The cottony cushion scale, Icerya purchasi, is considered to have originated in Australia. It was accidentally introduced to California in about 1868 and devastated the citrus industry there until a natural enemy from Australia was introduced in 1888. This was the first ever example of successful classical biological control. The cottony cushion scale has subsequently spread widely through most of the tropical and subtropical countries of the world. It is also periodically discovered in greenhouses in temperate regions, but is not generally a pest in these situations.
Life cycle and appearance of Cottony cushion scale
As with all scale insects, the females do not have wings and look similar to the immature stages. Immature cottony cushion scales have black limbs and an orange-brown body that is coated with white and yellow wax. The adult ‘females’ are easily recognized by their large size (up to 10 mm long), red-brown body colour and covering of granular, white wax. The legs, antennae and body hairs are conspicuously black. The nymphs and adult females produce long, hair-like, transparent rods of wax from the body. On reaching maturity, the ‘female’ produces a white, fluted, wax ovisac with a series of uniform ridges running lengthwise over the surface. The ovisac may reach the same length as the body, giving an overall combined length of up to 20 mm.
The 'females' are actually hermaphrodites with fertilization occurring between the eggs and the sperm of the same individual. Males are occasionally produced from unfertilized eggs, but mating is not necessary for reproduction. The adult male has well developed antennae and one pair of dusky wings,
The adult ‘females’ produce 500 to 2000 bright-red, oblong eggs over a period of 2 to 3 months. After leaving the egg sac, the crawlers settle along the midribs and veins of the leaves. The next two instars migrate to the larger twigs and branches and eventually moult into the adult 'female'. There are two to four generations per year.
Cottony cushion scale is a particular pest of citrus, Acacia spp., Casuarina spp. and Pittosporum spp., but it can damage many types of fruit and forest trees, and ornamental shrubs and trees. Damage is mostly caused by sap depletion; the shoots dry up and die, and defoliation occurs. In addition, the copious quantities of honeydew produced by the scales coat the leaves, blocking the stomata and impeding gas exchange. Sooty moulds grow on the honeydew, blocking the light from the leaves and reducing photosynthesis. Also due to the honeydew, cottony cushion scale infestations are often attended by ants.