Plants affected
All brassicas, including cabbage, cauliflower, Brussels sprout, calabrese and broccoli; swede and radish are also sometimes attacked.
Symptoms
Yellow patches appear on the foliage of infested plants.
On the underside of these areas dense colonies of grey-green
aphids covered with a powdering of white mealy wax can be
found. Infestations on young plants in early summer are particularly
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.
Biology
Mealy cabbage aphid can be present on brassicas throughout the year, but the heaviest infestations occur from mid-spring to mid-autumn, usually peaking in early autumn. The aphid overwinters as eggs on leaves and stems of brassicas but active aphids can survive the winter in milder areas.
Control
Brassicas that have finished cropping should be dug in or pulled up and removed to reduce the overwintering population.
Insecticides containing bifenthrin (Scotts Bug Clear Gun, Doff All In One Garden Pest Killer or Bayer Sprayday Greenfly Killer Plus) should give good control but can only be used once during the growing season on brassicas. The systemic insecticide thiacloprid (Provado Ultimate Bug Killer Ready To Use) can be used on leafy brassicas up to the eight true leaf stage of growth. Organic insecticides containing pyrethrum, fatty acids or plant oils and extracts can be used without restrictions on the number of applications or stage of growth.