Scale insects are small, brown, limpet-like, sap-sucking insects that attack a wide range of plants – mainly woody plants, and especially those with waxy leaves. Most are smaller than 6mm (¼in) in size, although wisteria scale for instance can be twice this size.
The actual insect is covered by the hard, scaly, shell-like covering, which helps protect it from predators – and most types of insecticides.
Some species produce highly visible white, waxy egg masses in summer – hydrangea scale being a good example – which also contains the young nymph stage of the lifecycle. This is often the first thing you notice, having been unaware that your plants have scale insects, since they are very well camouflaged.
Adult scale insects are sedentary – never moving – but the newly hatched young nymphs, or “crawlers”, crawl over the stems and leaves to find somewhere to spend the rest of their lives.
There are around 25 different species of scale insects. Some attack a wide range of plants, whereas others only attack one or two plant species, and some may only attack plants inside the house or greenhouse.