How to recognise it
The scales are most obvious in summer when the females mature. They appear as little white masses (the eggs) with yellow/brown caps on top, which are up to 5mm in size.
The females lay hundreds of eggs, either under the waxy scales, an additional covering of woolly wax, or directly under their bodies. The nymphs hatch in June or July and crawl over the plant before settling to feed on leaves until autumn. Then they settle on branches and trunks to overwinter. They mature in spring and are generally all female, reproducing parthenogenically, although males sometimes develop.
Why it’s a problem
These scale insects inflict little damage on plants, although the colonies of scales can be considered unsightly.
Where you are likely to find it
Affects the stems (branches and trunks) of horse chestnut, lime, elm, sycamore, magnolia, maple and cornus trees. It may also affect ornamental shrubs, such as bay laurel and skimmia. It’s found most frequently on plants near to roads and car parks due to the slightly raised temperatures in these conditions.
How to deter it
Check any new plants for infestations; introduction of plant material is the most common source of scales.
How to get rid of it
The parasitic wasp Metaphycus helvolus can be used as a biological control within confined areas. The wasp lays its eggs within the immature nymph scale, the eggs then develop within it.
Can be controlled chemically by spraying with a systemic insecticide such as thiacloprid, acetamiprid, imidacloprid or thiamethoxan. Apply in May to mid summer, when the nymphs are active and have not yet built up their protective scales, making the chemical more effective. Spray when necessary indoors, where the nymphs can hatch at any time due to the favourable conditions. A second application should be made after 2 weeks to deal with scales which escaped the first treatment.
Deciduous fruit trees affected can be washed with tar oil in winter.
Alternatively the affected plant can be washed with fatty acids (also called insecticidal soaps) or plant oils. This isn’t advisable for plants with delicate leaves as more damage than good can be done.
Is it good for anything?!
Other useful information
This scale insect is closely related to the woolly currant scale.