Northern Ireland's Priority Species

Eriogaster lanestris – small eggar

Eriogaster lanestris

Eriogaster lanestris (Linnaeus, 1758)
Family: Lasiocampidae

The small eggar is now only known from north County Londonderry and County Fermanagh and has certainly declined in range in the last fifty years. The adult has been seen only rarely in Northern Ireland. The distinctive larval webs and distinctively marked larvae betray the moths’ presence.

In brief

  • Recorded at Magilligan and Ballymaclarry Strands, County Londonderry and Monmurry, County Fermanagh

  • The Magilligan-Ballymaclarry population is known to be one of the largest extant populations in the UK

  • Its habitat is blackthorn thickets and hedges

  • Best seen when the adult flies in March and April; larval webs are most conspicuous in late June

  • Threatened by scrub removal and badly timed hedge cutting

  • From the historical records a serious decline is evident

  • Searches of former sites have so far proved fruitless.

Species description
The adults have light reddish-brown forewings with a white patch near the base and a pale postmedian line. There is also a distinctive white spot on the forewing. Egg-bound females have a disproportionately large body.

Life cycle
The adults are on the wing on mild nights in late February and March following a period of prolonged sunshine. Egg-bound females are virtually incapable of flight, relying on males to find them as they crawl towards the blackthorn’s growth points. Eggs are laid on the branch tips where the new growth will occur. Well-grown larvae are very distinctive with lines of tufts of long ginger-based hairs. A series of white dots and lines form a series of ‘U’-shaped marks along the sides. The larvae live communally in a dense silken web and often bask on the surface on sunny days. Once they have reached full size, the larvae leave the web to locate a safe area to pupate, often near the base of a blackthorn bush. The eggars can remain in the pupae for more than one year as they await suitable flying conditions.

Similar species
A number of other related species occur in Northern Ireland but none are especially similar or have the same flight season. A number of other species have larval webs and the bird cherry ermine is often found abundantly in the same habitat, but the larvae are much smaller and totally different in appearance. The webs are less dense and not as obviously ‘tent-shaped’.

How to see this species
Light trapping close to good blackthorn habitat in late February and March might be productive, but searching for larval webs in late June is undoubtedly the best method. The western North Coast and Fermanagh offer the best chances of success. Relevant access permissions should always be sought prior to visiting any sites.

Current status
The recently surveyed Magilligan-Ballymaclarry population stood at 54 webs in 2005, probably the largest concentration left in the UK. The species appears to be recently extinct at the nearby Umbra. In Fermanagh, it is only known from near Monmurry where a handful of adults have been trapped. No webs have been located in the area and these remain the only Northern Irish records of wild-caught adults. In the last hundred years it has been recorded at a number of scattered localities in Counties Down, Antrim and Fermanagh.

Why is this species a priority in Northern Ireland?

  • Rapid decline and rare, now only known from two localities, one of which is very significant in a UK context.

It is listed as Notable in the UK.

Threats/Causes of decline
Changes to hedge management have brought about the demise of this species. Summer cutting will destroy larval webs, winter cutting may remove pupae and spring cutting, eggs. The inability of the females to fly any distance greatly restricts the ability of this species to colonise suitable habitat. Fragmentation and isolation of colonies makes them more vulnerable to one-off incidents such as fire, as well as increasing the effects of predation and attacks by parasites.

Conservation of this species

Current action

  • Management recommendations have been identified for the main site where it is known to occur

  • Implementation of the Northern Ireland habitat action plan for Mixed Ashwoods.

Proposed objectives/actions

  • Maintain the number of viable populations of the species

  • Allow blackthorn scrub to develop in those areas where the majority of larval webs are found

  • Coppice established blackthorn to create new growth but ensure that bushes with larval webs are NOT coppiced

  • Monitor distribution and numbers of larval webs, preferably annually, but in at least every year before any coppicing/scrub removal commences.

What you can do

  • Report all moth sightings to the Moth Recorder for Northern Ireland, Andrew Crory, or use the Butterfly Conservation Northern Ireland (BCNI) sightings web page at The BCNI database is managed by CEDaR and these records will then be used to update the Butterflies and Moths of Northern Ireland website.
  • Join Butterfly Conservation. Butterflies and Moths are in serious decline — with your support Butterfly Conservation can take action to reverse this.

Further information

Butterflies and Moths of Northern Ireland

Excellent images and information on the species in the UK, including photos of larval webs.

Northern Ireland Habitat Action Plans

Baynes, E.S.A. (1964). A revised catalogue of Irish macrolepidoptera (Butterflies and Moths). E.W. Classey, Hampton, Middlesex.

Heath, J. and Emmet A.M. et al. (1992). The moths and butterflies of Great Britain and Ireland. Volume 7(2). Harley Books, England.

Porter, J. (1997). The colour identification guide to caterpillars of the British Isles. Viking, London.

Skinner, B. (1988). The colour identification guide to the moths of the British Isles. Viking, London.

Waring, P. and Townsend, M. (2003). Field Guide to the Moths of Great Britain and Ireland. British Wildlife Publishing, Hants.

Text written by:
Allen & Mellon Environmental Ltd.

iNaturalist: Species account : iNaturalist World Species Observations database