How to Manage Pests
UC Pest Management Guidelines
Cottony Cushion Scale
Scientific Name: Icerya purchasi
(Reviewed 9/08, updated 5/14)
In this Guideline:
Description of the Pest
The most distinguishing feature of the cottony cushion scale female is the fluted cottony egg sac that she secretes. About 600 to 800 eggs are laid in the sac. Hatching occurs within a few days in summer, but can take up to 2 months in winter. Newly hatched nymphs are red with dark legs and antennae. First and second instar feed on twigs and leaves, usually along the veins. Third instars and adults are found mainly on branches and the trunk, rarely the fruit. Third instars are covered with a thick, cottony secretion that disappears after they molt. Adult females settle and begin to form the white, elongated egg sac. Males are rare and females can reproduce without mating. There are three generations a year.
Cottony cushion scales extract plant sap from leaves, twigs, and branches, thus reducing tree vigor. If infestations are heavy, leaf and fruit drop can occur along with twig dieback. The scale secretes honeydew, which promotes the growth of sooty mold.
The cottony cushion scale was a major pest of citrus in the 1880s. Efforts at controlling this pest resulted in one of the earliest and most impressive examples of classical biological control (where natural enemies are imported from the pest's native country and introduced in areas to which it has spread). Today, infestations occur in the San Joaquin Valley because of the temporary destruction of the natural enemies by insecticide treatments such as pyrethroids, neonicotinoids, and insect growth regulators. If you encounter cottony cushion scale, look for its natural enemies. Insecticide treatments are usually not necessary unless the use of broad-spectrum insecticides has decimated vedalia beetle populations.
Two natural enemies effectively control cottony cushion scale. The vedalia beetle, Rodolia cardinalis, was introduced from Australia in the early 1890s. The adult and larva feed on all stages of the scale. Female beetles lay eggs underneath the scale or attached to the egg sac. Young larvae move into the egg mass and feed on eggs. Older stage larvae feed on all scale stages. For more information about the life cycle of the cottony cushion scale and the vedalia beetle, see UC ANR Publication 8051, Stages of Cottony Cushion Scale and its Natural Enemy, the Vedalia Beetle.
The parasitic fly, Cryptochaetum iceryae, was also introduced from Australia and is a very effective parasite of this scale in coastal areas. The fly deposits its eggs inside the scale body. Upon hatching, parasite larvae feed on the scale body and pupate within the remains of the scale. Control ants if they are tending cottony cushion scale because they can significantly disrupt natural enemy activity.
Cottony cushion scale like moist, cool conditions and do well in citrus trees with dense canopies. Open the tree by pruning the interior of the canopy to remove suckers, dead branches, and crossing limbs, especially in mandarin and grapefruit varieties. Also in young trees, pruning the lower scaffold area can help.
Organically Acceptable Methods
Biological and cultural control are acceptable for use on an organically certified crop.
Monitoring and Treatment Decisions
Early Spring Monitoring
Monitor cottony cushion scale by examining 25 trees in the orchard. Spread the branches apart and look into the interior of the tree for adult female cottony cushion scale during March-April. If you find an infestation, also look for vedalia beetle stages (red eggs or larvae of the vedalia beetle) on the white egg sac of the adult scales or vedalia pupal cases attached to leaves. The vedalia beetle is the best method of controlling cottony cushion scale. Vedalia beetles grow very rapidly (they can complete four generations in the time it takes cottony cushion scale to complete one generation) and consume large numbers of cottony cushion scale eggs and nymphs in a very short amount of time. When vedalia beetles arrive in an orchard, they can control a serious cottony cushion scale problem in 4 to 6 weeks.
Insecticides are often not as effective as vedalia beetle and are disruptive to the natural enemies needed for other pests. If you have vedalia beetle stages present, then it will most likely control cottony cushion scale, as long as you do not disrupt it with pesticides (pyrethroids, neonicotinoids, and insect growth regulators are toxic to vedalia). If vedalia does not arrive naturally in an infested orchard by the end of March, it is critical to find stages of this beetle from another source and release them in April in order to give them enough time (6 weeks) to build their numbers and control cottony cushion scale. As few as 20 vedalia adults or larvae can be used to establish a population in an orchard. Vedalia beetles are very sensitive to heat and halt egg production and larval development when San Joaquin Valley daily temperatures exceed 90°F (usually in June). Thus, if the release is made after April, there is often not enough time for the vedalia beetle population to exert full control of the scales before hot weather and pesticide applications reduce their effectiveness.
If vedalia beetles do not arrive early enough or establish well enough, or an insecticide treatment for another pest eliminates the vedalia beetle, a treatment using buprofezin (Applaud), an organophosphate (malathion, methidathion), or a carbamate (carbaryl) may be warranted. Monitor cottony cushion scale by examining 25 trees in the orchard, spreading branches apart and looking into the interior of the tree. Count the number of live adult female scales (make sure they are alive by pulling apart the scale bodies - they should have liquid inside) per 2 foot branch. If the number of live adult female scale exceeds 4per branch, a treatment is warranted.
In the San Joaquin Valley, cottony cushion scale can become a serious problem in spring after use of broad-spectrum insecticides such as organophosphates, carbamates, and pyrethroids for citrus thrips and worm control. This is because these pesticides kill the vedalia beetle during the period when it is most actively feeding and reproducing on cottony cushion scale (March–June). The vedalia beetle will often recover from these sprays and control cottony cushion scale infestations by early summer. Of greater concern is the effect of insect growth regulators (IGRs) such as pyriproxyfen (Esteem) and buprofezin (Applaud) for California red scale control and neonicotinoids (imidacloprid-Provado and Admire and acetamiprid-Assail) for citricola scale and glassy-winged sharpshooter control. Insect growth regulators prevent vedalia beetle from completing pupation and emerging as adults; pyriproxyfen also prevents the beetle eggs from hatching.
Neonicotinoids kill vedalia beetles when they contact the residues or feed on cottony cushion scale that have taken up the insecticide systemically. The residues from pyriproxyfen, buprofezin, and imidacloprid can last more than 5 months. A sign that IGRs are killing vedalia beetles is the presence of dead vedalia beetle pupae on the outside leaves of trees. IGRs will also kill the cottony cushion scale pest, but they kill it very slowly. The orchards that experience the worst cottony cushion problems are not the orchards that are sprayed with IGRs, because the IGRs kill the cottony cushion scale as well as the California red scale. The worst cottony cushion scale outbreaks are in neighboring orchards because the spray drift from the treated orchard kills the vedalia beetle but not the cottony cushion scale. The neonicotinoids do not have any effect on cottony cushion scale.
In coastal areas, the parasitic fly can usually be observed parasitizing cottony cushion scales. The emerging parasite leaves an exit hole in the mummified scales. Ants are attracted to the honeydew excreted by this scale but do not interfere greatly with its biological control.
UC IPM Pest Management Guidelines: Citrus
Insects, Mites, and Snails
Acknowledgments for contributions to Insect, Mites, and Snails: