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==Ecology== [[File:Scale Insects (2127992762).jpg|thumb|upright|A cluster of scale insects on a stem]] Scale insects are an ancient group, having originated in the [[Cretaceous]], the period in which [[angiosperm]]s came to dominance among plants, with only a few groups species found on [[gymnosperm]]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).<ref name=Capinera/> The parasite biologist [[Robert Poulin (zoologist)|Robert Poulin]] notes that the feeding behaviour of scale insects closely resembles that of [[ectoparasite]]s, 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.<ref name=Rollinson>{{cite book |last=Poulin |first=Robert |author-link=Robert Poulin (zoologist) |editor1=Rollinson, D. |editor2=Hay, S. I. |title=The Many Roads to Parasitism: A Tale of Convergence |chapter=The Many Roads to Parasitism |series=Advances in Parasitology |chapter-url=https://books.google.com/books?id=9y4AlXka7t0C&pg=PA28 |year=2011 |volume=74 |publisher=Academic Press |isbn=978-0-12-385897-9 |pages=27β28 |doi=10.1016/B978-0-12-385897-9.00001-X |pmid=21295676 }}</ref> For example, [[Dactylopiidae|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]].<ref name=Capinera/> A Hawaiian mealybug ''[[Clavicoccus erinaceus]]'' that fed solely on the now critically endangered ''[[Abutilon sandwicense]]'' has gone extinct as has another species ''[[Phyllococcus|Phyllococcus oahuensis]]''.<ref>{{Cite journal |last1=Moir |first1=Melinda L. |last2=Hughes |first2=Lesley|last3=Vesk |first3=Peter A. |last4=Leng |first4=Mei Chen |date=2014 |title=Which host-dependent insects are most prone to coextinction under changed climates? |journal=Ecology and Evolution |volume=4 |issue=8 |pages=1295β1312 |doi=10.1002/ece3.1021 |pmc=4020690 |pmid=24834327|bibcode=2014EcoEv...4.1295M }}</ref> Several other monophagous scale insects, especially those on islands, are threatened by [[coextinction]] due to threats faced by their host plants.<ref>{{Cite journal |last1=Thacker|first1=Jonathan I.|last2=Hopkins |first2=Graham W. |last3=Dixon |first3=Anthony F. G. |date=2006 |title=Aphids and scale insects on threatened trees: co-extinction is a minor threat|journal=Oryx |volume=40 |issue=2 |pages=233β236 |doi=10.1017/S0030605306000123 |doi-access=free}}</ref> Most scale insects are [[herbivore]]s, 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.<ref>{{Cite journal |last=Moran |first=Nancy A. |date=2001 |title=The coevolution of bacterial endosymbionts and phloem-feeding insects |journal=Annals of the Missouri Botanical Garden |volume=88 |issue=1 |pages=35β44 |doi=10.2307/2666130 |jstor=2666130 |url=https://www.biodiversitylibrary.org/part/38840 }}</ref> Scale insects secrete a large quantity of sticky viscid fluid known as "[[Honeydew (secretion)|honeydew]]". This includes sugars, amino acids and minerals, and is attractive to ants as well as acting as a [[Substrate (biology)|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.<ref>{{cite book |last1=Stauffer |first1=S. |last2=Rose |first2=M. |title=Soft Scale Insects |url=https://books.google.com/books?id=QjMzWX2hB1EC&pg=PA186 |year=1997 |publisher=Elsevier |isbn=978-0-08-054135-8 |pages=186β187}}</ref> [[File:Formica fusca and mealy bugs.jpg|thumb|upright|left|[[Mutualism (biology)|Mutualistic]] ''[[Formica fusca]]'' ants tending a herd of [[mealybugs]]]] Scale insects in the genus ''[[Cryptostigma]]'' live inside the nests of neotropical ant species.<ref>{{cite journal|doi=10.1590/S1519-566X2004000600009 |title=A new species of ant-tended soft scale of the genus ''Cryptostigma'' Ferris (Hemiptera: Coccidae) associated with bamboo in Peru |year=2004 |last1=Kondo |first1=Takumasa |last2=Gullan |first2=Penny J. |journal=Neotropical Entomology |volume=33 |issue=6 |pages=717β723 |doi-access=free }}</ref> Many tropical plants need ants to survive which in turn cultivate scale insects thus forming a [[tripartite symbiosis]].<ref>{{cite journal |doi=10.1098/rspb.2008.0573 |jstor=25249807 |title=An ancient tripartite symbiosis of plants, ants and scale insects |year=2008 |last1=Itino |first1=Takao |last2=Murase |first2=Kaori |last3=Sato |first3=Yumiko |last4=Inamori |first4=Keita |last5=Itioka |first5=Takao |last6=Quek |first6=Swee-Peck |last7=Ueda |first7=Shouhei |journal=Proceedings of the Royal Society B: Biological Sciences |volume=275 |issue=1649 |pmid=18611850 |pages=2319β26 |pmc=2603224}}</ref> Some ants and scale insects have a [[Mutualism (biology)|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.<ref name=Capinera/> This provides a means for the mealybug to be dispersed widely. Species of ''[[Hippeococcus]]'' have long clinging legs with claws to grip the ''[[Dolichoderus]]'' 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.<ref name=Capinera/> Another species of ant maintains a herd of scale insects inside the hollow stems of a ''[[Barteria]]'' 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.<ref>{{cite book |last1=HΓΆlldobler |first1=Bert |last2=Wilson |first2=Edward O. |author2-link=E. O. Wilson |title=The Ants |url=https://archive.org/details/ants0000hlld |url-access=registration |year=1990 |publisher=Harvard University Press |isbn=978-0-674-04075-5 |page=[https://archive.org/details/ants0000hlld/page/553 553]}}</ref> [[File:Ladybug(india).jpg|thumb|''[[Cheilomenes sexmaculata]]'' [[Predation|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 fungus|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.<ref>{{cite book |last1=Evans |first1=Harry C. |last2=Hywel-Jones |first2=Nigel L. |title=Soft Scale Insects |url=https://books.google.com/books?id=QjMzWX2hB1EC&pg=PA3 |year=1997 |publisher=Elsevier |isbn=978-0-08-054135-8 |pages=3β4}}</ref> 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.<ref>{{cite web |title=The genus ''Septobasidium'' |work=The genome portal of the Department of Energy Joint Genome Institute |url=https://genome.jgi.doe.gov/Sepsp1/Sepsp1.home.html |publisher=Fungal Genomics Resource |access-date=18 January 2020}}</ref> Natural enemies include [[parasitoid wasp]]s, mostly in the families [[Encyrtidae]] and [[Eulophidae]], and [[Predation|predatory]] beetles such as [[Anthribidae|fungus weevils]], [[Coccinellidae|ladybirds]] and [[sap beetle]]s.<ref name=Capinera/> 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.<ref>{{Cite web|url=http://www.entomology.wisc.edu/mbcn/kyf205.html|title=Know Your Friends - Mealybug Destroyer|date=14 February 2009|publisher=www.entomology.wisc.edu [[University of Wisconsin]]|access-date=16 January 2020|archive-url=https://web.archive.org/web/20120216004540/http://www.entomology.wisc.edu/mbcn/kyf205.html|archive-date=16 February 2012|url-status=dead}}</ref> 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.<ref name=Capinera/>
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