Mealy Cabbage Aphid
The mealy cabbage aphid (Brevicoryne brassicae) is a common pest of plants in the Brassica family. Found especially on cabbages, Brussels sprouts, cauliflowers and swedes.
Dense colonies occur on the leaves of brassica plants from July onwards and continue increasing in numbers until the winter. Leaves become distorted and turn yellow as a result of the aphids feeding. Severe infestations can check growth and kill young plants; the effect is less damaging on mature plants. This aphid is an important carrier of cauliflower and turnip mosaic viruses.
Description of pest
Cabbage aphid colony
Overwintering cabbage aphid eggs
The aphids are grey-green in colour and covered with a whitish-grey, mealy wax which repels water.
Aphids move from one brassica crop to another throughout the year. Although they can survive the winter as adults in milder areas, they usually overwinter as eggs on the stems and leaves of brassica plants.
Young aphids hatch out in April and begin feeding on leaves, moving to flower buds and stalks as the season progresses. During May-July winged aphids fly off and establish rapidly expanding colonies on younger plants, which often become substantial populations by the autumn. The build-up of aphid numbers is linked to temperature – years with mild winters see a much quicker increase in infestations. Predator numbers also increase in these conditions.
Prevention and control
- Encourage natural controls: Make your garden a friendly place for a range of helpful creatures by avoiding harmful sprays and providing suitable wildlife habitats. Grow flowers that attract beneficial insects. Small, simple flowers are best, such as members of the Umbelliferae and Compositae families, as well as Limnanthes douglasii (the poached egg plant), Convolvulus tricolor and Phacelia tanacetifolia. Cabbage aphids are attacked by all the predators and parasites that prey on other species of aphid.
- Cultural control: Always remove old brassica plants as soon as cropping has finished and bury them in a compost trench. If possible, remove all over-wintering plants by mid-April, before planting out any more brassicas. This can reduce local infestation considerably. However, if you have neighbours who leave their old brassicas in the ground, it will be less effective.
- Hand picking: When numbers of aphids are low, they can be squashed by hand. Alternatively, remove the leaves they have gathered on, if not too numerous.
- Undersowing the crop with clover: This can reduce the number of cabbage aphids by over 95%, as well as decreasing the amount of other pests like caterpillars and flea-beetles. The best way to do this is to sow a strip of wild white clover during April to August – clover takes longer to germinate than brassicas – and transplant your brassicas into the strip once the clover is established. Clover can become invasive and compete with the crop plants for food and water, so keep the clover trimmed with shears to a height of about 10cm. After you have finished cropping, the clover can be dug in as a green manure.
Chemical control – a last resort: Sprays, even those approved for use in an organic garden, can kill both the pest you are trying to control and beneficial natural predators, so think twice before using them.
Examine young plants regularly during June-September and spray with insecticidal soap if you see aphid numbers building up. Use a powerful spray to get through the aphids' waxy coating, and repeat several times if necessary. Spraying may be essential for seedlings and young plants, as they can suffer serious aphid damage, but it is usually unnecessary on mature plants as they can tolerate the aphids.
Spray flowering crops at dusk when bees are not active. Always follow the manufacturers instructions when making up and using sprays.