Mealy cabbage aphid

Symptoms
Infestations on young plants most damaging as the shoot tips may be killed, causing sideshoots to develop and resulting in multi-headed plants. This aphid can transmit cauliflower and turnip mosaic viruses.

Spot them
Dense colonies of grey-green aphids covered with a powdering of white mealy wax can be found under damaged and yellowed foliage of infested plants.