Wisteria scale

Wisteria scale is a sap sucking insect that was first found in Britain 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. 
Most active: April-June

What is wisteria scale?

There are many types of scale insects encountered by gardeners, they are sap sucking true bugs. 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. Wisteria scale mainly occurs on wisteria but also feeds on some Acer and Prunus species.

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 usually with a whitish dusting. It has a circular base and hemispherical shape. Wisteria scale can be up to 10mm in diameter, 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. Wisteria scale can develop very heavy infestations and stems can become thickly encrusted with scales, when this occurs host plants may 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

Check wisteria and other susceptible plants frequently so action can be taken before a damaging population has developed. When choosing control options you can minimise harm to non-target animals by starting with the methods in the non-pesticide control section. If this is not sufficient to reduce this insect to acceptable levels then you may choose to use pesticides. Within this group the shorter persistence pesticides (that are usually certified for organic growing) are likely to be less damaging to non-target wildlife than those with longer persistence and/or systemic action.
Light infestations are of little consequence and can be tolerated, but heavy attacks can be dealt with in late spring and early summer when the more vulnerable newly-hatched scales are present. Note that dead scales can remain firmly attached to the plants. The success of any treatment can be gauged by the extent to which new growth remains free of scale insects.
Wisterias are often not easy to treat because of their size. It is also difficult to access all areas of a plant  thoroughly when it is growing against a wall. It is usually not worthwhile treating a 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.

Non-pesticide control

  • Where possible tolerate populations of scale insects. Well-tended healthy plants are able to tolerate light populations of these insects and  so they do not necessarily require control
  • Adult scales can be removed when seen but this may not reduce large populations
  • Encourage predators  in the garden, some ladybirds, parasitoid wasps and some birds will eat scale insects
 
 

Pesticide control

  • For scales on deciduous plants including edible fruits, a plant oil winter wash (considered organic e.g. Growing Success Winter Tree Wash) can be used. This can control the overwintering scale nymphs in December-January when the plants are fully dormant
  • The best time for summer spraying is in May and June when the more vulnerable newly hatched scale nymphs are present 
  • Organic sprays, such as natural pyrethrum (e.g. Bug Clear Gun for Fruit & Veg, Neudorff Bug Free Bug and Larvae Killer), fatty acids (e.g. Solabiol Bug Free, Doff Greenfly & Blackfly Killer) or plant oils (e.g. Vitax Plant Guard 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 numbers in check. Plant oil and fatty acid products are less likely to affect larger insects such as ladybird adults 
  • Plant invigorators combine nutrients to stimulate plant growth with surfactants or fatty acids that have a physical mode of action (e.g. Ecofective Bug Control, RHS Bug and Mildew Control, SB Plant Invigorator and Westland Resolva Natural Power Bug & Mildew). These are not considered organic
  • More persistent contact-action insecticides include the synthetic pyrethroids lambda-cyhalothrin (e.g. Westland Resolva Bug Killer), deltamethrin (e.g. Provanto Ultimate Fruit & Vegetable Bug Killer, Provanto Sprayday Greenfly Killer) and cypermethrin (e.g. Py Bug Killer)
  • 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.
Plants in flower should not be sprayed due to the danger to bees and other pollinating insects.
Inclusion of a pesticide product does not indicate a recommendation or endorsement by RHS Gardening Advice. It is a list of products currently available to the home gardener.

Download

Pesticides for gardeners (pdf document)
 
 
 

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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