Silky wave moth  (Idaea dilutaria)

The Silky Wave is a pale tan-coloured moth, with a slightly darker-brown wavy pattern on both the fore- and hindwings (4). It is similar in appearance to the more common Idaea fuscovenosa, but lacks the black spot on the fore-wing (2). The caterpillar is dull greyish in colour with a pale line along the sides towards the rear and smaller lighter markings on the upper surface (6).

This species was first recorded from Durdham Down near Bristol in 1851 (2). It still occurs in the Bristol area today, as well as at a site on the Gower coast of Glamorgan, and a third population exists on the Great Orme in North Wales (1). This moth also occurs in central and southern Europe (1).

Occurs in open calcareous grasslands on steep south-facing slopes where grazing is either light or absent. The species requires a source of the larval foodplant, common rock-rose (Helianthemum) (1).

A single brood is produced each year, the adults of which fly from mid-June to mid-July. The caterpillars are present between August and May, feeding on withered and rotting leaves of the foodplant. The overwintering stage is the caterpillar (2).

The main factor thought to be responsible for the decline in this species is inappropriate grassland management, such as overgrazing or scrub encroachment (1). The Silky Wave is at the limit of its range in the UK and its preference for south-facing slopes may indicate that climatic factors may also affect this species (4).

The UK Biodiversity Action Plan (UK BAP) has identified this species as a priority for conservation. A Species Action Plan has been produced, which details the main actions required to conserve this moth. The plan for the Silky Wave moth aims to maintain the current known populations, enhance these populations by 2005 and reintroduce the species to three sites within the historic range by 2010 if no further existing populations are discovered. Proposed conservation management for this species includes increasing the area of suitable habitat at occupied sites and linking existing isolated fragments of habitat in order to allow numbers of the Silky Wave to increase (1).

Further reading on moths:
Leverton, R. (2001) Enjoying Moths. Poyser, London.
Skinner, B. (1984) Moths of the British Isles. Penguin Books, Harmondsworth

Information authenticated by Barry Stewart.