Woolly Aphid is a minor pest that affects apples.
What does it look like?
The aphids are blackish brown, but the fluffy white waxy covering they produce is what gives a colony away, coating affected bark from spring through early autumn. Apples and crab apples are the main hosts, and pyracantha and Cotoneaster horizontalis (RHS), hawthorn and sorbus (Gardeners' World) can carry it too.
What causes it
Colonies start in spring around old pruning cuts and other thin-barked areas, then spread to younger shoots, with populations peaking in mid to late summer.
Is this pest active now?
Watch from around April through October, the full window this pest stays active on the tree.
Symptoms
Soft, lumpy swellings develop in the bark where the aphids feed, caused by chemicals the aphids secrete. The swellings can split in frosty weather, and the splits are one of the routes apple canker uses to get in. Canker can girdle and kill small branches and fruiting spursWhether a fruit tree carries its fruit mainly on short side-shoots along older wood (spur-bearing) or at the tips of the previous year's growth (tip-bearing). Full definition → within a single season, and larger branches and the trunk can die back over time.
Life cycle
Aphids overwinterKeeping a plant alive and intact through the cold months, whether left outdoors, moved under cover, or stored dormant. Full definition → as immature nymphs sheltering in bark crevices, without producing wax during the cold months. Washington State and Utah State extension services add that they overwinter mainly on the roots, and that some root colonies move up into the tree each year. They become active again in spring around old pruning cuts, and by mid-to-late summer populations peak and spread to younger shoots. Winged forms develop in mid-summer and fly off to find new host trees.
Why this matters in Scotland
No Scottish records, incidence or timing figures were found for woolly aphid. The aphid often does not affect cropping, and the risk worth watching is apple canker getting in through frost-split swellings.
Canker enters through wounds, buds and frost damage, and woolly aphid swellings are one route among several (RHS apple canker).
Controls: what works
Not every method here works equally well: some are backed by real evidence, others are simply traditional and untested.
Hand control: squash colonies by finger and thumb
Crushing a visible colony by hand where practical needs no product and suits a new colony around a pruning cut before it spreads. Regular checks from spring mean colonies are caught early.
Scrub off the wool, blast it with a jet of water, or prune out badly affected areas
A stiff brush or a high-speed jet of water removes the sticky mass, and badly affected shoots can be pruned out. A jet will not cure the problem long-term, so it knocks a colony back rather than finishing it.
Natural predators: encourage ladybirds, ground beetles, hoverflies, parasitoid wasps and earwigs
Ladybirds, ground beetles, hoverflies, parasitoid wasps and earwigs all help keep populations down (RHS), and a garden that skips chemical control lets them do that job. Washington State University Extension credits the parasitoid wasp Aphelinus mali with most of the biological control and calls the generalist predators important but easily overlooked.
Earwig shelters: hang a hay-filled flower pot in the tree
A flower pot loosely stuffed with hay and hung in the branches gives earwigs somewhere to shelter close to the colonies.
When planting a new apple tree, choose a rootstockThe root system and lower stem a fruit tree is grafted onto, chosen to control the tree's eventual size rather than for its own fruit. Full definition → bred for woolly aphid resistance
Washington State University Extension says the Malling-Merton rootstocks, including MM.106, were developed to be resistant to woolly apple aphid, and names the Geneva rootstocks G.41 and G.213 as resistant. This is US advice and cannot help an established tree; ask the nursery which rootstock a tree is on before buying.
Myths about woolly aphid
“The white, woolly colonies on apple bark are a separate problem from any damage happening at the roots.”
Woolly apple aphid (Eriosoma lanigerum) is the same insect above and below ground. A US tree fruit extension service describes galls forming where colonies feed on twigs or roots, says the underground colonies cause the most damage, and notes that continued feeding can kill roots and cause reduced growth or even death of young trees.
Our read The wool most gardeners notice is the visible half of what this species can do. A trunk colony does not show the roots are clean, and a struggling young tree with no visible wool is worth a root check.
What to do: If wool shows up on the bark, also check exposed roots and the root collar for lumpy, gall-like swellings, particularly on a young or newly planted tree that seems to be growing poorly.
1 source
- Washington State University Tree FruitGalls form where aphid colonies feed on twigs or roots, and subterranean colonies cause the most damage.
“Encouraging general garden predators like ladybirds is enough on its own to keep a woolly aphid colony under control.”
Ladybirds, ground beetles, hoverflies, parasitoid wasps and earwigs are the natural enemies to encourage (RHS). A US tree fruit extension service gives most of the credit for biological control to the parasitoid wasp Aphelinus mali, with a complex of generalist predators important but easily overlooked because they leave no trace once a colony is eaten. A review of the species says effective limitation of populations relies on chemical and biological control together rather than the parasitoid alone.
Our read Predators help and are worth encouraging, but generalist predators alone are not shown to clear an established colony.
What to do: Keep gardening in ways that support predators generally, but check colonies through the season rather than assuming predators will clear an established one, and fall back on hand control for a colony that isn't shrinking.
2 sources
- Washington State University Tree FruitThe parasitoid Aphelinus mali is generally given the most credit for biological control, while a complex of generalist predators is also important but easily overlooked since they consume colonies without leaving a trace.
- PMCAphelinus mali is one of the most important natural enemies of Eriosoma lanigerum worldwide, but effective limitation of populations relies on synergy between chemical and biological control rather than the parasitoid working alone.
“The waxy wool coating shields a woolly aphid colony from insecticide sprays, so a normal spray application won't work well against it.”
A US extension service describes the wax mass as a physical barrier: it protects the aphids from insecticides, weather and natural enemies, and without added oil an insecticide has difficulty penetrating the waxy covering.
Our read There is no home-garden spray to add, so the practical point is that squashing, scrubbing or jetting off visible colonies bypasses the wax rather than trying to out-spray it.
What to do: Don't judge a spray's failure against this pest as a dosing mistake. The wax coating itself is the obstacle, which is one more reason hand control and encouraging predators are the more reliable route here.
1 source
- Utah State University ExtensionThe waxy mass protects the woolly aphid from insecticides, weather and natural enemies, and without added oil, insecticide has difficulty penetrating the waxy covering of the colony.
Frequently asked questions
What should I do first if I find woolly aphid in my garden?
The first thing to try: Hand control. Then: Natural predators.
Will woolly aphid kill my plants?
Usually not: it often does not affect cropping or the tree's future health. The indirect risk is that frost-split swellings let apple canker in, which can kill small branches and spurs.
When should I watch for woolly aphid?
Watch from around April through October, the full window this pest stays active on the tree.
Should I just leave woolly aphid alone?
Yes for small colonies: they can be tolerated while predators work, and crushing or scrubbing off a colony around a pruning cut is easy where practical.
Sources
Show the 6 sources
- RHS1: identification, symptoms, the pruning-wound trigger, life cycle and overwintering, hand control, encouraging predators, earwig shelters, absence of a chemical control
- RHS1: woolly aphid and frost damage as routes for apple canker infection; small branches and spurs can be killed within a season
- Gardener's World7: hosts including hawthorn and sorbus, frost-split swellings as a canker route, jet of water that will not cure long-term
- Gardener's World7: scrubbing off white fluff on shoots or pruning out badly affected areas
- treefruit.wsu.edu8: root galls and overwintering on roots, Aphelinus mali and generalist predators (US extension)
- Utah State University Extension8: the wax covering and insecticide penetration (US extension)