What are woolly aphids?
Woolly aphids are aphids that hide. Glands along the body push out fine white wax, and the insect builds that wax into a covering until the colony looks like scraps of cotton wool glued to a branch. People almost always spot the fluff first and the insect second, if at all.
Wipe a lump away and there is not much to see. Adults measure roughly 1 to 2.5 mm, and the body underneath is green, blue-grey, brown, reddish or purple depending on species. Several different genera carry the name, so “woolly aphid” describes a habit rather than a single insect.
The wax is not decoration. It hides the colony from predators, and it sheds water. That second property is the one that matters to you, because it is why sprayed treatments so often fail.
How to identify woolly aphids
Work down this list. The first two lines settle most cases on their own, since the combination of white fluff and a woody host is close to diagnostic.
Woolly aphid identification checklist
Check each clue below to narrow down what you saw.
White fluff, not an insect
What you notice first is the wax, in lumps like cotton wool or scraps of mould stuck to bark, twigs and leaf joints. The insects are underneath it.
On a tree or a woody shrub
Apple, crabapple, beech, elm, ash, oak, hackberry, pear, hawthorn, cotoneaster and pyracantha are the usual hosts. Garden aphids prefer soft new growth instead.
Tiny, and hard to see under the wax
Adults run about 1 to 2.5 mm. Wipe the wax away and the insect beneath is green, blue-grey, brown, reddish or purple, depending on species.
Short antennae
Usually less than a third of the body length. Ordinary garden aphids carry noticeably longer ones.
Almost no cornicles
This is the surprise. The two rear tubes that identify most aphids are reduced here to slight rings or pores, so the usual test does not apply.
Sticky honeydew below
Leaves, paths and cars under the tree turn tacky, and black sooty mould often follows on the honeydew.
Ants or yellowjackets in traffic
Both come for the honeydew. A steady trail up a trunk is worth following.
Lumpy galls on the wood
Swollen, warty growths on branches or trunk mean the woolly apple aphid, and that is the one species worth acting on.
Woolly aphids vs mealybugs
This is the single most common misidentification, and it is an understandable one. Both insects are small, both are soft, and both cover themselves in white wax.
Woolly aphids and mealybugs, side by side
Where you find it
- Woolly aphid
- Outdoors on trees and woody shrubs
- Mealybug
- Mostly houseplants and greenhouses
What the wax looks like
- Woolly aphid
- Loose fluffy threads, like cotton wool
- Mealybug
- A flat mealy powder coating the body
Body shape underneath
- Woolly aphid
- Rounded aphid body, legs tucked in
- Mealybug
- Distinctly oval, often with short filaments around the edge
Position on the plant
- Woolly aphid
- Bark, twigs, branch joints, sometimes roots
- Mealybug
- Leaf joints, stem crevices, undersides of leaves
Movement
- Woolly aphid
- Sedentary once feeding, colonies packed close
- Mealybug
- Slow crawlers, more scattered
Galls
- Woolly aphid
- The apple species raises warty galls on wood
- Mealybug
- Never causes galls
The quickest shortcut is the setting. Fluff on an apple or beech branch outdoors is a woolly aphid. White fluff in the leaf joints of a houseplant is a mealybug. For the indoor problem, see our tiny white bugs on plants guide.
Woolly aphid species, sorted by the tree they attack
The host narrows things down faster than the insect does, because most of these species are fussy. Find your tree in the left column.
Woolly aphid species by host plant
Apple, crabapple, pear, hawthorn, cotoneaster, pyracantha
- Species
- Woolly apple aphid, Eriosoma lanigerum
- What you see
- Fluff in bark cracks and pruning wounds, plus warty galls on wood and roots. The damaging one
American beech
- Species
- Beech blight aphid, Grylloprociphilus imbricator
- What you see
- Dense white colonies on twigs that wave their rear ends in unison when disturbed
Beech (leaf undersides)
- Species
- Woolly beech aphid, Phyllaphis fagi
- What you see
- Pale yellowish-green insects under the leaves, with wax
Elm, then serviceberry or hawthorn
- Species
- Woolly elm aphids, Eriosoma americanum and E. crataegi
- What you see
- Curled elm leaves in spring, then the colony moves to the second host
Ash, then true fir roots
- Species
- Prociphilus americanus
- What you see
- Black to dark green, on ash leaves in spring, underground by summer
Ash only
- Species
- Prociphilus fraxinifolii
- What you see
- Pale to yellowish green, stays on ash all year
Oak
- Species
- Woolly oak aphids, Stegophylla species
- What you see
- Pale grey-green to yellow-brown, on the leaves, all year on oak
Hackberry
- Species
- Woolly hackberry aphid, Shivaphis celti
- What you see
- Dark to pale green or pinkish. Heavy honeydew is the usual complaint
Pine needles
- Species
- Woolly pine needle aphid, Schizolachnus piniradiatae
- What you see
- Dark green or brown with grey wax tufts along the needles
Alder, then silver maple
- Species
- Woolly alder aphid, Paraprociphilus tessellatus
- What you see
- Conspicuous white colonies on alder twigs
The woolly apple aphid, the one that matters
If a woolly aphid is going to cause you real trouble, it is Eriosoma lanigerum. Every other species on that list is mostly a cosmetic problem. This one is different, for a reason that is easy to miss: it feeds below ground as well as above it.
Its feeding triggers the plant to grow globular, warty galls on branches, trunk and roots. Above ground those galls split the bark, and split bark is an open door for canker. Below ground, heavy root galling slows the tree’s growth, and that is the damage that actually costs you a crop.
Life cycle, and the species that change trees
Woolly aphids run through three stages: egg, nymph, adult. Nymphs pass through four instars before they can breed, and there is no pupal stage, so the young look like small adults throughout. Several generations follow one another in a single season, which is why a colony seems to appear from nowhere.
Winter is passed either as eggs laid on dormant buds or, in some species, underground on roots. That detail is the reason a dormant-season oil spray is so effective, and it is covered in the treatment steps below.
The genuinely odd part is host alternation. Several species spend the year on two completely different plants. Woolly elm aphids start on elm and finish on serviceberry or hawthorn. Prociphilus americanus feeds on ash leaves in spring, then moves down to the roots of true firs for the summer. Others, including the oak and hackberry species, never leave one host genus. So a colony vanishing in midsummer does not always mean you solved anything.
What damage do woolly aphids actually cause?
Start with the reassuring part, because it applies to most readers. UC IPM puts it plainly: most woolly aphid species are a nuisance and do not threaten the plant’s health or survival. What they produce is mess rather than injury.
- Honeydew. Sugary waste rains down onto whatever is below, which is why the path, the patio furniture or the car roof turns sticky.
- Sooty mould. A black fungus grows on the honeydew. It does not attack the tree, but it blocks light from leaves and looks awful.
- Ants and yellowjackets. Both come for the honeydew. The yellowjackets are the real nuisance here, since they sting and the aphids do not.
- Distorted growth. Curled leaves and twisted shoot tips where colonies feed on young tissue.
- Shoot-tip dieback. Only where populations run heavy for an extended period.
- Galls. The woolly apple aphid alone, and the one effect worth treating.
Do woolly aphids bite, and are they harmful to humans?
No to both. A woolly aphid feeds through fine stylets designed to pierce plant tissue and reach the sap. Those mouthparts cannot do anything to skin. They carry no disease that affects people, and one landing on you is simply lost.
There is one indirect catch worth knowing. The honeydew attracts yellowjackets, and yellowjackets do sting. So a tree dripping honeydew in late summer is worth approaching with a bit of care, though the aphids themselves are harmless. If something has bitten you, our bug bite identification guide covers what actually does.
What eats woolly aphids
More than you would think, given the wax. Four groups work through colonies on the parts of the plant above ground, and between them they end most outbreaks without any help.
Natural enemies of woolly aphids
Parasitoid wasps
- What to look for
- Swollen, papery aphids left behind in the colony
- Why it matters
- Each one is a wasp about to emerge and attack the rest
Lady beetle adults and larvae
- What to look for
- The larvae look like small spotted alligators, nothing like the adult
- Why it matters
- Larvae eat far more than the adults, and get squashed by people who do not recognise them
Green lacewing larvae
- What to look for
- Flat brown bodies with large curved jaws
- Why it matters
- Work through a colony quickly
Aphid fly larvae
- What to look for
- Small legless maggots sitting in among the wax
- Why it matters
- Easy to miss entirely
The practical point follows directly. If you can see any of these at work, the cheapest thing you can do is nothing at all.
How to get rid of woolly aphids
Work out whether it needs treating at all
Most woolly aphid colonies do no lasting harm. UC IPM is direct about this: the majority are a nuisance rather than a threat to the plant. If you see fluff on a mature beech, elm, oak or ash and nothing else, the honest answer is to leave it and let the predators work.
Check for galls before you decide
Look for swollen, warty lumps on branches, trunk or exposed roots. Those mean the woolly apple aphid, which is the exception worth treating, because root galls stunt growth and split bark opens the way to canker.
Knock small colonies off with water
On a shrub or a young tree, a firm jet of water breaks up the wax and dislodges the colony. Repeat every few days. The wax repels sprays, so physically breaking it is what makes anything else work.
Use horticultural oil or insecticidal soap
Both need direct contact, and the wax makes that hard, so coverage matters more than product choice. Spray thoroughly into the crevices where colonies sit. Expect partial control rather than a clean kill.
Treat dormant buds in winter
A dormant-season horticultural oil spray on shrubs and small trees reduces the eggs overwintering on buds. This is the highest-value timing of the year, and most people miss it.
Leave the predators alone
Parasitoid wasps, lady beetles, green lacewing larvae and aphid fly larvae all work through woolly aphid colonies. Broad-spectrum insecticide removes them and usually makes next season worse.
Pest control disclaimer. Photo-based identification can help, but it is not always perfect. For dangerous spiders, large infestations, or anything you are unsure about, contact a licensed pest control professional.
Frequently asked questions
What are woolly aphids?
What is the white fluffy stuff on my tree branches?
Are woolly aphids the same as mealybugs?
Do woolly aphids bite?
Are woolly aphids harmful to humans?
Do woolly aphids kill trees?
What is the woolly apple aphid?
How do I get rid of woolly aphids?
Why does spraying woolly aphids not work?
What is a boogie-woogie aphid?
Do woolly aphids move between different trees?
When are woolly aphids active?
Is it wooly aphids or woolly aphids?
Related guides
Aphid identifier
The main aphid guide: identify one from a photo, or by colour, size and cornicles.
How to get rid of aphids
Control for ordinary garden aphids, where contact sprays actually reach the insect.
Tiny white bugs on plants
Mealybugs, whiteflies, and the rest of the white things people find indoors.
Spider mites
Fine webbing and stippled leaves, treated differently from aphids.
Ladybug vs Asian beetle
Lady beetles eat woolly aphids. How to tell the native ladybug from the Asian species.
Bug bite identification
If something actually bit you, it was not an aphid.