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.
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This scale infests twigs and branches. The mature hermaphrodite is oval in shape, reddish-brown with black hairs, 5 mm long. When mature, the insect remains stationary, attaches itself to the plant by waxy secretions, and produces a white egg sac in grooves, by extrusion, in the body which encases hundreds of red eggs. The egg sac will grow to be two to three times as long as the body. Newly hatched nymphs are the primary dispersal stage, with dispersion known to occur by wind and by crawling. Early stage nymphs feed from the midrib veins of leaves and small twigs, and do the bulk of the damage. At each molt, they leave at the old feeding point the former skin and the waxy secretions in which they had covered themselves and from which their common name is derived. Unlike many other scale insects, they retain legs and a limited mobility in all life stages. Older nymphs migrate to larger twigs and eventually as adults to branches and the trunk. Their life cycle is highly temperature dependent, as the length of time in each stage of life is longer in cold temperatures than high temperatures.
True males are uncommon to rare overall, and in many infestations are not present. Pure females are unknown. Self-fertilization by a hermaphrodite will produce only hermaphrodites. Matings of a male and hermaphrodite will produce both males and hermaphrodites.
In addition to the direct damage from sap sucking, the insects also secrete honeydew, on which sooty mold often grows and causes further damage to the host plant. Some ants will also consume this honeydew.
Icerya purchasi is important as one of the first major successes of biological control. Importations of the vedalia ladybeetle (Rodolia cardinalis) in 1888-1889 by C. V. Riley, later head of the USDA's Division of Entomology, resulted in swift reductions of I. purchasi populations, saving the burgeoning Californian citrus industry from this destructive pest.
A second biological control, the parasitic fly Cryptochaetum iceryae has also been introduced to California as an additional control vector. Use of insecticides as a control is recommended only if no biological control species is present. Imidacloprid is especially not recommended, although it affects other scale insects, because it has no effect on this species but is very toxic to the vedalia ladybug.
^Maskell, W.M. 1879 (1878). On some Coccidae in New Zealand. Transactions and Proceedings of the New Zealand Institute 11: 187-228.
^Gardner, Andy; Ross, Laura (2011). "The Evolution of Hermaphroditism by an Infectious Male-Derived Cell Lineage: An Inclusive-Fitness Analysis". The American Naturalist 178 (2): 191–201. doi:10.1086/660823. PMID21750383. Lay summary – National Geographic News (August 17, 2011).
Baker, James R. (July 1994). "Cottony Cushion Scale". Ornamentals and Turf: Department of Entomology Insect Note. North Carolina State University: North Carolina Cooperative Extension. Retrieved 2008-07-16.
T. R. Fasulo and R. F. Brooks (October 1993, revised June 2004). "Scale Pests of Florida Citrus". Entomology and Nematology Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Retrieved 2008-07-16.Check date values in: |date= (help)
Grafton-Cardwell, E. E. (December 2003). "Cottony Cushion Scale". How to Manage Pests: Pests in Gardens and Landscapes. University of California Statewide Integrated Pest Management Program. Retrieved 2008-07-16.
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