What is woolly aphid?

Aphids, also known as greenfly and blackfly, are sap-sucking insects. Woolly aphid is black aphid that sucks sap from woody stems and covers itself in a white waxy secretion.

Symptoms

Woolly aphid is usually easy to spot;

• Between spring and early autumn, infested parts of the trunk and branches are covered with a fluffy white waxy material. This is secreted by the blackish brown aphids
• The thinner bark around old pruning cuts is a prime site for woolly aphid colonies in spring but by mid-summer the insect spreads to younger shoots
• Affected shoots usually develop soft, lumpy growths in the bark as a result of woolly aphid feeding. Such shoots are easily spotted during winter pruning. The swellings can split in frosty weather and create entry wounds for the fungal disease known as apple canker
• Woolly aphid is only found on apple, cotoneaster and pyracantha on other plants the white waxy deposits could be signs of other insects including scale insects, woolly beech aphid or in glasshouses mealybug

Control

Check susceptible plants frequently from spring onwards so action can be taken before a damaging population has developed. Little can be done to deal with aphids on tall trees as treatment is only likely to be successful if the entire plant can be reached. When choosing control options you can minimise harm to non-target animals by starting with the methods in the non-pesticide control section. If this is not sufficient to reduce damage to acceptable levels then you may choose to use pesticides. Within this group the shorter persistence pesticides (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.

Non-pesticide control

• On small trees with light infestations, it is possible to control woolly aphid by scrubbing the aphid colonies with a stiff-bristled brush. This is best done in spring or early summer before an extensive population has built up.
• Encourage aphid predators in the garden. Woolly aphid has a number of natural enemies which help to keep it in check, although they are rarely effective enough to prevent damage occurring. They are eaten by some ladybirds, lacewings and hoverfly larvae, and they are also attacked by a parasitoid wasp called Aphelinus mali. The parasitoid wasp can sometimes be found in gardens, particularly those where little pesticide spraying is done. It is fairly easy to recognise the parasite's presence, as parasitised aphids stop producing wax and become black. A circular hole can sometimes be seen in the aphid's upper surface where the adult parasitoid wasp has emerged. If the parasitoid is found, it can be encouraged by limiting the use of pesticides.
• Research indicates that earwigs on fruit trees can reduce aphid numbers and on fruit trees they do not cause damage. Providing shelters such as flower pots loosely stuffed with hay in trees can help increase numbers.

Pesticide control

Little can be done to deal with aphids on tall trees as treatment is only likely to be successful if the entire plant is sprayed

• Organic sprays, such as natural pyrethrum (e.g. Bug Clear Gun for Fruit & Veg, Neudorff Bug Free Bug and Larvae Killer), fatty acids (e.g. Solabiol Bug Free, Doff Greenfly & Blackfly Killer) or plant oils (e.g. Vitax Plant Guard Pest & Disease Control, Bug Clear for Fruit and Veg) 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, RHS Bug and Mildew Control, 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)
• The systemic neonicotinoid insecticide acetamiprid (e.g. Bug Clear Ultra) is also available but not for use on food plants
• Winter washes do not give good control of this aphid as it overwinters deep within bark crevices where the wash does not reach

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)

Biology

Woolly aphid overwinters on its host plants as nymphs that hide in cracks in the bark or in crevices around old feeding areas. During the winter months the aphids do not produce the waxy material that gives them the characteristic woolly coating in spring and summer.

In spring, the aphids become active again, mainly around old pruning cuts or other places on the trunk or larger branches where the bark is thinner. They begin sucking sap from beneath the bark, and start secreting the fluffy ‘wool’.

Infestations reach a peak in mid- to late summer, when the aphids spread onto the younger shoots. Chemicals secreted into the plant as the aphids feed induce lumpy growths in the bark, especially on the younger shoots.

In mid-summer, winged forms of the aphid develop and these will fly off in search of new host plants.