The scale insect, often simply known as scale, is a limpet-like sap sucking pest likely to infest just about any greenhouse, ornamental or house plant, as well as fruits and even vegetables, although some of its many species seem to have developed a particular taste for plants belonging to the fern and fig (ficus) families.
Most scale species will grow to lengths from 1 to 6 mm (less than 1/4 in), although the Wisteria scale, Eulecanium excrescens, can grow as long as 10 mm (about 1/2 in). Shapes and colours vary between species, but all of them feature shell-like coverings as adults.
Breeding continuously, some species, like the Cushion Scale (Pulvinaria) deposit their eggs somewhere out of the way, typically protecting them with a mass of white, waxy fibres. Others lay their eggs within their own protective shells.
Adults are typically fairly sedentary, but the newly hatched nymphs will happily explore new grounds all over the original host plant and any nearby plants they can reach, subsequently spreading the infestation.
They are typically found on plants' stems and the back of leaves, more often than not along the leaf's central veins. Here, they feed on the plant's sap and excrete honeydew, which may attract ants, other pests and sooty black mould.
Signs of Infestation
Scale infestations can be detected by patches of the fibrous white wax; patches of honeydew and/ or a blackened appearance of leaves/ stems due to sooty mould attracted by the honeydew; discard outer coverings (shell-like scales/ bumps), and, of course, insect activity.
Poor, stunted growth and a visible loss of vigour in the plant are also indications of scale presence, as is the appearance of weakened, yellowing areas. If left untreated, the affected plant/s may eventually die.
For obvious reasons, it is best to prevent infestations in the first place. Regular checking will ensure potential infestations can be dealt with as soon as possible.
Dealing with Infestations
Two scale species - the Hemispherical Scale, Saisettia coffeae, and the Soft Scale, Coccus hesperidum - can, if affecting greenhouse plants, be combated with the help of a parasitic wasp, Metaphycus helvolus, their natural enemy.
For indoor plants and the majority of scale species, other steps have to be taken. If infestations have been detected early, they can often be resolved by touching each one of the pests with a little cotton soaked in rubbing alcohol, although are is advised, as too much alcohol touching the plant may cause tissue damage.
Home-made sprays or Malathion may also help to deal with minor infestations. For heavy, stubborn problems, chemical solutions are often the only effective answer.
Contact sprays - like Deltamethrin or organic sprays - have only a limited effect due to the fact that adults are protected against them by their shell and, as a result of continual breeding, several life-cycle stages of this pest will be present on a plant at any given moment during the year. In addition, these pesticides work only for short periods, so application has to be repeated frequently over prolonged periods.Systemic pesticides - thiacloprid, acetamiprid or thiamethoxam - offer a more effective solution, but need to be treated with utmost care and should never be used if edible plants/ food items are kept nearby. Before using these sprays, it is therefore vitally important to read labels and heed all instructions/ warnings carefully.