How to recognise it
The scales appear as static ‘bumps’ on leaves, stems and fruits of plants. They are up to 5mm long (males are 1-2mm long but are absent or rare in some species which are parthenogenic) and white, yellow or brown in colour.
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 weeks or months later and crawl over the plant before settling to feed as a scale. They may move on again to find new feeding sites.
Why it’s a problem
The scale insects insert their ’stylet’ or feeding apparatus into the plant and draw out the sap, which can weaken the plant’s growth in extreme cases. The scales excrete honeydew which, in turn, attracts sooty mould. This makes the plant look unattractive and can reduce the photosynthetic abilities of leaves (by blocking their access to light).
Where you are likely to find it
In northern climates they are particular problems in glasshouses or houseplants, although some species will survive outdoors. In warmer climates they are widespread on outdoor plants.
The scales appear on leaves, stems or fruits of plants and predominantly infest ornamental plants (although fruits such as peaches, nectarines and grapes may also be affected).
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, 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 any time 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.
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Other useful information
We have information relating to the following types of scale insects: