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.

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 using the methods in the non-pesticide section below. Pesticide treatments are likely to kill natural enemies and are only likely to be successful if the entire plant can be reached.
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

• 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

Pesticides

The RHS recommends that  you don't use pesticides. Most pesticides (including organic types) reduce biodiversity, including natural enemies, impact soil health and have wider adverse environmental effects.

• 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 Ultra 2, Neudorff Bug Free Bug and Larvae Killer) or plant oils (e.g. Vitax Plant Guard Pest & Disease Control, Bug Clear 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 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 and SB Plant Invigorator). These are not considered organic
• Further information about the use of pesticides available for management of scale is available on the pesticides for gardeners leaflet 

Download

Pesticides for gardeners (pdf document)