There are two distinct groups of scale insects - soft scale and hard scale. The most commonly found soft scale insects are:

• Coccus hesperidum - the adults are 3-5 mm long, green to brown in colour, oval- shaped and appear slightly flattened.
• Saisettia coffeae - the adults are 5 mm or longer in length, deep brown in colour and dome-shaped.

Hard scale insects have an armoured body, and tend to be slightly flatter than soft scales.

Scale insects are often found massing along leaf veins or stems. They feed on plant sap and inject a poisonous substance into the leaves, which causes yellowing of the foliage and eventual defoliation. Soft scale insects also produce large amounts of sticky honeydew (excrement) that results in the growth of black sooty moulds that may ultimately kill the plant.

Control

Soft scale can be controlled biologically in greenhouses or conservatories by the parasitic wasp Metaphycus helvolus, whilst hard scale by be controlled by the predatory beetle, Chilocorus nigritus. Alternatively, spray affected plants with malathion or pirimiphos-methyl, or treat deciduous fruit trees with a tar oil wash in winter.