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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 confined to London and the surrounding areas.
Wisteria scale (Eulecanium exrescens). Credit: RHS/Entomology.
There are many types of scale insects encountered by gardeners, they are sap sucking true bugs.
Wisteria scale 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.
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 usually 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.
Wisterias are often not easy to treat 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 plant is infested with brown scale as that species of scale does not usually affect the vigour of wisteria. Wisteria scale is potentially more damaging and may need treatment.
Scrape off adult scales when they are seen, this may help reduce an infestation and determine level of re-infestation but it is unlikely to eliminate it.
Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)
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.
Horse chestnut scale
RHS statement on pesticides in horticulture
Scurfy rose scale
Woolly vine or currant scale
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