Identification
An aphid covered with dense, white, waxy fluff resembling cotton wool or mould, beneath which brownish-red insects hide. It forms compact colonies on the trunk, main branches, in bark cracks, on pruning wounds and also on the roots. At the feeding sites the bark grows abnormally, producing knotty swellings and cracks that serve as an entry point for rot and canker. It attacks mainly apple, less often pear. A distinguishing feature is the rust-red fluid released when a colony is crushed — it stains the fingers like blood, hence the German name Blutlaus (blood louse). The insects suck sap from the phloem; the honeydew they secrete may become coated with sooty mould. The woolly apple aphid overwinters as young larvae hidden in bark cracks at the base of the trunk and on the roots; in spring it migrates up the tree, colonising young shoots, pruning wounds, watersprouts and root suckers. In Europe it reproduces exclusively by parthenogenesis, producing several generations over a season — for it lacks its primary host (elm), which it uses in its native North America. Winged females appearing in summer carry the infestation to neighbouring trees. The root form produces knotty thickenings on the roots that weaken and deform young trees — it is especially dangerous in nurseries and new plantings.
Treatment
Cutting out and destroying infested shoot sections and swellings, brushing off the colonies, and spraying with paraffin oils or insecticides that penetrate beneath the waxy layer. Introducing the parasitic wasp Aphelinus mali is effective, as is planting on woolly-aphid-resistant rootstocks (e.g. the Malling-Merton series). The waxy fluff is a physical barrier — contact products run off it, so every spray needs an added oil or a wetting adjuvant that breaks through the woolly covering and lets the liquid reach the insects; treatment is most effective in spring and early summer, when the colonies are young and the fluff is thin. Oils are applied away from heat and full sun — at high temperatures they can be phytotoxic to the leaves, and on contact they also harm some of the beneficial insects. Aphids hidden deep in cracks and under thick wax are reached by a systemic insecticide, taken up by the plant and distributed with the phloem sap, from which it passes to the feeding insects. Protecting natural enemies is crucial: besides Aphelinus mali, the colonies are thinned by earwigs, ladybirds, lacewings and hoverfly larvae — which is why spraying with broad-spectrum products (especially pyrethroids) is avoided, as they would kill the beneficial insects while leaving the wax-protected aphids; by destroying Aphelinus mali and predators, such treatment often intensifies the woolly aphid infestation, so a pyrethroid remains a last resort rather than a routine spray. Cultural measures support control: removing watersprouts and root suckers (favourite feeding sites), smooth training and healing of pruning wounds, and destroying the cut-out, swelling-covered sections limit the pest's hiding places. The root form can practically not be controlled chemically — so when establishing an orchard the key is choosing resistant rootstocks (MM106, MM111 and the others from the Malling-Merton series).
How to tell it apart from look-alikes
| Feature | Woolly apple aphid | Mealybugs |
|---|---|---|
| Host and location | Mainly apple (less often pear) in the orchard — trunk, main branches, bark cracks, roots. | Mostly potted, greenhouse and ornamental plants; leaf axils and undersides, shoots. |
| Fluid when crushed | Rust-red — stains the fingers (hence 'blood louse', German Blutlaus). | Colourless or pale, without red pigment. |
| Symptom on the wood | Knotty swellings and cracking of the bark and roots at the feeding sites. | No woody swellings; mainly honeydew, sooty mould and weakening of the plant. |
| Insect under the wax | An aphid (brownish-red) in a compact colony covered with woolly fluff. | An oval, segmented scale insect with waxy projections along the sides of its body, often mobile. |
Control — organic and chemical methods
Organic and biological methods
| Method | How it works | When to use |
|---|---|---|
| Parasitoid Aphelinus mali and natural enemies | The parasitic wasp Aphelinus mali lays its eggs in the bodies of the aphids; it is supported by earwigs, ladybirds, lacewings and hoverfly larvae. The condition is giving up sprays that destroy beneficial insects. | Throughout the season; the parasitoid is most active in summer |
| Resistant rootstocks (Malling-Merton series) | Planting apple trees on MM106, MM111 or other Malling-Merton rootstocks limits feeding on the roots — the only real protection against the root form, which you cannot control by spraying. | At the stage of establishing the orchard and the nursery |
| Cutting out swellings and removing suckers | Cutting out and destroying infested sections bearing swellings, removing watersprouts and root suckers (favourite feeding sites), brushing off the colonies and smooth healing of pruning wounds. A supporting measure that limits hiding places. | Winter to early spring and on an ongoing basis during the season |
| Paraffin/summer oils | The oil dissolves the waxy fluff and blocks the insects' spiracles; it must include a wetting additive so the liquid penetrates under the wool. It works only by contact — you have to wash off all the colonies thoroughly and apply it away from heat, because in hot weather it can be phytotoxic. | In spring and early summer, when the colonies are young |
Chemical methods
| Method | How it works | When to use |
|---|---|---|
| Systemic insecticide (distributed in the phloem) | A substance taken up by the plant and distributed with the phloem sap reaches the aphids hidden under the wax and in cracks that a contact product cannot wet. Apply selectively under heavy infestation, to spare Aphelinus mali. | Under heavy infestation of the above-ground parts |
| Contact product with an oil adjuvant (pyrethroid) | Contact products (e.g. a pyrethroid) are of little use on their own — the wax protects the insects; they require an added oil that breaks through the fluff. The broad spectrum kills Aphelinus mali and predators, which often intensifies the infestation — a last resort, not a routine treatment. | As an intervention, spot-targeted on outbreaks |
We give active-ingredient groups and modes of action, not brand names — specific products and their approval vary by country and are sometimes withdrawn. Always check and follow the current label.
Sources
- Atlas of plant pests
- Institute of Horticulture – National Research Institute
