Scale insect

Scale insects vary dramatically in appearance, from very small organisms (1–2 mm) that grow beneath wax covers (some shaped like oysters, others like mussel shells), to shiny pearl-like objects (about 5 mm), to animals covered with mealy wax. Adult females are almost always immobile (apart from mealybugs) and permanently attached to the plant on which they are feeding. They secrete a waxy coating for defence, making them resemble reptilian or fish scales, and giving them their common name. The group is extremely sexually dimorphic; female scale insects, unusual for Hemiptera, retain the immature external morphology even when sexually mature, a condition known as neoteny. Adult females are pear-shaped, elliptical or circular, with no wings, and usually no constriction separating the head from the body. Segmentation of the body is indistinct, but may be indicated by the presence of marginal bristles. Legs are absent in the females of some families, and when present vary from single segment stubs to five-segmented limbs. Female scale insects have no compound eyes, but ocelli (simple eyes) are sometimes present in Margarodidae, Ortheziidae and Phenacoleachiidae. The family Beesoniidae lacks antennae, but other families possess antennae with from one to 13 segments. The mouthparts are adapted for piercing and sucking. In species with winged males, generally only the forewings are fully functional. This is unusual among insects; it most closely resembles the situation in the true flies, the Diptera. However, the Diptera and Hemiptera are not closely related, and do not closely resemble each other in morphology; for example, the tail filaments of the Coccomorpha do not resemble anything in the morphology of flies. The hind (metathoracic) wings are reduced, commonly to the point that they can easily be overlooked. In some species the hind wings have hamuli, hooklets, that couple the hind wings to the main wings, as in the Hymenoptera. The vestigial wings are often reduced to pseudo-halteres, club-like appendages, but these are not homologous with the control organs of Diptera that are called halteres, and it is not clear whether they have any substantial control function. Hermaphroditism is very rare in insects, but several species of Icerya exhibit an unusual form. The adult possesses an ovotestis, consisting of both female and male reproductive tissue, and sperm is transmitted to the young for their future use. The fact that a new population can be founded by a single individual may have contributed to the success of the cottony cushion scale which has spread around the world. ==Life cycle==

scale, Mytilaspis pomorum. a) underside of scale showing female and eggs, x24 b) scale upperside, x24 c) female scales on twig d) male scale, x12 e) male scales on twig Female scale insects in more advanced families develop from the egg through a first instar (crawler) stage and a second instar stage before becoming adult. In more primitive families there is an additional instar stage. Males pass through a first and second instar stage, a pre-pupal and a pupal stage before adulthood (actually a pseudopupa, as only holometabolous insects have a true pupa). Many species have the XX-XO system where the female is diploid and homogametic while the male is heterogametic and missing a sex chromosome. In some Diaspididae and Pseudococcidae, both sexes are produced from fertilized eggs but during development males eliminate the paternal genome and this system called paternal genome elimination (PGE) is found in nearly 14 scale insect families. This elimination is achieved with several variations. The commonest (known as the lecanoid system) involved deactivation of the paternal genome and elimination at the time of sperm production in males, this is seen in Pseudococcidae, Kerriidae and some Eriococcidae. In the other variant or Comstockiella system, the somatic cells have the paternal genome untouched. A third variant found in Diaspididae involves the paternal genome being completely removed at an early stage making males haploid both in somatic and germ cells even though they are formed from diploids, i.e., from fertilized eggs. In addition to this there is also true haplodiploidy with females born from fertilized eggs and males from unfertilized eggs. This is seen in the genus Icerya. In Parthenolecanium, males are born from unfertilized eggs but diploidy is briefly restored by fusion of haploid cleave nuclei and then one sex chromosome is lost through heterochromatinization. Females can reproduce parthenogenetically with six different variants based on whether males are entirely absent or not (obligate v. facultative parthenogenesis); the sex of fertilized v. unfertilized eggs; and based on how diploidy is restored in unfertilized eggs. The evolution of these systems are thought to be the result of intra-genomic conflict as well as possibly inter-genomic conflict with endosymbionts under varied selection pressures. The diversity of systems has made scale insects ideal models for research. ==Ecology==

Scale insects are an ancient group, having originated in the Cretaceous, the period in which angiosperms came to dominance among plants, with only a few groups species found on gymnosperms. They feed on a wide variety of plants but are unable to survive long away from their hosts. While some specialise on a single plant species (monophagous), and some on a single genus or plant family (oligophagous), others are less specialised and feed on several plant groups (polyphagous). For example, cochineal species are restricted to cactus hosts, and the gall-inducing Apiomorpha are restricted to Eucalyptus. Some species have certain habitat requirements; some Ortheziidae occur in damp meadows, among mosses and in woodland soil, and the boreal ensign scale (Newsteadia floccosa) inhabits plant litter. Several other monophagous scale insects, especially those on islands, are threatened by coextinction due to threats faced by their host plants. Most scale insects are herbivores, feeding on phloem sap drawn directly from the plant's vascular system, but a few species feed on fungal mats and fungi, such as some species in the genus Newsteadia in the family Ortheziidae. Plant sap provides a liquid diet which is rich in sugar and non-essential amino acids. In order to make up for the shortage of essential amino acids, they depend on endosymbiotic proteobacteria. Scale insects secrete a large quantity of sticky viscid fluid known as "honeydew". This includes sugars, amino acids and minerals, and is attractive to ants as well as acting as a substrate on which sooty mould can grow. The mould can reduce photosynthesis by the leaves and detracts from the appearance of ornamental plants. The scale's activities can result in stress for the plant, causing reduced growth and giving it a greater susceptibility to plant diseases. Formica fusca ants tending a herd of mealybugs Scale insects in the genus Cryptostigma live inside the nests of neotropical ant species. Many tropical plants need ants to survive which in turn cultivate scale insects thus forming a tripartite symbiosis. Some ants and scale insects have a mutualistic relationship; the ants feed on the honeydew and in return protect the scales. On a tulip tree, ants have been observed building a papery tent over the scales. In other instances, scale insects are carried inside the ant's nest; the ant Acropyga exsanguis takes this to an extreme by transporting a fertilised female mealybug with it on its nuptial flight, so that the nest it founds can be provisioned. '' preying on mealybugs Scale insects have various natural enemies, and research in this field is largely directed at the species that are crop pests. Entomopathogenic fungi can attack suitable scales and completely overgrow them. The identity of the host is not always apparent as many fungi are host-specific, and may destroy all the scales of one species present on a leaf while not affecting another species. Fungi in the genus Septobasidium have a more complex, mutualistic relationship with scale insects. The fungus lives on trees where it forms a mat which overgrows the scales, reducing the growth of the individual parasitised scales and sometimes rendering them infertile, but protecting the scale colony from environmental conditions and predators. The fungus benefits by metabolising the sap extracted from the tree by the insects. Natural enemies include parasitoid wasps, mostly in the families Encyrtidae and Eulophidae, and predatory beetles such as fungus weevils, ladybirds and sap beetles. Ants looking after their providers of honeydew tend to drive off predators, but the mealybug destroyer has outwitted the ants by developing cryptic camouflage, with their larvae mimicking scale larvae. ==Significance==

As pests Many scale species are serious crop pests and are particularly problematic for their ability to evade quarantine measures. In 1990, they caused around $5 billion of damage to crops in the United States. The waxy covering of many species of scale protects their adults effectively from contact insecticides, which are only effective against the first-instar nymph stage known as the crawler. However, scales can often be controlled using horticultural oils that suffocate them, systemic pesticides that poison the sap of the host plants, or by biological control agents such as tiny parasitoid wasps and ladybirds. Insecticidal soap may also be used against scales. One species, the cottony cushion scale, is a serious commercial pest on 65 families of woody plants, including Citrus fruits. It has spread worldwide from Australia. File:Icerya purchasi feeding on Citrus.jpg|Adult female cottony cushion scale (Icerya purchasi) with young crawlers. The species is a major commercial pest of crops such as Citrus fruits. File:Anagyrus lopezi.jpg|The tiny parasitic wasp Anagyrus lopezi, a highly effective biological control of the cassava mealybug As biological controls At the same time, some kinds of scale insects are themselves useful as biological control agents for pest plants, such as various species of cochineal insects that attack invasive species of prickly pear, which spread widely especially in Australia and Africa. Products Some types of scale insect are economically valuable for the substances they can yield under proper husbandry. Some, such as the cochineal, kermes, lac, Armenian cochineal, and Polish cochineal, have been used to produce red dyes for coloring foods and dyeing fabrics. Both the colour name "crimson" and the generic name Kermes are from Italian carmesi or cremesi for the dye used for Italian silk textiles, in turn from the Persian qirmizī (قرمز), meaning both the colour and the insect. The colour name "scarlet" is similarly derived from Arabic siklāt, denoting extremely expensive luxury silks dyed red using kermes. Some waxy scale species in the genera Ceroplastes and Ericerus produce materials such as Chinese wax, and several genera of lac scales produce shellac. File:Indian collecting cochineal.jpg|Collecting scale insects from a prickly pear for a dyestuff, cochineal, 1777 File:Weltliche Schatzkammer Wienc.jpg|Coronation cloak of King Roger II of Sicily, 1133. Silk scarlet cloth dyed with kermes, made from female Kermes scales File:Shellac varities.png|Some varieties of shellac File:02-Indian-Insect-Life - Harold Maxwell-Lefroy - Kerria-Lacca.jpg|Kerria lacca and its shellac tubes ==Evolution==

The containing group of the scale insects was formerly treated as the superfamily Coccoidea but taxonomic uncertainties have led workers to prefer the use of the infraorder Coccomorpha as the preferred name for the group. Scale insects are members of the Sternorrhyncha. The phylogeny of the extant Sternorrhyncha, from a 2024 study using ultraconserved genetic elements, is shown in the cladogram: }} mealybug Electromyrmococcus (in the jaws of an ant) in Miocene Dominican amber The timing of phylogenetic diversification within the Coccomorpha was estimated in a 2016 study based on molecular clock divergence time estimates, along with fossils being used for calibration. They suggested that the main scale insect lineages diverged before their angiosperm hosts, and suggested that the insects switched from feeding on gymnosperms once the angiosperms became common and widespread in the Cretaceous. They estimated that the Coccomorpha appeared at the start of the Triassic period, around 245 million years ago, and that the neococcoids appeared during the Early Jurassic, some 185 million years ago. They are abundantly preserved in amber from the Early Cretaceous, 130 mya, onwards; they were already highly diversified by Cretaceous times. All the families were monophyletic except for the Eriococcidae. The Coccomorpha are division into two clades the "Archaeococcoids" and "Neococcoids". The archaeococcoid families have adult males with either compound eyes or a row of unicorneal eyes and have abdominal spiracles in the females. In neoccoids, the females have no abdominal spiracles. In the cladogram below the genus Pityococcus is moved to the "Neococcoids". A cladogram showing the major families using this methodology is shown below. |1=Pityococcidae |2= }} }} }} Recognition of scale insect families has fluctuated over time, and the validity of many remains in flux, with several recognized families not included in the phylogeny presented above including extinct groups are listed below: • Archecoccoidea • Apticoccidae • Arnoldidae • Burmacoccidae • Callipappidae • Coelostomidiidae • Electrococcidae • Grimaldiellidae • Grohnidae • Hammanococcidae • Jankotejacoccidae • Marchalinidae • Margarodidae • Matsucoccidae • Monophlebidae • Ortheziidae • Pennygullaniidae • Phenacoleachiidae • Pityococcidae • Putoidae • Serafinidae • Steingeliidae • Stigmacoccidae • Termitococcidae • Weitschatidae • Xylococcidae • Neococcoidea • Aclerdidae • Albicoccidae • Asterolecaniidae • Beesoniidae • Calycicoccidae • Carayonemidae • Cerococcidae • Cissococcidae • Coccidae • Conchaspididae • Cryptococcidae • Dactylopiidae • Diaspididae • Eriococcoidae • Halimococcidae • Hodgsonicoccidae • Inkaidae • Kermesidae • Kerriidae • Lecanodiaspididae • Micrococcidae • Phoenicococcidae • Porphyrophoridae • Pseudococcidae • Rhizoecidae • Stictococcidae • Tachardiidae ==See also==