Wisteria scale

Wisteria scale is a sap sucking insect that was first found in the UK in a London garden in 2001. Since then it has spread but remains mainly a pest of London and the surrounding area.

Wisteria scale (Eulecanium exrescens). Credit: RHS/Entomology.

Quick facts

Common name: Wisteria scale
Scientific name: Eulecanium excrescens
Plants affected: Wisteria and some Prunus and Acer species
Main symptoms: Large, blackish-brown, hemispherical scales on stems. Heavily infested wisterias may die back.
Most active: April-June

What is wisteria scale?

Wisteria scale is a sap-sucking insect that mainly occurs on wisteria but also infests some Acer and Prunus species. Scale insects are soft-bodied insects that are covered by a hard shell or scale when mature. At up to 10mm in diameter wisteria scale is substantially larger than most other scale insects found in  Britain.

Symptoms

The mature scales are found in late spring on the stems of wisteria and other host plants. The shell or scale that covers the insect and its eggs is blackish brown with a whitish dusting. It has a circular base and hemispherical shape.  Wisteria scale can be up to 10mm in diameter, and it is substantially larger than most other scale insects found in Britain. The immature scales are less convex and are pale brown with pinkish-white encrustations that resemble sugar grains. The wisteria scale can develop very heavy infestations and stems can become thickly encrusted with scales.  Such infestations can make the plant lack vigour and die back.

Note that there are other scale insects commonly found on wisteria. The most common is brown scale, Parthenolecanium corni, which is widespread in Britain on a wide range of woody plants. Brown scale is chestnut brown and oval in shape at the base. The mature scales can be up to 6mm long but are often smaller. Nut scale, Eulecanium tiliae, has a similar hemispherical shape to wisteria scale but is brown and no more than 5-6mm in diameter. Brown scale and nut scale are generally not damaging on wisteria.

Control

Wisterias are often not easy to spray because of their size. It is also difficult to spray a plant thoroughly when it is growing against a wall. It is usually not worthwhile spraying if the pest is brown scale. Wisteria scale is potentially a damaging and may need treatment.

Chemical control

The best time to control scale insects is when the more vulnerable newly hatched scale nymphs are present.  For wisteria scale, this is in late May-June. Organic pesticides, such as fatty acids (e.g. Bayer Organic Bug Free, Doff Greenfly and Blackfly Killer) or plant oils and extracts (e.g. Bug Clear for Fruit & Veg, Vitax Organic Pest and Disease Control) will give some control of newly hatched scales but several thorough applications will be required.

However, because of the difficulty of spraying the far side of wall-grown plants, these short persistence contact sprays will miss many of the scale nymphs. Systemic insecticides, such as thiacloprid (e.g. Provado Ultimate Bug Killer) or acetamiprid (e.g. Bug Clear Ultra) are more effective and longer lasting. They are absorbed into the plant tissues and taken up by the scales when they feed.

Note that dead scales can remain firmly attached to the stems.  The success of any treatment can be gauged by the extent to which new growth remains free of infestation.

Plants in flower should not be sprayed due to the danger to pollinating insects.

Download

Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)

Biology

Wisteria scale has one generation a year. The mature females deposit their eggs underneath their shells in late spring. The eggs hatch in late May-June and the young nymphs crawl about until they find somewhere suitable to feed. After that they live sedentary lives. They overwinter as immature nymphs on the stems and reach maturity in late spring. Scale insects do not fly. It is likely that they spread to other plants by young nymphs being blown by the wind or being transported by other animals, such as birds.

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