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 confined to London and the surrounding areas.

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?

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.

Non-pesticide control

Well-tended healthy plants are able to tolerate light infestations of this insect.

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.

Pesticide 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
  • 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
  • Organic sprays, such as natural pyrethrum (e.g. Bug Clear Gun for Fruit & Veg, Ecofective Bug Killer), fatty acids (e.g. Solabiol Bug Free, Doff Greenfly & Blackfly Killer) or plant oils (e.g. Vitax Organic Pest & Disease Control, Bug Clear for Fruit and Veg) can give good control of scale insect nymphs. These pesticides have a very short persistence and so may require reapplication to keep scale insects in check. Plant oil and fatty acid products are less likely to affect larger insects such as ladybird adults. This can be used on ornamental and most edible plants
  • More persistent insecticides include the synthetic pyrethroids lambda-cyhalothrin (e.g. Westland Resolva Pest Killer), deltamethrin (e.g. Provanto Ultimate Fruit & Vegetable Bug Killer) and cypermethrin (e.g. Py Bug Killer). These products are suitable for use on some food plants. Label instructions on suitability for particular food plants, especially maximum number of applications and harvest interval must be followed
  • The systemic neonicotinoid insecticide acetamiprid (e.g. Bug Clear Ultra) is also available
  • Follow label instructions when using pesticides. On edible plants make sure the food plant is listed on the label and follow instructions on maximum number applications, spray interval and harvest interval
  • Do not spray plants in flower due to the danger to pollinating insects
  • Inclusion of a pesticide product does not indicate a recommendation or endorsement by the RHS. It is a list of products currently available to the home gardener


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.

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