Woolly aphid

Woolly aphid is common on edible and ornamental apple trees, pyracantha and Cotoneaster horizontalis. The insects are hidden under white fluff that can be mistaken for a mould.

Woolly aphid

Quick facts

Common name Woolly aphid also known as American blight
Latin name Eriosoma lanigerum
Plants affected Edible and crab apples, pyracantha and Cotoneaster horizontalis
Main symptoms Lumpy swellings on the bark and, in summer, colonies of aphids covered in white fluff on the trunk and branches
Caused by A sap-sucking insect (aphid)
Timing April-October

What is woolly aphid?

Woolly aphid is an insect that, unlike most aphids, sucks sap from woody stems, rather than foliage.

Symptoms

Due to its conspicuous appearance, 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 infestation in spring but by mid-summer the insect is likely to have spread to the 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. These swellings can split in frosty weather and create entry wounds for the fungal disease known as apple canker

Control

Non chemical

On small trees with light infestations, it is possible to control woolly aphid by scrubbing the aphid colonies with a stiff-bristled brush. This should be 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 parasitic wasp called Aphelinus mali. The parasitic wasp can sometimes be found in gardens, particularly those where little 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 parasite has emerged.  If the parasite is found, it can be encouraged by limiting the use of pesticides, but often the parasite only occurs in worthwhile numbers in late summer after heavy infestations of the aphid have already developed.

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.

Chemical

  • On edible apples, including crab apples used for jam making, the trees can be sprayed thoroughly with the pyrethroid insecticides deltamethrin (e.g. Bayer Sprayday Greenfly Killer), lambda-cyhalothrin (e.g. Westland Resolva Bug Killer) and the neonicotinoid acetamiprid (e.g. Scots Bug Clear), provided apples are listed on the label. Manufactures instructions must be followed, including the minimum period that needs to be left between treatment and picking the fruit on apple trees; for these pesticides it is seven days for deltamethrin and lambda-cyhalothrin and 14 days for acetamiprid
  • The products listed above can also be used on non-edible crab apples, pyracantha and Cotoneaster horizontalis
  • 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 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.

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