Cabbage aphid (brassica aphid, grey cabbage aphid)
Brevicoryne brassicae

Cabbage aphids, from the genus Brevicoryne of the family Aphididae, are grayish-green, but a waxy covering gives them a grayish-white to powdery blue appearance.

Cabbage aphids are green gray with a white, waxy coating. They commonly occur in dense colonies, often covered with waxy droplets. They prefer to feed on the youngest leaves and flowering parts and are often found deep within the heads of cabbages or Brussels sprouts. Both winged and wingless adults occur; the winged adults have a black thorax and lack the waxy coating. The aphid does not infest noncruciferous crops but can survive on related weed species when cole crops are not in the field.