The cottony cushion scale is a sap-sucking insect pest found on a wide variety of ornamental plants, fruit trees, and shrubs. Originally from Australia, this pest has spread to tropical and subtropical regions worldwide, occasionally appearing in greenhouses in temperate climates. It feeds on the sap of its host plants, and its presence is often made obvious by the visible white, cottony masses it produces.
What is Cottony Cushion Scale?
The most recognizable feature of the cottony cushion scale, Icerya purchasi, is the adult female. She has an orange-brown body and is conspicuous for the white, fluted egg sac she creates, attached to her body. This cottony ovisac can be two to three times as long as her 5 mm body and contains hundreds of bright red eggs.
The life cycle begins when eggs hatch into mobile first-instar nymphs, commonly called “crawlers.” These tiny red crawlers have black legs and antennae and spread the infestation as they move to find a suitable place to feed, often settling along the veins of leaves. As they grow, they molt, and subsequent instars migrate to twigs and larger branches. Adult males are winged and rarely seen; females are hermaphroditic and can reproduce without mating, completing two to four generations per year.
Affected Plants and Damage Caused
Cottony cushion scale infests a broad range of host plants and is particularly common on citrus trees, acacia, nandina, pittosporum, and roses. The damage is caused by the insects sucking phloem sap from the leaves, twigs, branches, and even the trunk of the host plant. This constant feeding depletes the plant of nutrients, reducing its overall vigor.
Plants may exhibit yellowing leaves, premature leaf drop, and dieback of twigs and branches. A significant secondary effect of their feeding is the production of a sugary, sticky waste product called honeydew. This substance coats the plant, leading to the growth of a black fungus called sooty mold. While sooty mold doesn’t directly harm the plant, it blocks sunlight and can interfere with photosynthesis, further weakening the plant.
Management and Control Methods
For minor infestations, physical removal can be an effective first step. This can involve pruning and disposing of heavily infested branches or simply wiping the scales off plant surfaces with a cloth or a soft brush. This method can help reduce their numbers, especially on smaller plants or in localized areas.
A sustainable long-term solution is biological control. The vedalia beetle, Rodolia cardinalis, is a natural predator that was introduced from Australia in the 1890s to control cottony cushion scale, saving California’s citrus industry. Both the adult beetles and their larvae feed on all life stages of the scale. The female beetle lays her red eggs on or under the scale’s egg sac, and the beetle larvae burrow in to consume the scale’s eggs and crawlers.
Horticultural oils, including neem oil, offer another control option. These oils work by smothering the insects upon direct contact. They are most effective against the vulnerable crawler stage. Thorough coverage of the entire plant is necessary for the oils to be effective, and repeat applications may be needed to manage newly hatched crawlers.
Chemical insecticides should be considered a last resort, as their use can disrupt the natural balance of an ecosystem. Broad-spectrum insecticides, such as pyrethroids and neonicotinoids, are problematic because they can kill beneficial insects like the vedalia beetle, which can make the infestation worse. If chemical intervention is deemed necessary, always read and follow label instructions carefully to minimize harm to non-target organisms.