Scale insects are small

insect Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three pairs ...

s of the order

Hemiptera Hemiptera (; ) is an order (biology), order of insects, commonly called true bugs, comprising over 80,000 species within groups such as the cicadas, aphids, planthoppers, leafhoppers, Reduviidae, assassin bugs, Cimex, bed bugs, and shield bugs. ...

, suborder

Sternorrhyncha The Sternorrhyncha suborder of the Hemiptera contains the aphids, whiteflies, and scale insects, groups which were traditionally included in the now-obsolete order "Homoptera". "Sternorrhyncha" refers to the rearward position of the mouthparts re ...

. Of dramatically variable appearance and extreme

sexual dimorphism Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most ani ...

, they comprise the infraorder Coccomorpha which is considered a more convenient grouping than the superfamily Coccoidea due to taxonomic uncertainties. Adult females typically have soft bodies and no limbs, and are concealed underneath domed scales, extruding quantities of wax for protection. Some species are hermaphroditic, with a combined ovotestis instead of separate ovaries and testes. Males, in the species where they occur, have legs and sometimes wings, and resemble small flies. Scale insects are

herbivore A herbivore is an animal anatomically and physiologically adapted to eating plant material, for example foliage or marine algae, for the main component of its diet. As a result of their plant diet, herbivorous animals typically have mouthpart ...

s, piercing plant tissues with their mouthparts and remaining in one place, feeding on

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 excess fluid they imbibe is secreted as honeydew on which sooty mold tends to grow. The insects often have a mutualistic relationship with

ant Ants are eusocial insects of the family Formicidae and, along with the related wasps and bees, belong to the order Hymenoptera. Ants evolved from vespoid wasp ancestors in the Cretaceous period. More than 13,800 of an estimated total of ...

s, which feed on the honeydew and protect them from

predators Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill th ...

. There are about 8,000 described species. The oldest fossils of the group date to the

Early Cretaceous The Early Cretaceous ( geochronological name) or the Lower Cretaceous (chronostratigraphic name), is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 145 Ma to 100.5 Ma. Geology Pro ...

, preserved in

amber Amber is fossilized tree resin that has been appreciated for its color and natural beauty since Neolithic times. Much valued from antiquity to the present as a gemstone, amber is made into a variety of decorative objects."Amber" (2004). In ...

. They were already substantially diversified by this time suggesting an earlier origin during the

Triassic The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest period ...

Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The J ...

. Their closest relatives are the jumping plant lice,

whiteflies Whiteflies are Hemipterans that typically feed on the undersides of plant leaves. They comprise the family Aleyrodidae, the only family in the superfamily Aleyrodoidea. More than 1550 species have been described. Description and taxonomy The ...

, phylloxera bugs and

aphids Aphids are small sap-sucking insects and members of the superfamily Aphidoidea. Common names include greenfly and blackfly, although individuals within a species can vary widely in color. The group includes the fluffy white woolly aphids. A t ...

. The majority of female scale insects remain in one place as adults, with newly hatched nymphs, known as "crawlers", being the only mobile life stage, apart from the short-lived males. The reproductive strategies of many species include at least some amount of

asexual reproduction Asexual reproduction is a type of reproduction that does not involve the fusion of gametes or change in the number of chromosomes. The offspring that arise by asexual reproduction from either unicellular or multicellular organisms inherit the fu ...

parthenogenesis Parthenogenesis (; from the Greek grc, παρθένος, translit=parthénos, lit=virgin, label=none + grc, γένεσις, translit=génesis, lit=creation, label=none) is a natural form of asexual reproduction in which growth and development ...

. Some scale insects are serious commercial pests, notably the cottony cushion scale (''

Icerya purchasi ''Icerya purchasi'' ( common name: cottony cushion scale) is a scale insect that feeds on more than 80 families of woody plants, most notably on ''Citrus'' and ''Pittosporum''. Originally described in 1878 from specimens collected in New Zealand ...

'') on ''

Citrus ''Citrus'' is a genus of flowering plant, flowering trees and shrubs in the rue family, Rutaceae. Plants in the genus produce citrus fruits, including important crops such as Orange (fruit), oranges, Lemon, lemons, grapefruits, pomelos, and lim ...

'' fruit trees; they are difficult to control as the scale and waxy covering protect them effectively from contact insecticides. Some species are used for

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 ...

of pest plants such as the prickly pear, '' Opuntia''. Others produce commercially valuable substances including carmine and kermes dyes, and

shellac Shellac () is a resin secreted by the female lac bug on trees in the forests of India and Thailand. It is processed and sold as dry flakes and dissolved in alcohol to make liquid shellac, which is used as a brush-on colorant, food glaze and ...

lacquer. The two red colour-names crimson and scarlet both derive from the names of ''Kermes'' products in other languages.

Description

Scale insects vary dramatically in appearance, from very small organisms (1–2 mm) that grow beneath

wax Waxes are a diverse class of organic compounds that are lipophilic, malleable solids near ambient temperatures. They include higher alkanes and lipids, typically with melting points above about 40 °C (104 °F), melting to giv ...

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

mealybug Mealybugs are insects in the family (biology), family Pseudococcidae, unarmored scale insects found in moist, warm habitats. Many species are considered pest (animal), pests as they feed on plant juices of greenhouse plants, house plants and sub ...

s) 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 key character that sets apart the Coccomorpha from all other Hemiptera is the single segmented tarsus on the legs with only one claw at the tip. The group is extremely

sexually dimorphic Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most ani ...

; female scale insects, unusually for Hemiptera, retain the immature external morphology even when sexually mature, a condition known as

neoteny Neoteny (), also called juvenilization,Montagu, A. (1989). Growing Young. Bergin & Garvey: CT. is the delaying or slowing of the physiological, or somatic, development of an organism, typically an animal. Neoteny is found in modern humans compa ...

. 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 A simple eye (sometimes called a pigment pit) refers to a form of eye or an optical arrangement composed of a single lens and without an elaborate retina such as occurs in most vertebrates. In this sense "simple eye" is distinct from a multi-l ...

(simple eyes) are sometimes present in

Margarodidae The Margarodidae (illegitimately as Margodidae) or ground pearls are a family of scale insects within the superfamily Coccoidea. Members of the family include the Polish cochineal and Armenian cochineal (genus ''Porphyrophora'') and the original ...

Ortheziidae Ortheziidae is a family of scale insects commonly known as the ensign scales or ortheziids. They occur in most parts of the world but the majority of the species are found in the Neotropical and Nearctic regions while there are not many species i ...

and

Phenacoleachiidae Phenacoleachiidae is a family of scale insects commonly known as the phenacoleachiids. They are found only in the South Island of New Zealand, and on certain offshore islands. There are two species in a single genus.Beesoniidae Beesoniidae is a family of scale insects commonly known as beesoniids. They typically cause galls on their plant hosts. Members of this family mostly come from southern Asia.antennae, but other families possess antennae with from one to thirteen segments. The mouthparts are adapted for piercing and sucking. Adult males in contrast have the typical head, thorax and abdomen of other insect groups, and are so different from females that pairing them as a species is challenging. They are usually slender insects resembling

aphid Aphids are small sap-sucking insects and members of the superfamily Aphidoidea. Common names include greenfly and blackfly, although individuals within a species can vary widely in color. The group includes the fluffy white woolly aphids. A t ...

s or small

flies Flies are insects of the order Diptera, the name being derived from the Greek δι- ''di-'' "two", and πτερόν ''pteron'' "wing". Insects of this order use only a single pair of wings to fly, the hindwings having evolved into advanced m ...

. They have antennae with nine or ten segments, compound eyes (Margarodidae and Ortheziidae) or simple eyes (most other families), and legs with five segments. Most species have wings, and in some, generations may alternate between being winged and wingless. Adult males do not feed, and die within two or three days of emergence. 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 Flies are insects of the order Diptera, the name being derived from the Greek δι- ''di-'' "two", and πτερόν ''pteron'' "wing". Insects of this order use only a single pair of wings to fly, the hindwings having evolved into advanced ...

, the Diptera. However, the Diptera and Hemiptera are not closely related, and do not closely resemble each other in

morphology Morphology, from the Greek and meaning "study of shape", may refer to: Disciplines * Morphology (archaeology), study of the shapes or forms of artifacts * Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies ...

; 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 A hamus or hamulus is a structure functioning as, or in the form of, hooks or hooklets. Etymology The terms are directly from Latin, in which ''hamus'' means "hook". The plural is ''hami''. ''Hamulus'' is the diminutive – hooklet or little h ...

, hooklets, that couple the hind wings to the main wings, as in the

Hymenoptera Hymenoptera is a large order (biology), order of insects, comprising the sawfly, sawflies, wasps, bees, and ants. Over 150,000 living species of Hymenoptera have been described, in addition to over 2,000 extinct ones. Many of the species are Par ...

. The

vestigial Vestigiality is the retention, during the process of evolution, of genetically determined structures or attributes that have lost some or all of the ancestral function in a given species. Assessment of the vestigiality must generally rely on co ...

wings are often reduced to pseudo-

halteres ''Halteres'' (; singular ''halter'' or ''haltere'') (from grc, ἁλτῆρες, weights held in the hands to give an impetus in leaping) are a pair of small club-shaped organs on the body of two orders of flying insects that provide infor ...

, club-like appendages, but these are not homologous with the control organs of Diptera, and it is not clear whether they have any substantial control function.

Hermaphroditism In reproductive biology, a hermaphrodite () is an organism that has both kinds of reproductive organs and can produce both gametes associated with male and female sexes. Many taxonomic groups of animals (mostly invertebrates) do not have s ...

is very rare in insects, but several species of ''

Icerya ''Icerya'' is a genus of scale insects in the family Monophlebidae. It is named after physician-naturalist Dr. Edmond Icery of British Mauritius. Hermaphroditism Hermaphroditism In reproductive biology, a hermaphrodite () is an organism ...

'' 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

Female scale insects in more advanced families develop from the egg through a first

instar An instar (, from the Latin '' īnstar'', "form", "likeness") is a developmental stage of arthropods, such as insects, between each moult (''ecdysis''), until sexual maturity is reached. Arthropods must shed the exoskeleton in order to grow or ...

(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 Holometabolism, also called complete metamorphosis, is a form of insect development which includes four life stages: egg, larva, pupa, and imago (or adult). Holometabolism is a synapomorphic trait of all insects in the superorder Endopterygota. ...

insects have a true pupa). The first instars of most species of scale insects emerge from the egg with functional legs, and are informally called "crawlers". They immediately crawl around in search of a suitable spot to settle down and feed. In some species they delay settling down either until they are starving, or until they have been blown away by wind onto what presumably is another plant, where they may establish a new colony. There are many variations on such themes, such as scale insects that are associated with species of ants that act as herders and carry the young ones to protected sites to feed. In either case, many such species of crawlers, when they moult, lose the use of their legs if they are female, and stay put for life. Only the males retain legs, and in some species wings, and use them in seeking females. To do this they usually walk, as their ability to fly is limited, but they may get carried to new locations by the wind. EB1911 Hemiptera - Fig

Adult females of the families Margarodidae, Ortheziidae and Pseudococcidae are mobile and can move to other parts of the host plant or even adjoining plants, but the mobile period is limited to a short period between moults. Some of these overwinter in crevices in the bark or among plant litter, moving in spring to tender young growth. However, the majority of female scale insects are sedentary as adults. Their dispersal ability depends on how far a crawler can crawl before it needs to shed its skin and start feeding. There are various strategies for dealing with deciduous trees. On these, males often feed on the leaves, usually beside the veins, while females select the twigs. Where there are several generations in the year, there may be a general retreat onto the twigs as fall approaches. On branches, the underside is usually preferred as giving protection against predation and adverse weather. The solenopsis mealybug feeds on the foliage of its host in summer and the roots in winter, and large numbers of scale species feed invisibly, year-round on roots.

Reproduction and the genetics of sex determination

Scale insects show a very wide range of variations in the genetics of sex determination and the modes of reproduction. Besides

sexual reproduction Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete ( haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote tha ...

, a number of different forms of reproductive systems are employed, including

. In some species, sexual and asexual populations are found in different locations, and in general, species with a wide geographic range and a diversity of plant hosts are more likely to be asexual. Large population size is hypothesized to protect an asexual population from becoming extinct, but nevertheless, parthenogenesis is uncommon among scale insects, with the most widespread generalist feeders reproducing sexually, the majority of these being pest species. Drosicha_Lefroy

Many species have the XX-XO system where the female is

diploid Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectively ...

and homogametic while the male is

heterogametic Heterogametic sex (digametic sex) refers to the individuals of a species in which the sex chromosomes are not the same. For example, in humans, males with an X and a Y sex chromosome would be referred to as the heterogametic sex, and females ...

and missing a sex chromosome. In some

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 ...

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 ''Kerriidae'' is a family of scale insects, commonly known as lac insects or lac scales. Some members of the genera ''Metatachardia'', ''Tachardiella'', ''Austrotacharidia'', ''Afrotachardina'', ''Tachardina'', and ''Kerria'' are raised for comme ...

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 ''

''. 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 Intragenomic conflict refers to the evolutionary phenomenon where genes have phenotypic effects that promote their own transmission in detriment of the transmission of other genes that reside in the same genome. The selfish gene theory postulates ...

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 Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of th ...

, the period in which

angiosperm Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek words ('container, vessel') and ('seed'), and refers to those plants th ...

s came to dominance among plants, with only a few groups species found on

gymnosperm The gymnosperms ( lit. revealed seeds) are a group of seed-producing plants that includes conifers, cycads, ''Ginkgo'', and gnetophytes, forming the clade Gymnospermae. The term ''gymnosperm'' comes from the composite word in el, γυμνό� ...

s. 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). The parasite biologist Robert Poulin notes that the feeding behaviour of scale insects closely resembles that of ectoparasites, living on the outside of their host and feeding only on them, even if they have not traditionally been so described; in his view, those species that remain immobile on a single host and feed only on it behave as obligate ectoparasites. For example, cochineal species are restricted to cactus hosts, and the gall-inducing '' Apiomorpha'' are restricted to ''

Eucalyptus ''Eucalyptus'' () is a genus of over seven hundred species of flowering trees, shrubs or mallees in the myrtle family, Myrtaceae. Along with several other genera in the tribe Eucalypteae, including '' Corymbia'', they are commonly known as euca ...

''. 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'', the boreal ensign scale, is a species of scale insect in the family Ortheziidae. It is native to Europe and is found in the soil and amongst plant litter and mosses on the ground. Description The adult female is about ...

(''Newsteadia floccosa'') inhabits

plant litter Plant litter (also leaf litter, tree litter, soil litter, litterfall or duff) is dead plant material (such as leaves, bark, needles, twigs, and cladodes) that have fallen to the ground. This detritus or dead organic material and its constitue ...

. A Hawaiian mealybug ''

Clavicoccus erinaceus ''Clavicoccus erinaceus'' is an extinct species of mealybug in the family Pseudococcidae. It was endemic to Oʻahu Oahu () ( Hawaiian: ''Oʻahu'' ()), also known as "The Gathering Place", is the third-largest of the Hawaiian Islands. It is ...

'' that fed solely on the now critically endangered ''

Abutilon sandwicense ''Abutilon sandwicense'', commonly known as the greenflower Indian mallow, is a species of flowering plant in the mallow family, Malvaceae, that is endemic to the island of Oahu, Hawaii, in the United States. It inhabits dry forests on the slop ...

'' has gone extinct as has another species ''

Phyllococcus oahuensis ''Phyllococcus oahuensis'' was a species of mealybug in the family Pseudococcidae Mealybugs are insects in the family Pseudococcidae, unarmored scale insects found in moist, warm habitats. Many species are considered pests as they feed on pl ...

''. Several other monophagous scale insects, especially those on islands, are threatened by

coextinction Coextinction and cothreatened refer to the phenomena of the loss or decline of a host species resulting in the loss or endangerment of an other species that depends on it, potentially leading to cascading effects across trophic levels. The term or ...

due to threats faced by their host plants. Most scale insects are

s, feeding on

phloem Phloem (, ) is the living biological tissue, tissue in vascular plants that transports the soluble organic compounds made during photosynthesis and known as ''photosynthates'', in particular the sugar sucrose, to the rest of the plant. This tran ...

drawn directly from the plant's vascular system, but a few species feed on fungal mats and

fungi A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...

, such as some species in the genus ''

Newsteadia ''Newsteadia'' is a genus of ensign scale insects in the superfamily Coccoidea. Most species are inconspicuous, measuring under two millimeters long and found in leaf litter. Species In 1962, J. M. Hoy stated there were 11 species in the genus. ...

'' 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 Sooty mold (also spelled sooty mould) is a collective term for different Ascomycete fungi, which includes many genera, commonly ''Cladosporium'' and ''Alternaria''. It grows on plants and their fruit, but also environmental objects, like fences, ...

can grow. The mould can reduce

photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...

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 and mealy bugs

Scale insect 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 ''Liriodendron'' () is a genus of two species of characteristically large trees, deciduous over most of their populations, in the magnolia family (Magnoliaceae). These trees are widely known by the common name tulip tree or tuliptree for their ...

, 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. This provides a means for the mealybug to be dispersed widely. Species of '' Hippeococcus'' have long clinging legs with claws to grip the ''

Dolichoderus ''Dolichoderus'' is a genus of ants found worldwide. Taxonomy The ants of the Neotropical genus ''Monacis'' were revised in 1959 by Kempf. However, Brown in 1973 and G. C. Wheeler and J. Wheeler in 1973 and 1976 considered both ''Monacis'' and ' ...

'' ants which tend them; they allow themselves to be carried into the ant colony. Here the mealybugs are safe from predation and environmental hazards, while the ants have a source of nourishment. Another species of ant maintains a herd of scale insects inside the hollow stems of a ''

Barteria The genus ''Barteria'' is in the family Passifloraceae in the major group Angiosperms (Flowering plants). It contains 8 described species, however, only 6 are accepted. They are native to Tropical Africa and found in the countries of Angola, Beni ...

'' tree; the scale insects feed on the sap and the ants, while benefiting from the honeydew, drive away other herbivorous insects from the tree as well as preventing vines from smothering it. Ladybug(india)

Scale insects have various natural enemies, and research in this field is largely directed at the species that are crop pests.

Entomopathogenic fungi An entomopathogenic fungus is a fungus that can kill or seriously disable insects. Typical life cycle These fungi usually attach to the external body surface of insects in the form of microscopic spores (usually asexual, mitosporic spores also ...

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 ''Septobasidium'' is a fungal genus within the family Septobasidiaceae. Approximately 175 described species are associated with this genus. ''Septobasidium'' species are known to be entomopathogens. Description ''Septobasidium'' spp. are charac ...

'' 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 wasp Parasitoid wasps are a large group of hymenopteran superfamilies, with all but the wood wasps (Orussoidea) being in the wasp-waisted Apocrita. As parasitoids, they lay their eggs on or in the bodies of other arthropods, sooner or later causin ...

s, mostly in the families

Encyrtidae Encyrtidae is a large family of parasitic wasps, with some 3710 described species in about 455 genera. The larvae of the majority are primary parasitoids on Hemiptera, though other hosts are attacked, and details of the life history can be variab ...

and

Eulophidae The Eulophidae are a large family of hymenopteran insects, with over 4,300 described species in some 300 genera. The family includes the genus '' Elasmus'', which used to be treated as a separate family, "Elasmidae", and is now treated as a s ...

, and

predatory Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill th ...

beetles such as fungus weevils,

ladybirds Coccinellidae () is a widespread family of small beetles ranging in size from . They are commonly known as ladybugs in North America and ladybirds in Great Britain. Some entomologists prefer the names ladybird beetles or lady beetles as they ...

and

sap beetle The sap beetles, also known as Nitidulidae, are a family of beetles. They are small (2–6 mm) ovoid, usually dull-coloured beetles, with knobbed antennae. Some have red or yellow spots or bands. They feed mainly on decaying vegetable ma ...

s. Ladybirds feed on aphids and scale insects, laying their eggs near their prey to ensure their larvae have immediate access to food. The ladybird ''

Cryptolaemus montrouzieri ''Cryptolaemus montrouzieri'', common name mealybug ladybird or mealybug destroyer, is a ladybird species. Etymology Étienne Mulsant described ''C. montrouzieri'', naming the new species after a Marist brother and missionary, Abbe Montrouzi ...

'' is known as the "mealybug destroyer" because both adults and larvae feed on mealybugs and some soft scales. 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 A quarantine is a restriction on the movement of people, animals and goods which is intended to prevent the spread of disease or pests. It is often used in connection to disease and illness, preventing the movement of those who may have been ...

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 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 ...

stage known as the ''crawler''. However, scales can often be controlled using

horticultural oil Horticultural oils or narrow range oils are lightweight oils, either petroleum or vegetable based. They are used in both horticulture and agriculture, where they are applied as a dilute spray on plant surfaces to control insects and mites. They are ...

s that suffocate them, systemic pesticides that poison the sap of the host plants, or by

agents such as tiny

parasitoid In evolutionary ecology, a parasitoid is an organism that lives in close association with its host (biology), host at the host's expense, eventually resulting in the death of the host. Parasitoidism is one of six major evolutionarily stable str ...

wasps and ladybirds.

Insecticidal soap Insecticidal soap is used to control many plant insect pests. Soap has been used for more than 200 years as an insect control. Because insecticidal soap works on direct contact with pests via the disruption of cell membranes when the insect is pen ...

may also be used against scales. One species, the cottony cushion scale, is a serious commercial pest on 65 families of woody plants, including ''

'' fruits. It has spread worldwide from Australia. File:Scale insect.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 Parasitoid wasps are a large group of hymenopteran superfamilies, with all but the wood wasps (Orussoidea) being in the wasp-waisted Apocrita. As parasitoids, they lay their eggs on or in the bodies of other arthropods, sooner or later causi ...

'' Anagyrus lopezi'', a highly effective biological control of the

cassava ''Manihot esculenta'', common name, commonly called cassava (), manioc, or yuca (among numerous regional names), is a woody shrub of the spurge family, Euphorbiaceae, native to South America. Although a perennial plant, cassava is extensively ...

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 Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...

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 The cochineal ( , ; ''Dactylopius coccus'') is a scale insect in the suborder Sternorrhyncha, from which the natural dye carmine is derived. A primarily sessile parasite native to tropical and subtropical South America through North Americ ...

, kermes,

lac Lac is the resinous secretion of a number of species of lac insects, of which the most commonly cultivated is ''Kerria lacca''. Cultivation begins when a farmer gets a stick that contains eggs ready to hatch and ties it to the tree to be infes ...

, 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 ''Ceroplastes'' is a genus of wax scales in the family Coccidae. There are more than 130 described species in ''Ceroplastes''. See also * List of Ceroplastes species, List of ''Ceroplastes'' species References External links

* Cocci ...

'' and '' Ericerus'' produce materials such as Chinese wax, and several genera of lac scales produce

. File:Indian collecting cochineal.jpg, Collecting scale insects from a prickly pear for a

dyestuff A dye is a colored substance that chemically bonds to the substrate to which it is being applied. This distinguishes dyes from pigments which do not chemically bind to the material they color. Dye is generally applied in an aqueous solution and ...

, 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 ''Kerria lacca'' is a species of insect in the family Kerriidae, the lac insects. These are in the superfamily Coccoidea, the scale insects. This species is perhaps the most commercially important lac insect, being a main source of lac, a resin ...

'' 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

. The

phylogeny A phylogenetic tree (also phylogeny or evolutionary tree Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA.) is a branching diagram or a tree showing the evolutionary relationships among various biological spe ...

of the extant groups, inferred from analysis of small subunit (18S)

ribosomal RNA Ribosomal ribonucleic acid (rRNA) is a type of non-coding RNA which is the primary component of ribosomes, essential to all cells. rRNA is a ribozyme which carries out protein synthesis in ribosomes. Ribosomal RNA is transcribed from ribosom ...

, is shown in the first

cladogram A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to d ...

Phylogenetic diversification within the Coccomorpha has been analysed by the taxonomist Isabelle Vea and the entomologist David Grimaldi in 2016, combining DNA (3 gene regions) and 174 morphological characters (to allow

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

evidence to be incorporated). They showed 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. The Coccomorpha appeared at the start of the

period, some 245 mya; the neococcoids some 185 mya. Scale insects are very well represented in the fossil record, being abundantly preserved in

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. 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 ''Burmacoccus'' is an extinct genus of scale insect in the extinct monotypic family Burmacoccidae, containing a single species, ''Burmacoccus danyi''. The genus is solely known from the Albian – Cenomanian Burmese amber deposits. History a ...

** Callipappidae ** Coelostomidiidae ** Electrococcidae ** Grimaldiellidae ** Grohnidae ** Hammanococcidae ** Jersicoccidae ** Kozariidae ** Kukaspididae ** Kuwaniidae ** Labiococcidae ** Lebanococcidae ** Lithuanicoccidae ** Macrodrilidae ** Marchalinidae **

** Matsucoccidae ** Monophlebidae **

** Pennygullaniidae **

Phenacoleachiidae Phenacoleachiidae is a family of scale insects commonly known as the phenacoleachiids. They are found only in the South Island of New Zealand, and on certain offshore islands. There are two species in a single genus.Pityococcidae ** Putoidae ** Serafinidae ** Steingeliidae ** Stigmacoccidae ** Termitococcidae ** Weitschatidae ** Xylococcidae * Neococcoidea ** Aclerdidae **

Albicoccidae ''Albicoccus'' is an extinct genus of scale insect in the extinct monotypic family Albicoccidae, containing a single species, ''Albicoccus dimai''. The genus is solely known from the Albian - Cenomanian Burmese amber deposits. History and class ...

** Asterolecaniidae **

Beesoniidae Beesoniidae is a family of scale insects commonly known as beesoniids. They typically cause galls on their plant hosts. Members of this family mostly come from southern Asia.Calycicoccidae ** Carayonemidae **

Cerococcidae Cerococcidae is a family of scale insects commonly known as ornate pit scales or cerococcids. There are 83 species in 5 genera. Members of this family occur in all regions of the world. Description Adult females produce a protective waxy test ...

** Cissococcidae **

Coccidae The Coccidae are a family of scale insects belonging to the superfamily Coccoidea. They are commonly known as soft scales, wax scales or tortoise scales. The females are flat with elongated oval bodies and a smooth integument which may be cover ...

** Conchaspididae ** Cryptococcidae **

Dactylopiidae ''Dactylopius'' is a genus of insect in the superfamily Coccoidea, the scale insects. It is the only genus in the family Dactylopiidae.Van Dam, A. R. and B. May. (2012)A new species of ''Dactylopius'' Costa (''Dactylopius gracilipilus'' sp. nov. ...

** Eriococcoidae ** Halimococcidae ** Hodgsonicoccidae ** Inkaidae **

Kermesidae The Kermesidae are a family of scale insects belonging to the superfamily Coccoidea. The type genus, '' Kermes'', includes the kermes scale insects, from which a red dye, also called ''kermes'' (a.k.a. crimson Crimson is a rich, deep red ...

** Kerridae ** Lecanodiaspididae ** Micrococcidae ** Phoenicococcidae ** Porphyrophoridae ** Pseudococcidae ** Rhizoecidae ** Stictococcidae ** Tachardiidae

References

External links

ScaleNet homepage

* ttp://nlbif.eti.uva.nl/bis/diaspididae.php Diaspididae of the World
Scale Insect Forum

On the

University of Florida The University of Florida (Florida or UF) is a public land-grant research university in Gainesville, Florida. It is a senior member of the State University System of Florida, traces its origins to 1853, and has operated continuously on its ...

/ Institute of Food and Agricultural Sciences ''Featured Creatures'' website:
''Ceroplastes rubens'', red wax scale

{{DEFAULTSORT:Scale Insect Sternorrhyncha Agricultural pest insects Insect pests of ornamental plants Insect pests of tropical forests Extant Triassic first appearances