Scale insect
Scale insect Temporal range:
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Waxy scales on cycad leaf | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Arthropoda |
Class: | Insecta |
Order: | Hemiptera |
Suborder: | Sternorrhyncha |
Infraorder: | Coccomorpha Heslop-Harrison, 1952 |
Superfamily: | Coccoidea Handlirsch, 1903 [1] |
Families | |
Scale insects are small insects of the order Hemiptera, suborder Sternorrhyncha. Of dramatically variable appearance and extreme sexual dimorphism, 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 herbivores, piercing plant tissues with their mouthparts and remaining in one place, feeding on sap. The excess fluid they imbibe is secreted as honeydew on which sooty mold tends to grow. The insects often have a mutualistic relationship with ants, which feed on the honeydew and protect them from predators. There are about 8,000 described species.
The oldest fossils of the group date to the Early Cretaceous, preserved in amber. They were already substantially diversified by this time suggesting an earlier origin during the Triassic or Jurassic. Their closest relatives are the jumping plant lice, whiteflies, phylloxera bugs and aphids. 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 by parthenogenesis.
Some scale insects are serious commercial pests, notably the cottony cushion scale ( products in other languages.
Description
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.[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 extremely sexually dimorphic; female scale insects, unusually 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 thirteen 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 resembling aphids or small flies. 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 the
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.[5]
Life cycle
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).[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]
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.[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. Besides sexual reproduction, a number of different forms of reproductive systems are employed, including asexual reproduction by parthenogenesis. 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.[6]
Many species have the XX-XO system where the female is
Ecology
Scale insects are an ancient group, having originated in the
Most scale insects are
Scale insects in the genus
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.[16] 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.[17]
Natural enemies include parasitoid wasps, mostly in the families Encyrtidae and Eulophidae, and predatory beetles such as fungus weevils, ladybirds and sap 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 ladybird Cryptolaemus montrouzieri is known as the "mealybug destroyer" because both adults and larvae feed on mealybugs and some soft scales.[18] 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]
Significance
As pests
Many scale species are serious crop
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.[23][24]
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Adult femalecottony cushion scale(Icerya purchasi) with young crawlers. The species is a major commercial pest of crops such as Citrus fruits.
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The tinyparasitic wasp Anagyrus lopezi, a highly effective biological control of the cassavamealybug
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.[25][26]
Products
Some types of scale insect are economically valuable for the substances they can yield under proper husbandry. Some, such as the
Some waxy scale species in the genera
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Collecting scale insects from a prickly pear for adyestuff, cochineal, 1777
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Coronation cloak of King Roger II of Sicily, 1133. Silk scarlet cloth dyed with kermes, made from female Kermes scales
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Some varieties of shellac
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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.
Sternorrhyncha |
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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 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 Triassic period, some 245 mya; the neococcoids some 185 mya. Scale insects are very well represented in the fossil record, being 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.[37] In the cladogram below the genus Pityococcus is moved to the "Neococcoids". A cladogram showing the major families using this methodology is shown below.[38]
Coccomorpha |
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Recognition of scale insect families has fluctuated over time, and the validity of many remains in flux,[39][40] with several recognized families not included in the phylogeny presented above including extinct groups are listed below:[41][42][43]
- Archecoccoidea Borchsenius, 1958
- †Apticoccidae Vea & Grimaldi, 2015
- †Arnoldidae Koteja, 2008
- †BurmacoccidaeKoteja, 2004
- Callipappidae MacGillivray, 1921
- Coelostomidiidae Morrison, 1927
- †Electrococcidae Koteja, 2000
- †Grimaldiellidae Koteja, 2000
- †Grohnidae Koteja, 2008
- †Hammanococcidae Koteja & Azar, 2008
- †Jersicoccidae Koteja, 2000
- †Kozariidae Vea & Grimaldi, 2015
- †Kukaspididae Koteja & Poinar, 2001
- Kuwaniidae MacGillivray, 1921
- †Labiococcidae Koteja, 2000b
- †Lebanococcidae Koteja & Azar, 2008
- Lithuanicoccidae Koteja, 2008
- Macrodrilidae Poinar, 2020[44]
- Marchalinidae Morrison, 1927
- Margarodidae Cockerell, 1899
- Matsucoccidae Morrison, 1927
- Monophlebidae Morrison, 1927
- Ortheziidae Amyot & Audinet-Serville, 1843
- †Pennygullaniidae Koteja & Azar, 2008
- Phenacoleachiidae Cockerell, 1902
- Pityococcidae McKenzie, 1942
- Putoidae Tang, 1992
- †Serafinidae Koteja, 2008
- Steingeliidae Morrison, 1927
- StigmacoccidaeMorrison, 1927
- Termitococcidae Jakubski, 1965
- †Weitschatidae Koteja, 2008
- Xylococcidae Pergande in Hubbard & Pergande, 1898
- Neococcoidea Borchsenius, 1950
- Aclerdidae Cockerell, 1905
- †AlbicoccidaeKoteja, 2004
- Asterolecaniidae Cockerell, 1896
- Beesoniidae Ferris, 1950
- Calycicoccidae Brain, 1918
- Carayonemidae Richard, 1986
- Cerococcidae Balachowsky, 1942
- Cissococcidae Brain, 1918
- Coccidae Fallen, 1814
- Conchaspididae Green, 1896
- Cryptococcidae Kosztarab, 1968
- DactylopiidaeSignoret, 1875
- Diaspididae Targioni-Tozzetti, 1868
- Eriococcoidae Cockerell, 1899
- Halimococcidae Brown & McKenzie, 1962
- †Hodgsonicoccidae Vea & Grimaldi, 2015
- †Inkaidae Koteja, 1989
- Kermesidae Signoret, 1875
- Kerriidae Lindinger, 1937
- Lecanodiaspididae Targioni-Tozzetti, 1869
- Micrococcidae Silvestri, 1939
- Phoenicococcidae Stickney, 1934
- Porphyrophoridae Signoret, 1875
- PseudococcidaeCockerell, 1905
- Rhizoecidae Williams, 1969
- Stictococcidae Lindinger, 1913
- Tachardiidae Green, 1896
See also
References
- ^ "Coccoidea Handlirsch, 1903". Integrated Taxonomic Information System.
- ^ ISBN 978-1-4020-6242-1.
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- ISBN 978-1-4020-8644-1.
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- ISBN 978-0-08-054135-8.
- ^ "The genus Septobasidium". The genome portal of the Department of Energy Joint Genome Institute. Fungal Genomics Resource. Retrieved 18 January 2020.
- University of Wisconsin. 14 February 2009. Archived from the originalon 16 February 2012. Retrieved 16 January 2020.
- S2CID 84171305.
- ^ "Scale Insects". Iowa State University. Retrieved 14 January 2020.
- ISBN 978-0-313-38426-4.
- ^ "Scale insects". Gardeners' World. Retrieved 16 January 2020.
- ^ ScaleNet
- ISBN 9781139464857.
- S2CID 5816903. Archived from the original(PDF) on 2015-09-23.
- ^ "Opuntia ficus-indica (prickly pear)". CABI. 3 January 2018.
- ^ "Cochineal and Carmine". Major colourants and dyestuffs, mainly produced in horticultural systems. FAO. Retrieved June 16, 2015.
- ^ "Guidance for Industry: Cochineal Extract and Carmine". FDA. Retrieved 6 July 2016.
- )
- ^ ویکی, پارسی. "معنی قرمز | لغت نامه دهخدا". پارسی ویکی (in Persian). Retrieved 3 April 2021.
- ^ "Crimson (n.)". Etymology Online. Retrieved 17 January 2020.
- ISBN 978-9004124356.
- ISBN 978-0-521-18646-9.
- ^ "How Shellac Is Manufactured". The Mail (Adelaide, SA : 1912 – 1954). 18 Dec 1937.
- PMID 25283922.
- ^ Poinar, G.; Heiss, E. (2011). "New Termitaphididae and Aradidae (Hemiptera) in Mexican and Dominican amber" (PDF). Palaeodiversity. 4: 51–62.
- hdl:1885/63136.
- PMID 27000526.
- .
- .
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- ^ Ben-Dov, Y.; Miller, D. R.; Gibson, G.A.P. "Home". ScaleNet. Archived from the original on 2013-04-04. Retrieved 2013-04-04.
- S2CID 55067700.
- S2CID 216480240.
External links
- ScaleNet homepage
- Cottony cushion scale: the pest that launched a pest control revolution
- Diaspididae of the World
- Scale Insect Forum Archived 2020-09-30 at the Wayback Machine
- Scales of southeastern U.S. woody ornamentals
On the University of Florida / Institute of Food and Agricultural Sciences Featured Creatures website: