The Cottony Cushion Scale Icerya purchasi is a scale insect often found on citrus trees. It’s an Australian native but has spread to most parts of the World where citrus is grown.
The actual insects are oval, orange-brown with wispy hairs, and only about 5mm long when fully mature. The adult females attach themselves to the tree by waxy secretions and produce a white, grooved egg sac, which is the part that most easily comes to notice. They produce hundreds of eggs and can quickly multiply to cover most parts of a tree. The newly hatched nymphs are relatively mobile and can be seen crawling among the branches.
Like other species of scale insects, they cause damage by sucking sap, in the process excreting large quantities of honeydew, which drips onto leaves leading to sooty mould. Where ants are present, they will harvest the honeydew and actively protect the scales from predators, so a first step for biological control is to exclude ants from the tree by putting some type of barrier at the base of the trunk. Also making sure different parts of the tree don’t contact fences or adjacent trees or structures.
Natural predators include ladybirds, lacewings, and wasps. The predatory insects that get the most mention in literature are the Vedalia Ladybird Rodolia cardinalis and the parasitic fly Cryptochetum iceryae. Both are Australian natives as well and have been introduced to many parts of the World where Icerya purchasi has become a problem.
Where natural predators are not present or not giving control, the most common control method is to spray with white oil or its commercial equivalents. Insecticides are best avoided as they will harm any natural predators that may be present.