In the wild: Wild radish, shepherd’s purse, charlock
On Crops: Brussels sprouts, cabbage, kale, broccoli, cauliflower, swede oil seed rape
Worldwide, wherever cabbage family crops are grown
Cabbage aphids are bluish-grey in colour with a pale powdery waxy coating. They are often found in dense clusters underneath the leaves. Infested leaves are often curled and become yellow leading to stunted plant growth. While present in spring and summer, populations increase by late summer and persist well into the autumn and winter season.
Cabbage aphids have good camouflage, and may not be seen until they have become quite numerous. In addition to the aphids themselves, look for black deposits of honeydew where the aphids have been feeding. Late-maturing Brussels sprouts and cabbage are at high risk for damage by cabbage aphids.
Pull up and compost old plants, away from new growing sites because cabbage aphids can overwinter in dead plant tissue. Remove, contain and destroy leaves holding aphid clusters. Also collect, contain and destroy any leaves that have fallen onto the soil. Harvest Brussels sprouts often, because sprouts left too long can serve as aphid nurseries. Encourage beneficial insects including ladybirds, hoverflies, and lacewings, which are important aphid predators.
In small outbreaks, a high-pressure spray from the garden hose can help remove aphids from plants. Where aphid problems persist, as a last resort organic pyrethrum-based pesticides are available from garden centres. These need to be applied following the instructions on the label.
Ladybirds and their larvae are very effective predators of aphids and should be welcomed into your garden. Other predators such as hoverfly larvae and lacewings also provide effective natural ways of controlling aphids. Parasitic wasps will help to control aphid infestations by injecting aphids with their eggs. The eggs hatch into maggots that eat the aphids from the inside out. Eventually the wasp maggots kill the aphids, turning them into ‘mummies’ before emerging from the mummified bodies as adult wasps. Ants will often farm aphids and collect the sugary honeydew that aphids produce. The ants protect aphids from predators and can move them to new plants to establish new aphid colonies.