The cottony cushion scale (Icerya purchasi) is a homopteran pest native to Australia. Its Australian host plant is kangaroo acacia, but it is found on a wide variety of woody plant species. It is especially common on Citrus and Pittosporum (cheesewoods). Icerya purchasi is notable in that it was a pest overcome by one of the first major successes of effective biological control. In 1886 or 1887, the cottony cushion scale made its way into California on imported acacia plants. Not long after this, I. purchasi became a severe threat to the Californian citrus industry, which was just starting out at that time. In order to manage the overwhelming I. purchasi populations, C. V. Riley, later the head of the USDA’s Division of Entomology, imported the vedalia ladybeetle (Rodolia cardinalis), a natural enemy from Australia. The ladybug predators very effective and quickly controlled I. purchasi populations, saving the California citrus groves. At about the same time as it reached California, Icerya purchasi was also introduced into New Zealand and South Africa. It is now found world-wide, especially where citrus grows.

Covered with fluffy-looking white or yellow wax, the adult insects are large (up to 10 mm long) and usually have the conspicuous feature of a large egg sac (2-3 times the scale’s body length) with ridges running the length of it. Icerya purchasi are mostly hermaphroditic and finding males in a population is rare. Unlike most scale insects which permanently settle at later stages, cottony cushion scales are somewhat mobile at all stages of development, yet their dispersal stage is still the early instar (crawler) stage, when they can be carried by wind, crawling or hitchhiking on other animals to new host plants. Especially at the early nymph stages, this pest can badly damage citrus trees and reduce fruit production by sucking significant quantities of plant juices from veins on the leaf midrib and also from small twigs of the plant. Because they also produce a honeydew, cottony cushion scales make plants susceptible to sooty molds, which inhibit photosynthesis and further damage the plant.

The vedalia ladybeetle and the (also introduced) parasitic fly Cryptochaetum iceryae are very abundant and specific attackers of the cottony cushion scale. With these insects around, I. purchasi is considered only a minor pest of citrus orchards and no chemical control is needed or recommended, as pesticides are often toxic to these predators. A means of further controlling I. purchasi populations is to control their attendant ants, which protect the scales from predators in exchange for eating honeydew. Ant populations can often be reduced using tanglefoot or ant baiting.

(CABI, 2011; Grafton-Cardwell 2011; Hamon and Fasulo 2005; Wikipedia 2011)