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 is likely to have spread 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

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 infestation has built up.

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

• 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 applications, spray interval and harvest interval. The products highlighted in bold below can be used on edible apples, including crab apples used for jam making. All products listed can be used on non-edible crab apples, pyracantha and Cotoneaster horizontalis
• Organic sprays, such as natural pyrethrum (e.g. Bug Clear Gun for Fruit & Veg, Ecofective Bug Killer), fatty acids (e.g. Solabiol Bug Free, Doff Greenfly & Blackfly Killer) or plant oils (e.g. Vitax Organic 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
• More persistent insecticides include the synthetic pyrethroids lambda-cyhalothrin (e.g. Westland Resolva Pest Killer), deltamethrin (e.g. Provanto Ultimate Fruit & Vegetable Bug Killer) and cypermethrin (e.g. Py Bug Killer)
• The systemic neonicotinoid insecticide acetamiprid (e.g. Bug Clear Ultra) is also available
• The minimum period that needs to be left between treatment and picking the fruit on apple trees is seven days for deltamethrin and lambda-cyhalothrin and 14 days for acetamiprid (always check the label)
• Winter washes do not give good control of this aphid as it overwinters deep within bark crevices where the wash does not reach
• Plants in flower should not be sprayed due to the danger to pollinating insects
• Inclusion of a pesticide product does not indicate a recommendation or endorsement by the RHS. It is a list of products currently available to the home gardener

Download

Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)

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.