The cottony cushion scale (Icerya purchasi) is a sap-sucking insect that poses a serious threat to a wide variety of woody plants. This invasive pest originated in Australia and has since spread globally to subtropical and tropical environments. The insect gained notoriety after its accidental introduction to California in the late 1860s, where it rapidly developed into a devastating pest. Its unchecked populations threatened the complete destruction of the citrus industry in the 1880s.
Recognizing the Pest
The most distinctive feature of an infestation is the waxy, fluted white egg sac (ovisac) produced by the adult female. This structure, which gives the scale its common name, is elongated and ridged, often growing to be two to three times the length of the female’s body. When fully mature, the combined female and egg sac can reach nearly a half-inch in length and is easily visible.
The adult female’s body, often partially obscured by this waxy secretion, is typically bright orange-brown or reddish-brown. Inside the ovisac, the female deposits hundreds of bright red eggs, which hatch into first-stage nymphs, known as crawlers. These crawlers are bright red with dark legs and antennae, and unlike many other scale insects, they retain their mobility throughout all life stages.
Symptoms of Infestation
Cottony cushion scales cause damage through both direct feeding and indirect effects. The insects use piercing-sucking mouthparts to extract phloem sap from the leaves, twigs, and branches. This depletion of nutrients results in a decline in the plant’s vigor, presenting as reduced growth, defoliation, and the dieback of small twigs and branches.
The indirect damage results from the sticky, sugary liquid the scales excrete, known as honeydew. Honeydew coats the leaves and branches, creating a medium for the growth of a black fungus called sooty mold. While the mold does not directly infect plant tissue, the dark covering blocks sunlight. This impairs the plant’s ability to perform photosynthesis, further weakening the host.
The scale has a wide host range, including:
- Citrus
- Pittosporum
- Acacia
- Rose
Eradicating the Scale
Mechanical and Cultural Control
Initial control efforts should focus on non-chemical methods. For light infestations, direct removal is effective, involving pruning heavily infested branches and disposing of the material. A strong jet of water can also dislodge the scales and crawlers from the plant’s surface.
The application of horticultural oil is an effective mechanical control method that avoids synthetic pesticides. These narrow-range petroleum spray oils (PSOs) work by coating and suffocating the scales. Oils are most effective when applied during the period when the vulnerable first-instar crawlers are most active, typically soon after egg hatch.
Biological Control
The preferred method for managing this pest relies on biological control: the introduction of the Vedalia Beetle (Rodolia cardinalis). This predatory lady beetle was imported from Australia in 1888 and is credited with saving the Californian citrus industry. Both the adult beetles and their larvae are specialized predators that feed on all stages of the cottony cushion scale.
Female Vedalia Beetles lay their red, oblong eggs directly on or underneath the scale’s ovisac. The beetle larvae burrow into the sac to consume the scale eggs and crawlers. The beetle develops rapidly, completing multiple generations faster than the scale, allowing it to quickly overwhelm an infestation. A serious scale problem can often be controlled by the Vedalia Beetle within four to six weeks, making insecticide use unnecessary.
Chemical Control
Chemical treatments are challenging and should be considered a final option due to the risk of secondary outbreaks. The scale’s waxy coating and protective egg sac shield it from many traditional contact insecticides. Sprays are most effective when targeting the vulnerable crawler stage, when the insects are mobile and lack full waxy protection.
Broad-spectrum insecticides, such as pyrethroids and neonicotinoids, should be avoided because they are toxic to the beneficial Vedalia Beetle. Eliminating the beetle population can lead to a rapid resurgence of the scale, as its natural enemy is no longer available. Systemic insecticides like imidacloprid are ineffective against the scale while simultaneously poisoning the Vedalia Beetle, further complicating control efforts.