On first appearances this insect may be confused with cabbage whitefly. However, mealy cabbage aphid does not fly up in a white cloud when disturbed. Other symptoms to look out for include:

• Dense colonies of greyish-white aphids, up to 2.5 mm long, clustered on the underside of leaves and on the growing points
• The leaves develop a whitish-yellow discolouration where the aphids are feeding
• On young plants, the foliage develops in a distorted manner and the growing point may be killed
• In addition to edible and ornamental brassicas, this aphid can also be found on some related wild plants, such as charlock and shepherd’s purse
• On most brassicas these insects can be tolerated and are a food source for many aphid predators, which can wipe out aphid colonies. On kale and young plants palatability and growth can however be affected. 

Aphids form the basis of many food chains in the garden and it is not unusual to have some of these animals in a healthy balanced garden ecosystem.  Check brassicas frequently from spring onwards so action can be taken before a damaging population has developed, especially on more vulnerable young plants and kale. On older established plants this aphid can usually be tolerated and is a food source for many other invertebrates. When choosing management options you can minimise harm to non-target animals by starting with the methods in the non-pesticide control section and avoiding pesticides. Within pesticides the shorter persistence products (that are usually certified for organic growing) are likely to be less damaging to non-target wildlife than those with longer persistence and/or systemic action. Pesticide treatments are likely to kill natural enemies and are only likely to be successful if the entire plant can be reached.

Non-pesticide control

• Where possible tolerate populations of aphids they form an important part of many food chains and can be part of a healthy garden ecosystem
• Use finger and thumb to squash aphid colonies where practical
• Encourage aphid predators in the garden, such as ladybirds, ground beetles, hoverflies, parasitoid wasps and earwigs. Be aware that in spring aphid populations often build up before natural enemies are active in sufficient numbers and then give good control
• Dispose of old brassica plants once they have finished cropping to reduce the risk of the next season’s plants becoming affected

Pesticide control

• Organic sprays, such as natural pyrethrum (e.g. Bug Clear Ultra 2, Neudorff Bug Free Bug and Larvae Killer) or plant oils (e.g. Vitax Plant Guard Pest & Disease Control, Bug Clear Fruit & Veg, Vitax Rose Guard) can give good control of aphids. These pesticides have a very short persistence and so may require reapplication to keep aphid numbers in check. Plant oil and fatty acid products are less likely to affect larger insects such as ladybird adults
• Plant invigorators combine nutrients to stimulate plant growth with surfactants or fatty acids that have a physical mode of action against aphids (e.g. Ecofective Bug Control, Growing Success Bug Stop, Rose Clear 3 in 1 Action SB Plant Invigorator and Westland Resolva Natural Power Bug & Mildew). These products contain some synthetic ingredients and so are not considered organic
• More persistent contact-action insecticides include the synthetic pyrethroids lambda-cyhalothrin (e.g. Westland Resolva Bug Killer), deltamethrin (e.g. Provanto Ultimate Fruit & Vegetable Bug Killer, Provanto Sprayday Greenfly Killer) and cypermethrin (e.g. Py Bug Killer).
• A systemic containing the active ingredient Flupyradifurone (Provanto Smart Bug Killer) is available for use on ornamentals and selected edibles

Follow label instructions when using pesticides. On edible plants make sure the food plant is listed on the label and follow instructions on maximum number of applications, spray interval and harvest interval
Plants in flower should not be sprayed due to the danger to bees and other pollinating insects
Inclusion of a pesticide product does not indicate a recommendation or endorsement by RHS Gardening Advice. It is a list of products currently available to the home gardener

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Pesticides for gardeners (pdf document)

Mealy cabbage aphid overwinters on its host plants as eggs that are laid on stems in late autumn, although in mild winters active aphids can be throughout winter.

For most of spring and summer, the aphids are present as wingless females that give birth to live young.

Winged forms develop when plants become heavily infested, allowing the aphids to migrate to new host plants.