Editing Parasitism (section)

== Evolutionary ecology ==

[[File:Tyrannosaurus with infection.jpg|thumb|Restoration of a ''[[Tyrannosaurus]]'' with holes possibly caused by a ''[[Trichomonas]]''-like parasite]]

{{further|Evolutionary ecology}}

Parasitism is a major aspect of evolutionary ecology; for example, almost all free-living animals are host to at least one species of parasite. Vertebrates, the best-studied group, are hosts to between 75,000 and 300,000 species of helminths and an uncounted number of parasitic microorganisms. On average, a mammal species hosts four species of nematode, two of trematodes, and two of cestodes.<ref name="Dobson2008">{{cite journal |last1=Dobson |first1=A. |last2=Lafferty |first2=K. D. |last3=Kuris |first3=A. M. |last4=Hechinger |first4=R. F. |last5=Jetz |first5=W. |title=Homage to Linnaeus: How many parasites? How many hosts? |journal=Proceedings of the National Academy of Sciences |volume=105 |issue=Supplement 1 |date=2008 |doi=10.1073/pnas.0803232105 |pmid=18695218 |pages=11482–11489|pmc=2556407 |bibcode=2008PNAS..10511482D |doi-access=free }}</ref> Humans have 342 species of helminth parasites, and 70 species of protozoan parasites.<ref name="Sukhdeo2012"/> Some three-quarters of the links in [[food web]]s include a parasite, important in regulating host numbers. Perhaps 40 per cent of described species are parasitic.<ref name="Dobson2008"/>

=== Fossil record ===

{{further|Paleoparasitology}}

Parasitism is hard to demonstrate from the [[fossil record]], but holes in the [[mandible]]s of several specimens of ''[[Tyrannosaurus]]'' may have been caused by ''[[Trichomonas]]''-like parasites.<ref>{{cite journal |volume=4 |issue=9 |pages=e7288 |last=Wolff |first=Ewan D. S. |author2=Salisbury, Steven W. |author3=Horner, John R. |author4=Varrichio, David J. |title=Common Avian Infection Plagued the Tyrant Dinosaurs |journal=PLOS ONE |year=2009 |pmid=19789646 |pmc=2748709 |doi=10.1371/journal.pone.0007288|bibcode=2009PLoSO...4.7288W |doi-access=free }}</ref> ''[[Saurophthirus]]'', the Early Cretaceous [[flea]], parasitized [[Pterosauria|pterosaurs]].<ref>Ponomarenko, A.G. (1976) A new insect from the Cretaceous of Transbaikalia, a possible parasite of pterosaurians. ''Paleontological Journal''  10(3):339-343 (English) / ''Paleontologicheskii Zhurnal''  1976(3):102-106 (Russian)</ref><ref>{{Cite journal|last1=Zhang|first1=Yanjie|last2=Shih|first2=Chungkun|last3=Rasnitsyn|first3=Alexandr|last4=Ren|first4=Dong|last5=Gao|first5=Taiping|date=2020|title=A new flea from the Early Cretaceous of China|url=http://www.app.pan.pl/article/item/app006802019.html|journal=Acta Palaeontologica Polonica|volume=65|doi=10.4202/app.00680.2019|doi-access=free}}</ref> Eggs that belonged to [[nematode]] worms and probably [[protozoa]]n [[cyst]]s were found in the Late Triassic [[coprolite]] of [[phytosaur]]. This rare find in Thailand reveals more about the ecology of prehistoric parasites.<ref>{{cite journal|author=Thanit Nonsrirach, Serge Morand, Alexis Ribas, Sita Manitkoon, Komsorn Lauprasert, Julien Claude|date=9 August 2023|title=First discovery of parasite eggs in a vertebrate coprolite of the Late Triassic in Thailand|journal=PLOS ONE|volume=18 |issue=8 |pages=e0287891 |doi=10.1371/journal.pone.0287891|pmid=37556448|pmc=10411797 |bibcode=2023PLoSO..1887891N |doi-access=free }}</ref>

=== Coevolution ===

{{further|Host–parasite coevolution}}

As hosts and parasites evolve together, their relationships often change. When a parasite is in a sole relationship with a host, selection drives the relationship to become more benign, even mutualistic, as the parasite can reproduce for longer if its host lives longer.<ref name=Rook2007/> But where parasites are competing, selection favours the parasite that reproduces fastest, leading to increased virulence. There are thus varied possibilities in [[host–parasite coevolution]].<ref name=MasseyBuckling2004/>

[[Evolutionary epidemiology]] analyses how parasites spread and evolve, whereas [[Darwinian medicine]] applies similar evolutionary thinking to non-parasitic diseases like [[cancer]] and [[Autoimmune disease|autoimmune conditions]].<ref name="Ewald1994">{{cite book |last=Ewald |first=Paul W. |title=Evolution of Infectious Disease |url=https://archive.org/details/evolutionofinfec0000ewal|url-access=registration |year=1994 |publisher=Oxford University Press |isbn=978-0-19-534519-3 |page=8}}</ref>

==== Long-term partnerships favouring mutualism ====

[[File:Wolbachia.png|thumb|left|''[[Wolbachia]]'' bacteria within an insect cell]]

Long-term partnerships can lead to a relatively stable relationship tending to [[commensalism]] or [[Mutualism (biology)|mutualism]], as, all else being equal, it is in the evolutionary interest of the parasite that its host thrives. A parasite may evolve to become less harmful for its host or a host may evolve to cope with the unavoidable presence of a parasite—to the point that the parasite's absence causes the host harm. For example, although animals parasitised by [[helminth|worms]] are often clearly harmed, such infections may also reduce the prevalence and effects of [[Autoimmunity|autoimmune]] disorders in animal hosts, including humans.<ref name=Rook2007>{{cite journal |last=Rook |first=G. A. |url=https://www.academia.edu/580118 |title=The hygiene hypothesis and the increasing prevalence of chronic inflammatory disorders |journal=Transactions of the Royal Society of Tropical Medicine and Hygiene |year=2007 |volume=101 |issue=11 |pages=1072–1074 |pmid=17619029 |doi=10.1016/j.trstmh.2007.05.014}}</ref> In a more extreme example, some [[nematode]] worms cannot reproduce, or even survive, without infection by ''[[Wolbachia]]'' bacteria.<ref>{{Cite journal |last=Werren |first=John H. |date=February 2003 |title=Invasion of the Gender Benders: by manipulating sex and reproduction in their hosts, many parasites improve their own odds of survival and may shape the evolution of sex itself |journal=[[Natural History (magazine)|Natural History]] |volume=112 |issue=1 |page=58 |oclc=1759475 |url=http://findarticles.com/p/articles/mi_m1134/is_1_112/ai_97174198 |access-date=15 November 2008 |url-status=dead |archive-url=https://archive.today/20120708190307/http://findarticles.com/p/articles/mi_m1134/is_1_112/ai_97174198/ |archive-date=8 July 2012 }}</ref>

[[Lynn Margulis]] and others have argued, following [[Peter Kropotkin]]'s 1902 ''[[Mutual Aid: A Factor of Evolution]]'', that natural selection drives relationships from parasitism to mutualism when resources are limited. This process may have been involved in the [[symbiogenesis]] which formed the [[eukaryote]]s from an intracellular relationship between [[archaea]] and bacteria, though the sequence of events remains largely undefined.<ref>{{cite book |last1=Margulis |first1=Lynn |author1-link=Lynn Margulis |last2=Sagan |first2=Dorion |author2-link=Dorion Sagan |author3=Eldredge, Niles |author3-link=Niles Eldredge |year=1995 |url=https://books.google.com/books?id=4IIpAQAAMAAJ |title=What Is Life? |publisher=Simon and Schuster |isbn=978-0-684-81087-4}}</ref><ref>{{cite book |last1=Sarkar |first1=Sahotra |last2=Plutynski |first2=Anya |title=A Companion to the Philosophy of Biology |url=https://books.google.com/books?id=iN7UYNjbxsYC&pg=PA358 |year=2008 |publisher=John Wiley & Sons |isbn=978-0-470-69584-5 |page=358}}</ref>

==== Competition favouring virulence ====

Competition between parasites can be expected to favour faster reproducing and therefore more [[Virulence|virulent]] parasites, by [[natural selection]].<ref name=MasseyBuckling2004>{{cite journal |last1=Massey |first1=R. C. |last2=Buckling |first2=A. |last3=ffrench-Constant |first3=R.|author1-link=Ruth Massey |title=Interference competition and parasite virulence |journal=Proceedings of the Royal Society B: Biological Sciences |volume=271 |issue=1541 |year=2004 |pages=785–788 |doi=10.1098/rspb.2004.2676 |pmc=1691666 |pmid=15255095}}</ref><ref name=Rigaud2010>{{cite journal |last1=Rigaud |first1=T. |last2=Perrot-Minnot |first2=M.-J. |last3=Brown |first3=M. J. F. |title=Parasite and host assemblages: embracing the reality will improve our knowledge of parasite transmission and virulence |journal=Proceedings of the Royal Society B: Biological Sciences |volume=277 |issue=1701 |year=2010 |pages=3693–3702 |doi=10.1098/rspb.2010.1163|pmid=20667874 |pmc=2992712 }}</ref>

[[File:Pink Flamingos with Duck - Camargue, France - April 2007 (cropped).jpg|thumb|Biologists long suspected [[cospeciation]] of [[flamingo]]s and [[duck]]s with their parasitic [[louse|lice]], which were similar in the two families. Cospeciation did occur, but it led to flamingos and [[grebe]]s, with a later [[host switch]] of flamingo lice to ducks.]]

Among competing parasitic insect-killing bacteria of the genera ''[[Photorhabdus]]'' and ''[[Xenorhabdus]]'', virulence depended on the relative potency of the antimicrobial [[toxin]]s ([[bacteriocins]]) produced by the two strains involved. When only one bacterium could kill the other, the other strain was excluded by the competition. But when [[caterpillar]]s were infected with bacteria both of which had toxins able to kill the other strain, neither strain was excluded, and their virulence was less than when the insect was infected by a single strain.<ref name=MasseyBuckling2004/>

==== Cospeciation ====

A parasite sometimes undergoes [[cospeciation]] with its host, resulting in the pattern described in [[Fahrenholz's rule]], that the phylogenies of the host and parasite come to mirror each other.<ref name=Page>{{cite book |last=Page |first=Roderic D. M. |publisher=John Wiley |date=27 January 2006 |isbn=978-0-470-01617-6 |doi=10.1038/npg.els.0004124|title=Encyclopedia of Life Sciences |chapter=Cospeciation }}</ref>

An example is between the [[simian foamy virus]] (SFV) and its primate hosts. The phylogenies of SFV polymerase and the mitochondrial [[cytochrome c oxidase subunit II]] from African and Asian primates were found to be closely congruent in branching order and divergence times, implying that the simian foamy viruses cospeciated with Old World primates for at least 30&nbsp;million years.<ref name="SwitzerSalemi2005">{{cite journal |last1=Switzer |first1=William M. |last2=Salemi |first2=Marco |last3=Shanmugam |first3=Vedapuri |last4=Gao |first4=Feng |last5=Cong |first5=Mian-er |last6=Kuiken |first6=Carla |last7=Bhullar |first7=Vinod |last8=Beer |first8=Brigitte E. |last9=Vallet |first9=Dominique |last10=Gautier-Hion |first10=Annie |last11=Tooze |first11=Zena |last12=Villinger |first12=Francois |last13=Holmes |first13=Edward C. |last14=Heneine |first14=Walid |display-authors=3 |title=Ancient co-speciation of simian foamy viruses and primates |journal=Nature |volume=434 |issue=7031 |year=2005 |pages=376–380 |doi=10.1038/nature03341|pmid=15772660 |bibcode=2005Natur.434..376S |s2cid=4326578 |url=https://zenodo.org/record/1233279 }}</ref>

The presumption of a shared evolutionary history between parasites and hosts can help elucidate how host taxa are related. For instance, there has been a dispute about whether [[Phoenicopteriformes|flamingos]] are more closely related to [[Ciconiiformes|storks]] or [[Anseriformes|ducks]]. The fact that flamingos share parasites with ducks and geese was initially taken as evidence that these groups were more closely related to each other than either is to storks. However, evolutionary events such as the duplication, or the extinction of parasite species (without similar events on the host phylogeny) often erode similarities between host and parasite phylogenies. In the case of flamingos, they have similar lice to those of [[grebe]]s. Flamingos and grebes do have a common ancestor, implying cospeciation of birds and lice in these groups. Flamingo lice then [[host switch|switched hosts]] to ducks, creating the situation which had confused biologists.<ref name=JohnsonKennedy2006>{{cite journal|last1=Johnson|first1=K. P.|last2=Kennedy|first2=M.|last3=McCracken|first3=K. G|title=Reinterpreting the origins of flamingo lice: cospeciation or host-switching?|journal=Biology Letters|volume=2|issue=2|year=2006|pages=275–278|doi=10.1098/rsbl.2005.0427|pmc=1618896|pmid=17148381}}</ref>

[[File:Toxoplasma gondii (2).jpg|thumb|The protozoan ''[[Toxoplasma gondii]]'' facilitates its transmission by [[Behavior-altering parasites|inducing behavioral changes]] in rats through infection of neurons in their [[central nervous system]].]]

Parasites infect [[sympatry|sympatric]] hosts (those within their same geographical area) more effectively, as has been shown with [[Digenea|digenetic trematodes]] infecting lake snails.<ref name="Lively00"/> This is in line with the [[Red Queen hypothesis]], which states that interactions between species lead to constant natural selection for coadaptation. Parasites track the locally common hosts' phenotypes, so the parasites are less infective to [[allopatric speciation|allopatric]] hosts, those from different geographical regions.<ref name="Lively00">{{cite journal |last1=Lively |first1=C. M. |last2=Dybdahl |first2=M. F. |url=https://public.wsu.edu/~dybdahl/nature00.pdf |archive-url=https://web.archive.org/web/20160607131917/http://public.wsu.edu/~dybdahl/nature00.pdf |archive-date=7 June 2016 |url-status=live |title=Parasite adaptation to locally common host genotypes |journal=Nature |volume=405 |issue=6787 |pages=679–81 |year=2000 |pmid=10864323 |doi=10.1038/35015069 |bibcode=2000Natur.405..679L |s2cid=4387547 }}</ref>

=== Modifying host behaviour ===

{{further|Behavior-altering parasites}}
<!--
[[File:Reclinervellus nielseni.jpg|thumb|left|A larva of ''[[Reclinervellus nielseni]]'' parasitizing ''[[Cyclosa argenteoalba]]'' in Japan]]-->

Some parasites [[Behavior-altering parasites|modify host behaviour]] in order to increase their transmission between hosts, often in relation to predator and prey ([[parasite increased trophic transmission]]). For example, in the [[California coastal salt marsh]], the fluke ''[[Euhaplorchis californiensis]]'' reduces the ability of its [[killifish]] host to avoid predators.<ref>{{cite journal |last1=Lafferty |first1=K. D. |last2=Morris |first2=A. K. |year=1996 |url=http://parasitology.msi.ucsb.edu/sites/parasitology.msi.ucsb.edu/files/docs/publications/Altered%20Behavior.pdf |archive-url=https://web.archive.org/web/20190303123301/http://parasitology.msi.ucsb.edu/sites/parasitology.msi.ucsb.edu/files/docs/publications/Altered%20Behavior.pdf |archive-date=3 March 2019 |url-status=live |title=Altered behavior of parasitized killifish increases susceptibility to predation by bird final hosts |journal=Ecology |volume=77 |issue=5 |pages=1390–1397 |doi=10.2307/2265536 |jstor=2265536 |bibcode=1996Ecol...77.1390L }}</ref> This parasite matures in [[egret]]s, which are more likely to feed on infected killifish than on uninfected fish. Another example is the protozoan ''[[Toxoplasma gondii]]'', a parasite that matures in [[Felis silvestris catus|cats]] but can be carried by many other [[Mammalia|mammals]]. Uninfected [[Rattus rattus|rats]] avoid cat odors, but rats infected with ''T. gondii'' are drawn to this scent, which may increase transmission to feline hosts.<ref>{{cite journal |last1=Berdoy |first1=M. |last2=Webster |first2=J. P. |last3=Macdonald |first3=D. W. |title=Fatal attraction in rats infected with ''Toxoplasma gondii'' |journal=Proc. Biol. Sci. |volume=267 |issue=1452 |pages=1591–4 |year=2000 |pmid=11007336 |pmc=1690701 |doi=10.1098/rspb.2000.1182 }}</ref> The malaria parasite modifies the skin odour of its human hosts, increasing their attractiveness to mosquitoes and hence improving the chance for the parasite to be transmitted.<ref name="de Boer2017"/> The spider ''[[Cyclosa argenteoalba]]'' often have parasitoid wasp larvae attached to them which alter their web-building behavior. Instead of producing their normal sticky spiral shaped webs, they made simplified webs when the parasites were attached. This manipulated behavior lasted longer and was more prominent the longer the parasites were left on the spiders.<ref>{{cite journal |last=Takasuka |first=Keizo |date=16 September 2019 |title=Evaluation of manipulative effects by an ichneumonid spider-ectoparasitoid larva upon an orb-weaving spider host (Araneidae: Cyclosa argenteoalba) by means of surgical removal and transplantation |url=http://dx.doi.org/10.1636/joa-s-18-082 |journal=The Journal of Arachnology |volume=47 |issue=2 |page=181 |doi=10.1636/joa-s-18-082|s2cid=202579182 |issn=0161-8202}}</ref>

[[File:Bed bug, Cimex lectularius.jpg|thumb|Trait loss: bedbug ''[[Cimex lectularius]]'' is flightless, like many insect ectoparasites.]]

=== Trait loss ===

Parasites can exploit their hosts to carry out a number of functions that they would otherwise have to carry out for themselves. Parasites which lose those functions then have a selective advantage, as they can divert resources to reproduction. Many insect ectoparasites including [[Cimex|bedbugs]], [[Polyctenidae|batbugs]], [[lice]] and [[flea]]s have lost their [[insect flight|ability to fly]], relying instead on their hosts for transport.<ref>{{cite book |last=Alexander |first=David E. |title=On the Wing: Insects, Pterosaurs, Birds, Bats and the Evolution of Animal Flight |url=https://books.google.com/books?id=H6xUCgAAQBAJ&pg=PT119 |year=2015 |publisher=Oxford University Press |isbn=978-0-19-999679-7 |page=119}}</ref> Trait loss more generally is widespread among parasites.<ref>{{cite journal |last=Poulin |first=Robert |author-link=Robert Poulin (zoologist) |url=http://faculty.uml.edu/rhochberg/hochberglab/Courses/Parasite/PDF%20Papers/Parasitology%20Papers/Myths-Realities%20Parasite%20life%20histories.pdf |title=Evolution of parasite life history traits: myths and reality |journal=Parasitology Today |date=September 1995 |volume=11 |issue=9 |pages=342–345 |pmid=15275316 |doi=10.1016/0169-4758(95)80187-1|archive-url=https://web.archive.org/web/20120216194337/http://faculty.uml.edu/rhochberg/hochberglab/Courses/Parasite/PDF%20Papers/Parasitology%20Papers/Myths-Realities%20Parasite%20life%20histories.pdf |archive-date=16 February 2012 }}</ref> An extreme example is the [[myxosporea]]n ''[[Henneguya zschokkei]]'', an ectoparasite of fish and the only animal known to have lost the ability to respire aerobically: its cells lack [[mitochondria]].<ref name="PNAS">{{Cite journal |last1=Yahalom |first1=Dayana |last2=Atkinson |first2=Stephen D.|last3=Neuhof |first3=Moran |last4=Chang |first4=E. Sally |last5=Philippe |first5=Hervé |last6=Cartwright |first6=Paulyn |last7=Bartholomew |first7=Jerri L. |last8=Huchon |first8=Dorothée |date=19 February 2020 |title=A cnidarian parasite of salmon (Myxozoa: Henneguya) lacks a mitochondrial genome |journal=Proceedings of the National Academy of Sciences |volume=117 |issue=10 |pages=5358–5363 |doi=10.1073/pnas.1909907117 |pmid=32094163|pmc=7071853 |bibcode=2020PNAS..117.5358Y |doi-access=free }}</ref>

=== Host defences ===

Hosts have evolved a variety of defensive measures against their parasites, including physical barriers like the skin of vertebrates,<ref name=Colorado/> the immune system of mammals,<ref name=Maizels2009/> insects actively removing parasites,<ref name=Jeanne1979/> and defensive chemicals in plants.<ref name=Runyon2010/>

The evolutionary biologist [[W. D. Hamilton]] suggested that [[sexual reproduction]] could have evolved to help to defeat multiple parasites by enabling [[genetic recombination]], the shuffling of genes to create varied combinations. Hamilton showed by mathematical modelling that sexual reproduction would be evolutionarily stable in different situations, and that the theory's predictions matched the actual ecology of sexual reproduction.<ref name="Hamilton1990">{{cite journal |last1=Hamilton |first1=W. D. |author1-link=W. D. Hamilton|last2=Axelrod |first2=R. |last3=Tanese |first3=R. |title=Sexual reproduction as an adaptation to resist parasites (a review). |journal=Proceedings of the National Academy of Sciences |volume=87 |issue=9 |date=May 1990 |doi=10.1073/pnas.87.9.3566 |pmid=2185476 |pmc=53943 |pages=3566–3573 |bibcode=1990PNAS...87.3566H |doi-access=free }}</ref><ref name="EbertHamilton1996">{{cite journal |last1=Ebert |first1=Dieter |last2=Hamilton |first2=William D. |author2-link=W. D. Hamilton |title=Sex against virulence: the coevolution of parasitic diseases |journal=Trends in Ecology & Evolution |volume=11 |issue=2 |year=1996 |doi=10.1016/0169-5347(96)81047-0 |pmid=21237766 |pages=79–82|bibcode=1996TEcoE..11...79E }}</ref> However, there may be a trade-off between [[immunocompetence]] and breeding male vertebrate hosts' [[secondary sex characteristic]]s, such as the plumage of [[peacock]]s and the manes of [[lion]]s. This is because the male hormone [[testosterone]] encourages the growth of secondary sex characteristics, favouring such males in [[sexual selection]], at the price of reducing their immune defences.<ref name="FolstadKarter1992">{{cite journal |last1=Folstad |first1=Ivar |last2=Karter |first2=Andrew John |url=https://www.academia.edu/17248251 |title=Parasites, Bright Males, and the Immunocompetence Handicap |journal=The American Naturalist |volume=139 |issue=3 |year=1992 |pages=603–622 |doi=10.1086/285346 |bibcode=1992ANat..139..603F |s2cid=85266542 }}{{Dead link|date=July 2024 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>

==== Vertebrates ====

[[File:Short Horned Lizard (4457945238).jpg|thumb|left|The dry [[skin]] of vertebrates such as the [[short-horned lizard]] prevents the entry of many parasites.]]

The physical barrier of the tough and often dry and waterproof [[skin]] of reptiles, birds and mammals keeps invading microorganisms from entering the body. [[Human skin]] also secretes [[sebum]], which is toxic to most microorganisms.<ref name=Colorado>{{cite web |title=Host–Parasite Interactions Innate Defenses of the Host |publisher=University of Colorado |url=http://www.colorado.edu/outreach/BSI/k12activities/interactive/innatedefenses.pdf |access-date=7 May 2014 |archive-url=https://web.archive.org/web/20160304000852/http://www.colorado.edu/outreach/BSI/k12activities/interactive/innatedefenses.pdf |archive-date=4 March 2016 |url-status=dead }}</ref> On the other hand, larger parasites such as [[trematode]]s detect chemicals produced by the skin to locate their hosts when they enter the water. Vertebrate [[saliva]] and tears contain [[lysozyme]], an enzyme that breaks down the [[Bacterial cell structure#Cell wall|cell walls]] of invading bacteria.<ref name=Colorado/> Should the organism pass the mouth, the [[stomach]] with its [[hydrochloric acid]], toxic to most microorganisms, is the next line of defence.<ref name=Colorado/> Some intestinal parasites have a thick, tough outer coating which is digested slowly or not at all, allowing the parasite to pass through the stomach alive, at which point they enter the intestine and begin the next stage of their life. Once inside the body, parasites must overcome the [[immune system]]'s [[serum proteins]] and [[pattern recognition receptor]]s, intracellular and cellular, that trigger the adaptive immune system's [[lymphocyte]]s such as [[T cell]]s and antibody-producing [[B cell]]s. These have receptors that recognise parasites.<ref name=Maizels2009>{{cite journal |last=Maizels |first=R. M. |title=Parasite immunomodulation and polymorphisms of the immune system |journal=J. Biol. |year=2009 |volume=8 |issue=7 |pages=62 |pmid=19664200 |pmc=2736671 |doi=10.1186/jbiol166 |doi-access=free }}</ref>

==== Insects ====

[[File:Leaf Spot on Oak in Gunnersbury Triangle.jpg|thumb|[[Leaf spot]] on [[oak]]. The spread of the parasitic fungus is limited by defensive chemicals produced by the tree, resulting in circular patches of damaged tissue.]]

Insects often adapt their nests to reduce parasitism. For example, one of the key reasons why the wasp ''[[Polistes canadensis]]'' nests across multiple [[honeycomb|combs]], rather than building a single comb like much of the rest of its genus, is to avoid infestation by [[tineid moth]]s. The tineid moth lays its eggs within the wasps' nests and then these eggs hatch into larvae that can burrow from cell to cell and prey on wasp pupae. Adult wasps attempt to remove and kill moth eggs and larvae by chewing down the edges of cells, coating the cells with an oral secretion that gives the nest a dark brownish appearance.<ref name=Jeanne1979>{{cite journal |last=Jeanne |first=Robert L. |year=1979 |url=https://www.researchgate.net/publication/226226828 |title=Construction and Utilization of Multiple Combs in Polistes canadensis in Relation to the Biology of a Predaceous Moth |journal=Behavioral Ecology and Sociobiology |volume=4 |issue=3|pages=293–310 |doi=10.1007/bf00297649|bibcode=1979BEcoS...4..293J |s2cid=36132488 }}</ref>

==== Plants ====

Plants respond to parasite attack with a series of chemical defences, such as [[polyphenol oxidase]], under the control of the [[jasmonic acid]]-insensitive (JA) and [[salicylic acid]] (SA) signalling pathways.<ref name=Runyon2010>{{cite journal |last=Runyon |first=J. B. |author2=Mescher, M. C. |author3=De Moraes, C. M. |title=Plant defenses against parasitic plants show similarities to those induced by herbivores and pathogens |journal=Plant Signal Behav |volume=5 |issue=8 |pages=929–31 |year=2010 |pmid=20495380 |pmc=3115164 |doi=10.4161/psb.5.8.11772|bibcode=2010PlSiB...5..929R }}</ref><ref name=Thaler2002>{{cite journal |last1=Thaler |first1=Jennifer S. |last2=Karban |first2=Richard |last3=Ullman |first3=Diane E. |last4=Boege |first4=Karina |last5=Bostock |first5=Richard M. |url=https://www.researchgate.net/publication/226325654 |title=Cross-talk between jasmonate and salicylate plant defense pathways: effects on several plant parasites |journal=Oecologia |volume=131 |issue=2 |year=2002 |doi=10.1007/s00442-002-0885-9 |pmid=28547690 |pages=227–235|bibcode=2002Oecol.131..227T |s2cid=25912204 }}</ref> The different biochemical pathways are activated by different attacks, and the two pathways can interact positively or negatively. In general, plants can either initiate a specific or a non-specific response.<ref name=Thaler2002/><ref name="Frank_2000">{{cite journal |last=Frank |first=S. A. |url=https://stevefrank.org/reprints-pdf/00JTB-Defense.pdf |archive-url=https://web.archive.org/web/20010614080530/http://stevefrank.org/reprints-pdf/00JTB-Defense.pdf |archive-date=14 June 2001 |url-status=live |title=Specific and non-specific defense against parasitic attack |journal=J. Theor. Biol. |volume=202 |issue=4 |pages=283–304 |year=2000 |pmid=10666361 |doi=10.1006/jtbi.1999.1054|bibcode=2000JThBi.202..283F |citeseerx=10.1.1.212.7024 }}</ref> Specific responses involve recognition of a parasite by the plant's cellular receptors, leading to a strong but localised response: defensive chemicals are produced around the area where the parasite was detected, blocking its spread, and avoiding wasting defensive production where it is not needed.<ref name="Frank_2000"/> Non-specific defensive responses are systemic, meaning that the responses are not confined to an area of the plant, but spread throughout the plant, making them costly in energy. These are effective against a wide range of parasites.<ref name="Frank_2000"/> When damaged, such as by [[lepidoptera]]n [[caterpillar]]s, leaves of plants including [[maize]] and [[cotton]] release increased amounts of volatile chemicals such as [[terpene]]s that signal they are being attacked; one effect of this is to attract parasitoid wasps, which in turn attack the caterpillars.<ref name="Paré Tumlinson pp. 325–332">{{cite journal |last1=Paré |first1=Paul W. |last2=Tumlinson |first2=James H. |title=Plant Volatiles as a Defense against Insect Herbivores |journal=Plant Physiology |volume=121 |issue=2 |date=1 October 1999|doi=10.1104/pp.121.2.325 |pmid=10517823 |pages=325–332|pmc=1539229 }}</ref>
{{anchor|Ecology}}{{anchor|Evolution}}