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.