Scale insects are smallinsects of theorderHemiptera, suborderSternorrhyncha. Of dramatically variable appearance and extremesexual dimorphism, they comprise the infraorderCoccomorpha which is considered a more convenient grouping than the superfamilyCoccoidea 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 areherbivores, piercing plant tissues with their mouthparts and remaining in one place, feeding onsap. The excess fluid they imbibe is secreted ashoneydew on which sooty mold tends to grow. The insects often have amutualistic relationship withants, which feed on the honeydew and protect them frompredators. There are about 8,000 described species.
The oldest fossils of the group date to theLate Jurassic, preserved inamber. They were already substantially diversified by theEarly Cretaceous suggesting an earlier origin during theTriassic orJurassic. Their closest relatives are thejumping plant lice,whiteflies,phylloxera bugs andaphids. 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 ofasexual reproduction byparthenogenesis.
Some scale insects are serious commercial pests, notably the cottony cushion scale (Icerya purchasi) onCitrus fruit trees; they are difficult to control as the scale and waxy covering protect them effectively from contact insecticides. Some species are used forbiological control of pest plants such as the prickly pear,Opuntia. Others produce commercially valuable substances includingcarmine andkermes dyes, andshellac lacquer. The two red colour-namescrimson andscarlet both derive from the names ofKermes products in other languages.
Armoured scale insects:(A)Lepidosaphes gloverii, adult females. (B)Parlatoria oleae, adult females (circular, with dark spot) and immatures (oblong). (C)Diaspidiotus juglansregiae, adult female walnut scale with waxy scale cover removed.
Scale insects vary dramatically in appearance, from very small organisms (1–2 mm) that grow beneathwax 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 frommealybugs) and permanently attached to the plant on which they are feeding. They secrete a waxy coating for defence, making them resemblereptilian or fish scales, and giving them their common name.[2] 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.[3]
The group is extremelysexually dimorphic; female scale insects, unusual for Hemiptera, retain the immature external morphology even when sexually mature, a condition known asneoteny. 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, butocelli (simple eyes) are sometimes present inMargarodidae,Ortheziidae andPhenacoleachiidae. The familyBeesoniidae lacksantennae, but other families possess antennae with from one to 13 segments. The mouthparts are adapted for piercing and sucking.[2]
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 resemblingaphids or smallflies. 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.[2]
In species with winged males, generally only the forewings are fully functional. This is unusual among insects; it most closely resembles the situation in thetrue flies, the Diptera. However, the Diptera and Hemiptera are not closely related, and do not closely resemble each other inmorphology; 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 havehamuli, hooklets, that couple the hind wings to the main wings, as in theHymenoptera. Thevestigial wings are often reduced to pseudo-halteres,[a] club-like appendages, but these are nothomologous with the control organs of Diptera that are called halteres, and it is not clear whether they have any substantial control function.[5]
Hermaphroditism is very rare in insects, but several species ofIcerya 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 thecottony cushion scale which has spread around the world.[6]
Life-cycle of theapple 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 firstinstar (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 onlyholometabolous insects have a true pupa).[2]
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.[2]
Apple scale. a) male, with legs and wings b) foot of male c) larva, x20 d) antenna of larva e) immobile female (removed from scale)
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. Thesolenopsis 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.[2]
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. Besidessexual reproduction, a number of different forms of reproductive systems are employed, includingasexual reproduction byparthenogenesis. 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.[7]
A winged maleDrosicha sp
Many species have the XX-XO system where the female isdiploid and homogametic while the male isheterogametic and missing a sex chromosome. In someDiaspididae andPseudococcidae, 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 someEriococcidae. In the other variant orComstockiella 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 genusIcerya. InParthenolecanium, 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.[8]
Scale insects are an ancient group, having originated in theCretaceous, the period in whichangiosperms came to dominance among plants, with only a few groups species found ongymnosperms. 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).[2] The parasite biologistRobert Poulin notes that the feeding behaviour of scale insects closely resembles that ofectoparasites, 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.[9] For example,cochineal species are restricted to cactus hosts, and the gall-inducingApiomorpha are restricted toEucalyptus. Some species have certain habitat requirements; some Ortheziidae occur in damp meadows, among mosses and in woodland soil, and theboreal ensign scale (Newsteadia floccosa) inhabitsplant litter.[2] A Hawaiian mealybugClavicoccus erinaceus that fed solely on the now critically endangeredAbutilon sandwicense has gone extinct as has another speciesPhyllococcus oahuensis.[10] Several other monophagous scale insects, especially those on islands, are threatened bycoextinction due to threats faced by their host plants.[11]
Most scale insects areherbivores, feeding onphloemsap drawn directly from the plant's vascular system, but a few species feed on fungal mats andfungi, such as some species in the genusNewsteadia 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.[12] 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 asubstrate on whichsooty mould can grow. The mould can reducephotosynthesis 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.[13]
Scale insects in the genusCryptostigma live inside the nests of neotropical ant species.[14] Many tropical plants need ants to survive which in turn cultivate scale insects thus forming atripartite symbiosis.[15] Some ants and scale insects have amutualistic relationship; the ants feed on the honeydew and in return protect the scales. On atulip 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 antAcropyga 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.[2] This provides a means for the mealybug to be dispersed widely. Species ofHippeococcus have long clinging legs with claws to grip theDolichoderus 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.[2] Another species of ant maintains a herd of scale insects inside the hollow stems of aBarteria 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.[16]
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.[17] Fungi in the genusSeptobasidium 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.[18]
Natural enemies includeparasitoid wasps, mostly in the familiesEncyrtidae andEulophidae, andpredatory beetles such asfungus weevils,ladybirds andsap beetles.[2] Ladybirds feed on aphids and scale insects, laying their eggs near their prey to ensure their larvae have immediate access to food. The ladybirdCryptolaemus montrouzieri is known as the "mealybug destroyer" because both adults and larvae feed on mealybugs and some soft scales.[19] 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.[2]
Many scale species are serious croppests and are particularly problematic for their ability to evadequarantine measures.[20][21] In 1990, they caused around $5 billion of damage to crops in the United States.[22]The waxy covering of many species of scale protects their adults effectively from contactinsecticides, which are only effective against the first-instarnymph stage known as thecrawler. However, scales can often be controlled usinghorticultural oils thatsuffocate them, systemic pesticides that poison the sap of the host plants, or bybiological control agents such as tinyparasitoid wasps and ladybirds.Insecticidal soap may also be used against scales.[23]
One species, the cottony cushion scale, is a serious commercial pest on 65 families of woody plants, includingCitrus fruits. It has spread worldwide from Australia.[24][25]
Adult femalecottony cushion scale (Icerya purchasi) with young crawlers. The species is a major commercial pest of crops such asCitrus fruits.
At the same time, some kinds of scale insects are themselves useful as biological control agents for pest plants, such as various species ofcochineal insects that attackinvasive species ofprickly pear, which spread widely especially inAustralia and Africa.[26][27]
Some types of scale insect are economically valuable for the substances they can yield under proper husbandry. Some, such as thecochineal,kermes,lac,Armenian cochineal, andPolish cochineal, have been used to produce red dyes for coloring foods and dyeing fabrics.[28][29][30] Both the colour name "crimson" and the generic nameKermes are from Italiancarmesi orcremesi for the dye used for Italian silk textiles, in turn from the Persian[31]qirmizī (قرمز), meaning both the colour and the insect.[32] The colour name "scarlet" is similarly derived from Arabicsiklāt, denoting extremely expensive luxury silks dyed red using kermes.[33]
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.[36] Scale insects are members of theSternorrhyncha. Thephylogeny of the extant Sternorrhyncha, from a 2024 study using ultraconserved genetic elements, is shown in the cladogram:[37]
The timing of phylogenetic diversification within the Coccomorpha was estimated in a 2016 study based onmolecular 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 theTriassic period, around 245 million years ago, and that the neococcoids appeared during the Early Jurassic, some 185 million years ago.[39] Scale insects are very well represented in the fossil record, with the oldest known member of the group reported from theLate Jurassicamber from Lebanon.[40] 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 theEriococcidae. 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.[41] In the cladogram below the genusPityococcus is moved to the "Neococcoids". A cladogram showing the major families using this methodology is shown below.[39]
Recognition of scale insect families has fluctuated over time, and the validity of many remains in flux,[42][43] with several recognized families not included in the phylogeny presented above including extinct groups are listed below:[44][45][46]
^Szwedo, J. (2018). "The unity, diversity and conformity of bugs (Hemiptera) through time".Earth and Environmental Science Transactions of the Royal Society of Edinburgh.107 (2–3):109–128.doi:10.1017/s175569101700038x.S2CID134243346.
^Ben-Dov, Y.; Miller, D. R.; Gibson, G.A.P."Home".ScaleNet. Archived fromthe original on 2013-04-04. Retrieved2013-04-04.
^Poinar, G. O. Jr (2020). "Macrodrilidae fam. nov. (Hemiptera: Sternorrhyncha: Coccoidea), a new family of scale insects in mid-Cretaceous Burmese amber".Historical Biology: An International Journal of Paleobiology.33 (9):1726–1730.doi:10.1080/08912963.2020.1733549.S2CID216480240.
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