Essex emerald

{{Short description|Species of moth}}

{{Speciesbox

| image = Thetidia.smaragdaria.jpg

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| image2 = Thetidia_smaragdaira.jpg

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| taxon = Thetidia smaragdaria

| authority = (Fabricius, 1787)

| synonyms =

• Phalaena smaragdaria Fabricius, 1787
• Phorodesma castiliaria Staudinger, 1892
• Euchloris maritima Prout, 1935
• Geometra prasinaria Eversmann, 1837
• Geometra gigantea Milliere, 1874
• Geometra volgaria Guenee, 1858

}}

The Essex emerald (Thetidia smaragdaria) is a moth of the family Geometridae. The species was first described by Johan Christian Fabricius in 1787 as Phalaena smaragdaria.{{Cite web |title=Thetidia smaragdaria (Fabricius, 1787) |url=https://www.gbif.org/species/1957300 |access-date=2022-03-17 |website=www.gbif.org |language=en}} It is distributed throughout the Palearctic region with records from many European countries. The British subspecies Thetidia smaragdaria maritima was last seen in 1991 in Kent and is now presumed extinct.{{Cite web|url=https://butterfly-conservation.org/moths/essex-emerald|title=Essex Emerald}} In 2004 the moth was first recorded from Sweden.

Image:Britishentomologyvolume6Plate300.jpg's British Entomology Volume 6]]

The wingspan is 27–35 mm. There is one generation per year with adults on wing from mid-June to mid-July.

The larvae feed on Artemisia maritima and Achillea millefolium. Larvae can be found from July to June the following year. It overwinters in the larval stage.