Royal Horticultural Society

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.

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, making it 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.

Note that there is another scale insect commonly found on wisteria. This 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. Brown scale is generally not a damaging pest on wisteria. The wisteria scale can develop very heavy infestations with the stems thickly encrusted with scales.  Such infestations can make the plant lack vigour and die back.

Control

Wisterias are 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 pest and may need treatment.

Chemical control

Organic pesticides, such as those based on soft soaps or plant oils and extracts will give some control of newly hatched scales but several thorough applications will be required during the scale’s egg hatching period in late May-June. Systemic insecticides, such as thiacloprid (Provado Ultimate Bug Killer) or acetamiprid (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.

Biology

Wisteria scale has one generation a year. The mature females deposit their eggs underneath their protective shell. 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.