. After overwintering as pupae in the soil, the flies emerge in spring, feed on nectar, and lay eggs close to plants of the genus Brassica. The eggs are white and about 1 mm in diameter. They hatch into white maggots after about six days and the larvae feed for about three weeks on the roots and stems of the cabbage plants. After this, the larvae are typically 0.9 to 1 cm in length and form reddish-brown pupae which hatch into adult flies after around 20 days. This species is univoltine (has one generation per year) in northern Europe and bi- or trivoltine in central Europe.

from Wikipedia