{{short description|Gall found on oak trees}} {{refimprove|date=May 2023}} [[File:Oak apples - Keila.jpg|thumb|Oak apples on ''Quercus robur'' leaf]] [[Image:Gallarutos.jpg|thumb|right|250px|Oak galls on a Pyrenean oak (''Quercus pyrenaica'') in León, Spain.]] [[Image:Oak apple.jpg|thumb|right|250px|An oak apple on a tree in Worcestershire, England]]
An '''oak apple''' or '''oak gall''' is a large, round, vaguely apple-like gall commonly found on many species of oak. Oak apples range in size from {{convert|2|to|4|cm|inch|0}} in diameter and are caused by chemicals injected by the larva of certain kinds of gall wasp in the family Cynipidae.<ref name=cranshaw />
==Formation== The adult female wasp lays single eggs in developing leaf buds. The wasp larvae feed on the gall tissue resulting from their secretions, which modify the oak bud into the gall, a structure that protects the developing larvae until they undergo metamorphosis into adults.
Some common oak-apple-forming species are the ''Biorhiza pallida'' gall wasp in Europe, ''Amphibolips confluenta'' in eastern North America, and ''Atrusca bella'' in western North America.<ref name=cranshaw>{{cite book |last=Cranshaw |first=Whitney |title=Garden Insects of North America |year=2004 |publisher=Princeton University Press |location=Princeton, New Jersey |isbn=0-691-09560-4 |url-access=registration |url=https://archive.org/details/gardeninsectsofn0000cran }}</ref> Oak apples may be brownish, yellowish, greenish, pinkish, or reddish.
==Other galls== Considerable confusion exists in the general literature between the oak apple and the oak marble gall. The oak marble is frequently called the oak apple due to the superficial resemblance and the preponderance of the oak marble gall in the wild. Other galls found on oak trees include the oak artichoke gall and the acorn cup gall, but each of these has its own distinctive form.
==Iron gall ink== {{main|Iron gall ink}} Oak galls have been used in the production of ink since at least the time of the Roman Empire. From the Middle Ages to the early twentieth century, iron gall ink was the main medium used for writing in the Western world.<ref>{{cite book |last=Diringer |first=David |title=The Book Before Printing: Ancient, Medieval and Oriental |publisher=Dover Publications |date=1 March 1982 |pages=551–2}}</ref>
Gall nuts are a source of tannin in the production of iron gall ink. Tannins belong to a group of molecules known as polyphenols and can be taken from different parts of plants such as leaves, pods, fruits, and gall nuts.
Along with gall nuts, other important ingredients in the production of iron gall ink include iron sulfate and gum arabic. The reaction between the tannins from the gall nut and the iron produces a complex that gives the iron gall ink its color. The gum arabic makes the ink more viscous and helps bind the ink to the writing surface.<ref>{{cite web |url=https://travelingscriptorium.library.yale.edu/2013/03/21/iron-gall-ink/ |title=Iron Gall Ink |work=travelingscriptorium.library.yale.edu |date=21 March 2013 |access-date=24 May 2023}}</ref>
==Folklore== It is said that if a "worm" is found inside the gall on Michaelmas Day, then the year will be pleasant and unexceptional, and if a spider is found, then it will be a bad year with shortages and ruined crops. If a fly is found inside, then it will be a moderate season, and if nothing is found, then serious diseases will occur all that year.<ref name="Roud">Roud, Steven (2003). ''The Penguin Guide to the Superstitions of Britain and Ireland''. Penguin Books. p. 344.</ref>
Oak Apple Day (or Royal Oak Day) is a former public holiday in England on 29 May that commemorated the Restoration of Charles II in 1660. The popular name refers to the event during the English Civil War when Charles hid in an oak tree. The commemoration persists in some areas today, although festivities have little to do with the Restoration.
==Gallery== <gallery mode=packed> File:Oak apple BW 2014-11-16 20-35-06.jpg|An oak apple on an oak leaf. File:Andricus kollari oak marble gall, knikkergal.jpg|Oak apples File:Oakgall3800ppx.JPG|An oak tree with multiple oak apples. Image:Oak Gall.jpg|''Andricus kollari'' oak gall File:Galle.jpg|Oak apples on an oak tree. File:Gallrazorback.jpg|Oak apple gall on Garry oak (''Quercus garryana'') File:Oak apples on oak leaf and in cross section.JPG|Oak apples on oak leaf and in cross section File:Biorhiza pallida male.jpg|''Biorhiza pallida'' male File:Eikenblad galappel. 16-12-2025. (d.j.b.).jpg|An oak leaf gall on the underside of an oak leaf. </gallery>
== List of galls called oak apples == {{incomplete-list|date=October 2023}}
{| class="wikitable" |+ Oak apples |- ! Image !! Binomial name !! Host species !! Region |- |frameless|153x153px |''Amphibolips confluenta''<ref>{{Cite web |title=Amphibolips confluenta (agamic) |url=https://www.gallformers.org/gall/581 |access-date=2024-09-21 |website=www.gallformers.org}}</ref> |various |North America |- |frameless|205x205px |''Amphibolips quercuspomiformis''<ref name="Russo 2021" /> |coast live oak, interior live oak |North America |- | 150px | ''Andricus quercuscalifornicus''<ref name="Russo 2021" /> |valley oak, California scrub oak |North America |- | | ''Atrusca bella''<ref name="Russo 2021">{{Cite book |last=Russo |first=Ronald A. |author-link = Ronald A. Russo| url=https://www.degruyter.com/document/doi/10.1515/9780691213408/html |title=Plant Galls of the Western United States |publisher=Princeton University Press |year=2021 |isbn=978-0-691-21340-8 |lccn=2020949502 |pages=124 |language=en-us |doi=10.1515/9780691213408}}</ref> | Arizona white oak, Mexican blue oak, netleaf oak, Toumey oak, and shrub live oak | North America |- | 150px | ''Atrusca brevipennata''<ref name="Russo 2021" /> |shrub live oak, Gambel oak | North America |- | 150px || ''Atrusca capronae''<ref name="Russo 2021" />|| shrub live oak|| North America |- | 150px | ''Besbicus mirabilis''<ref name="Russo 2021" /> | Oregon oak | North America |- |frameless|215x215px |''Biorhiza pallida''<ref>{{Cite web |title=Oak Apple Gall Wasp (Biorhiza pallida) |url=https://uk.inaturalist.org/taxa/127837-Biorhiza-pallida |access-date=2024-09-21 |website=iNaturalist United Kingdom |language=en-GB}}</ref> |English oak, sessile oak |Europe |- | 150px || ''Trichoteras coquilletti''<ref name="Russo 2021" />|| Huckleberry oak, canyon live oak|| North America |- | 150px || ''Trichoteras vacciniifoliae''<ref name="Russo 2021" />|| Huckleberry oak, canyon live oak|| North America |}
==See also== * Knopper gall * Pineapple gall
==References== <references/>
==External links== {{Commons category|Galls on Quercus}} * [https://web.archive.org/web/20060427083644/http://www.fcps.k12.va.us/StratfordLandingES/Ecology/mpages/oak_apple_gall_wasp.htm Amphibolips confluenta] * [https://web.archive.org/web/20040906222401/http://www.oldknobbley.com/woodland_ecology/insects/insect_gall_wasp.php Biorhiza pallida] * [https://web.archive.org/web/20060515072048/http://www.glosnats.org.uk/images/news/oak_apple.jpg Gloucestershire Naturalists' Society photograph of galls with 'red apple' appearance] See also: [https://web.archive.org/web/20060615054923/http://glosnats.org.uk/news/archive041.php] * [http://www.english-country-garden.com/trees/oak.htm The Oak Apple Gall]
{{DEFAULTSORT:Oak Apple}} Category:Cynipoidea Category:Oak galls