''Aonidiella citrina'' or yellow scale is an armored scale insect from the

family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...

Diaspididae Diaspididae is the largest family of scale insects with over 2650 described species in around 400 genera. As with all scale insects, the female produces a waxy protective scale beneath which it feeds on its host plant. Diaspidid scales are far mo ...

. It feeds by sucking sap from plants in a number of plant families, but is mostly known for being a pest of

citrus ''Citrus'' is a genus of flowering trees and shrubs in the rue family, Rutaceae. Plants in the genus produce citrus fruits, including important crops such as oranges, lemons, grapefruits, pomelos, and limes. The genus ''Citrus'' is native to ...

Description

Armoured scales are so-called because they are covered by a hard, plate-like structure, the scale. The adult female yellow scale insect is concealed under a thin, pale circular scale through which its yellowish body colour can be seen. The female is legless and remains in the same location permanently, anchored to the surface. The male is a similar colour and has an elongate oval scale. The

exuviae In biology, exuviae are the remains of an exoskeleton and related structures that are left after ecdysozoans (including insects, crustaceans and arachnids) have moulted. The exuviae of an animal can be important to biologists as they can often b ...

(shed nymphal skins) are located near the centre in the female scale but are near one end in the male insect. When the adult male emerges from under its scale after four moults, it has limbs and a single pair of wings and is able to fly rather weakly; it lives for only a few hours, and its sole purpose is to find and mate with a female.

Life cycle

The adult female scale insect is

viviparous Among animals, viviparity is development of the embryo inside the body of the parent. This is opposed to oviparity which is a reproductive mode in which females lay developing eggs that complete their development and hatch externally from the ...

, the eggs hatching internally, and produces up to 150 young at the rate of two or three per day. The first-stage larva is mobile and is known as a crawler; it emerges from under the mother's scale and disperses to other parts of the plant; it may be blown by the wind or transported elsewhere by animals and is the only mobile life stage except for the adult male. When it has found a depression or other suitable location, it pierce the host plant with its mouthparts and sucks

sap Sap is a fluid transported in xylem cells (vessel elements or tracheids) or phloem sieve tube elements of a plant. These cells transport water and nutrients throughout the plant. Sap is distinct from latex, resin, or cell sap; it is a separ ...

. The female

nymph A nymph ( grc, νύμφη, nýmphē, el, script=Latn, nímfi, label= Modern Greek; , ) in ancient Greek folklore is a minor female nature deity. Different from Greek goddesses, nymphs are generally regarded as personifications of nature, are ...

moults twice as it develops while the male moults four times.

Distribution

Yellow scale has a widespread distribution in Asia, the Mediterranean area, Africa and South America, and is a serious pest of citrus in the southern United States, China, Japan, India, Iran and Australia. When it first appeared in California it was thought to be a yellow variant of red scale (''Aonidiella aurantii'') because the two are so similar in morphology. However, later various microscopic differences between them were noticed. Yellow scale affects fewer host plants than does red scale, and it is more susceptible to insecticides and biological pest control.

Damage

Yellow scale insects seldom feed on the woody parts of plants, concentrating on the leaves and fruits. It is a less serious pest of citrus than red scale because its effects are more cosmetic, and because it is easier to control. Heavy infestations of the insect can cause yellowing of the leaves, leaf fall, dieback of twigs and branches, or even tree death. Fruits may be disfigured and attain lower prices. In California, many populations of both red and yellow scale have become resistant to some of the most commonly used pesticides, but

biological control Biological control or biocontrol is a method of controlling pests, such as insects, mites, weeds, and plant diseases, using other organisms. It relies on predation, parasitism, herbivory, or other natural mechanisms, but typically also i ...

using chalcid wasp parasitoids is very effective; '' Aphytis melinus'' and '' Aphytis lingnanensis'' are used in mild coastal areas and '' Comperiella bifasciata '' in inland areas such as the

San Joaquin Valley The San Joaquin Valley ( ; es, Valle de San Joaquín) is the area of the Central Valley of the U.S. state of California that lies south of the Sacramento–San Joaquin River Delta and is drained by the San Joaquin River. It comprises seven ...

References

{{Taxonbar, from=Q10414113 Aspidiotina Agricultural pest insects Citrus pests Insects of Asia Insects described in 1891