The Essex Emerald moth is one that could be mistaken for a butterfly in shape. The upper wings are a light, dusky emerald in colour, the upper edge bordered with pale yellow. There is a cream-coloured uneven line that echoes the line of the outer edge of the wing, a cream spot inboard of this line, and another faint line at the base of the wing. The hind wings are a much paler green, again with faint white lines echoing the outer edges. The body of the insect matches the wings in colouration, but the thorax is more strongly coloured. The yellow line of the upper wings is continued across the top of the thorax. The caterpillar grows to about 20-23 mm, and has a greyish body and darker grey lines running down its length. The skin is very rough and usually covered with particles of leaf.
The Essex Emerald moth is on the wing from mid-June to early July, but has not often been found as the adult insect. The eggs are laid on high up the stem of sea wormwood, Artemisia maritima, although, on the continent, the nominate species feeds on other artemisia plants, as well as yarrow and groundsel.
The caterpillars, on hatching, cover themselves in pieces of the foodplant, presumably as daytime camouflage against predators. They feed at night and are able to survive inundation by salt water during high tides. The caterpillars hibernate over winter and resume feeding on sea wormwood in spring. By the end of May they are fully-grown and pupate attached to the stem of the plant.
The Essex Emerald moth is recognised as a subspecies of Thetidia smaragdaria, which has a much wider distribution, ranging across most of Europe from southern Scandinavia to the Mediterranean, and eastwards to Siberia, northern China and Amur, to Japan. In the UK, Thetidia smaragdaria maritima was last found only around the coastlines of Essex and Kent.
This moth has now been declared extinct in the UK, and the nominate species found in Europe and Asia has also declined to the point where none have been seen in recent years. Factors that may have contributed to the disappearance of this moth include: a loss of habitat through inappropriate grazing, unnecessary removal of vegetation including the foodplant plus fires along sea walls; coastal development together with re-building and maintenance of seawalls as well as agricultural improvements adjacent to sites; and finally, genetic instability due to the inbreeding of a small captive population.
The Essex Emerald moth is still listed in the UK Biodiversity Action Plan, and included in English Nature's Species Recovery Programme. The last known site for the species was monitored each year until 1993. Although a captive-breeding population survived until 1996, and five re-introduction attempts have been made, it is feared this moth is now extinct in the UK.
Any future plans for this species rely on a hitherto unknown population being found and protected. At the present time, none have appeared but the search will continue in the hope that this attractive insect might once again be included in the list of British wildlife.
The UK Biodiversity Action Plan for this species is available at UK BAP.
Hibernation is a winter survival strategy characteristic of some mammals in which an animal’s metabolic rate slows down and a state of deep sleep is attained. Whilst hibernating, animals survive on stored reserves of fat that they have accumulated in summer. In insects, the correct term for hibernation is ‘diapause’, a temporary pause in development and growth. Any stage of the lifecycle (eggs, larvae, pupae or adults) may enter diapause, which is typically associated with winter.
Interbreeding amongst close relatives.
The process of forming a pupa, the stage in an insect’s development, when huge changes occur that reorganise the larval form into the adult form. In butterflies the pupa is also called a chrysalis.
A population usually restricted to a geographical area that differs from other populations of the same species, but not to the extent of being classified as a separate species.
Part of the body located near the head in animals. In insects, the three segments between the head and the abdomen, each of which has a pair of legs.
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