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.[1] 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.[2] In 2004 the moth was first recorded from Sweden.
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.
