Syrphidae
Hoverflies- Kind
- Wild — attract it
- Best for
- Among the commonest predators in colonies; attract with flowers
Pest
Woolly apple aphid
Eriosoma lanigerum
The short answer
A small reddish-purple aphid that covers itself in long white strands of wax, so its colonies look like tufts of cotton wool on apple bark, pruning cuts and shoot tips. It feeds on the bark, and its feeding raises knobbly galls on twigs and, more seriously, on roots, where UC IPM says the main injury — stunting from root galls — is done. Above ground, the colonies are unsightly and shed honeydew, and both UC IPM and the RHS advise tolerating them on healthy trees where possible. The parasitic wasp Aphelinus mali, native to the north-eastern US and established in the Pacific Northwest since 1928, can completely control the colonies on the branches, according to UC IPM, as long as broad-spectrum sprays do not kill it. The wasp never parasitises the root colonies, and NC State notes there is no insecticidal fix for them, so resistant rootstocks are the long-term answer for new trees.
hero macro · woolly aphid colony at a pruning cut
White waxy colony of Eriosoma lanigerum round an old apple pruning wound. Photo: pending.
Identification
The aphids themselves are small, about 1/20–1/10 inch (1.2–2.5 mm), and blackish, reddish-brown or purple, but they are hidden under long white waxy filaments that make each colony look like a tuft of cotton wool. Young nymphs are purple with a powdery grey wax. Colonies sit on bark, especially round old pruning cuts and wounds, on root suckers and in the leaf axils of new shoots.
Parasitised aphids are easy to recognise once you know what to look for: hard, black mummies in or near the colony, each with a neat round hole where an adult Aphelinus mali wasp has cut its way out. Seeing them means the wasp is present.
Easily confused with
- MealybugsAlso a pestBrush the wax aside and look at the insect. Woolly apple aphids underneath are reddish, brown or purple, with antennae under a third of the body length, and they raise warty galls on bark and roots.
- Woolly adelgidsAlso a pestCheck the host. Adelgids feed only on conifers such as hemlock, pine, spruce and fir; woolly apple aphid lives on apple, cotoneaster and pyracantha.
- Lacewing larvaeBeneficial — leave itLacewing larvae are alligator-shaped, with three pairs of distinct legs and prominent, curved, pincer-like jaws. UC IPM lists lacewing larvae among the predators that eat woolly aphids.
Damage symptoms
The white wax is what people notice, but the damage is done by the galls. UC IPM and Washington State both say the root colonies cause most of the injury, stunting young trees. Above ground the colonies are unsightly, sticky with honeydew, and gall the twigs.
- White, cotton-wool tufts on bark, round pruning cuts and wounds, and in leaf axils on new shoots
- Soft, knobbly swellings on twigs that can split in frost and let in apple canker (RHS)
- Galls on the roots, with a shallow fibrous root system, yellowing foliage and poor growth on young trees
- Young trees that are easily uprooted
- Sticky honeydew and black sooty mould on leaves and fruit below heavy colonies
Lifecycle & degree days
Its original winter host is American elm, and where elm is present it alternates between elm and apple (NC State). In most fruit-growing areas, Washington State says, it lives on apple all year, reproducing without mating. It overwinters mainly as nymphs on the roots, with some surviving in protected places above ground. In spring and summer the aphids move up and down the trunk, and colonies build on shoots and water sprouts through midsummer and autumn. In the UK the RHS gives April to October as the active period, peaking in mid to late summer.
- Overwinters
- Mainly nymphs on roots; eggs on elm where elm is present
- Active
- April–October (RHS); aerial colonies peak midsummer to autumn
- Summer sites
- Leaf axils on new shoots, pruning cuts, wounds, root suckers
- Other hosts
- Pear, hawthorn, mountain ash, cotoneaster, pyracantha; American elm
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Host crops
Apple is the main host. NC State adds pear, hawthorn and mountain ash, and UC IPM and the RHS cotoneaster and pyracantha. American elm is the aphid's original winter host (Washington State). Resistance in the rootstock protects the roots, where the damage is done: Washington State lists MM.106, MM.111 and Merton 793, and Geneva rootstocks including G.41, G.202, G.210, G.213, G.214, G.890 and G.969. UC IPM names Winter Banana as one of the most susceptible varieties to aerial galls.
Monitoring & action thresholds
Action threshold
No threshold set
Washington State says no monitoring procedure or treatment threshold has been developed, and NC State agrees. In a garden the question is whether the tree is young and struggling (root colonies matter) or established and healthy (aerial colonies can usually be tolerated, and the RHS recommends that where possible). Black, holed mummies in the colonies are a reason to wait rather than spray.
From mid-spring, check root suckers and the edges of pruning cuts and wounds for new white colonies (Utah State). Washington State suggests watching from midsummer onward, when aerial colonies are commonest. Look for black mummies with round exit holes: they show that Aphelinus mali is present.
Beneficial
Biological control
The key natural enemy is Aphelinus mali, a tiny black parasitic wasp, slightly shorter than the aphid, that jumps rather than flies. It is native to the north-eastern US and was introduced to the Pacific Northwest in 1928, one of the early successes of biological control. UC IPM says it can completely control aerial colonies. It has limits: it never reaches the root colonies, and Washington State notes it may fall behind after mild winters, when many aphids survive. It is a wild parasitoid, not a product you buy. Generalist predators matter too. A 2006–2008 Washington survey found hoverfly larvae, green lacewings and lady beetles the commonest predators in the colonies, and Washington State and the RHS both credit the European earwig. Outbreaks follow broad-spectrum sprays: UC IPM links them to pyrethroids, and Washington State to some codling moth insecticides.
Hippodamia convergens and others
Ladybugs- Kind
- Wild — attract it
- Best for
- Wild lady beetles feed in colonies; conserve, do not buy
Chrysoperla carnea
Green lacewing- Rate
- 2–20 /m² per release, weekly
- Temp
- 20–28 °C best; works above a 12 °C average
- Best for
- Wild lacewings eat colonies; releases are untested on this pest
Cultural controls
For a backyard tree, the aim is to keep young roots healthy and let natural enemies deal with the colonies above ground.
- Plant new apples on resistant rootstocks such as MM.106, MM.111 or a resistant Geneva stock (for example G.41 or G.890), and do not plant infested nursery stock
- Squash small colonies between finger and thumb, as the RHS suggests
- Prune out galled and heavily infested shoots in winter
- Do not spray pyrethroids or other broad-spectrum insecticides, which kill Aphelinus mali and cause outbreaks
- Where colonies are heavy, UC IPM suggests horticultural oil on the shoots in the dormant season, and oil or insecticidal soap in the growing season
- Earwigs eat woolly aphids; in Spanish organic orchards, 30 earwigs per tree released in cardboard shelters shortened colonies from the second year
Chemical, last resort
Broad-spectrum sprays make it worse — they kill Aphelinus mali and trigger outbreaksCheck any product before it goes on. Open the compatibility matrix →
- Horticultural oilREI 4 h
- ImidaclopridREI 12 h
Rating: the effect on this pest's natural enemies if a spray hits them or they feed on treated plants. REI is the re-entry interval on farm and greenhouse labels; home labels usually give none, so follow the label.
Sources
UC IPM Pest Management Guidelines: Apple — Woolly Apple Aphid (UC ANR 3432, text 01/11)
UC IPM Home and Landscape, Woolly Aphids (web page)
WSU Tree Fruit, Woolly Apple Aphid (Beers, Hoyt & Willett 1993; rev. 2010, 2019)
WSU Tree Fruit, Aphelinus mali (Beers, Hoyt & Willett 1993; rev. 2010)
NC State Extension, Woolly Apple Aphid (Walgenbach, 2015)
Utah State University Extension, Woolly Apple Aphid (IPM fruit pest page)
Pacific Northwest Insect Management Handbook, Apple — Woolly apple aphid
RHS, Woolly aphid (advice page)
Gontijo, Cockfield & Beers, Environ. Entomol. 41:1364 (2012)
Alins et al., Earwig Releases Provide Accumulative Biocontrol of WAA, Insects 14:890 (2023)
UC IPM, Relative Toxicities of Insecticides Used in Apples to Natural Enemies (10/15)
UC IPM Home and Landscape, Adelgids (web page)
UC IPM Natural Enemies Gallery, Green lacewings (web page)
Page status
Drafted from the sources below, then checked claim by claim against those sources on 23 September 2026.
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