There has been considerable progress with the Large Blue butterfly reintroduction programme since 2001 as Matthew Oates, our butterfly specialist, explains.
Facts about the Large Blue:
- It is the only British representative of six species globally, five of which occur in Europe. All six are in serious decline and classified as either endangered or vulnerable at global and European levels.
- Large Blue butterflies can be ranked amongst the most vulnerable species in the world.
- In Europe, the Large Blue still occurs in 37 countries but it has declined by some 50-80 percent in recent decades overall and is deemed stable in only seven countries.This decline is particularly severe in Northern Europe.
- Conservationist scientists in the UK are spearheading attempts to conserve the Large Blue in Northern Europe.
The Large Blue Recovery Programme
The Recovery Programme is a partnership made up of, among others, organisations and individuals including Butterfly Conservation, the Centre for Ecology & Hydrology (formerly ITE), English Nature, and the Wildlife Trusts for Cornwall, Gloucestershire and Somerset, and of course the National Trust.
The partnership seeks to further the conservation of the Large Blue in Europe and, in particular, to re-establish the butterfly at sustainably-managed sites in England.
The Large Blue season
- In the UK, the Large Blue adult season lasts for some three or four weeks during June and July.
- The timing of the flight season varies from region to region and from year to year, with much depending on seasonal weather.
- Individual butterflies live for four to five days, on average, but they do not all emerge at once; individuals tend to emerge over a period of 10 to 20 days.
Young Large Blues
- The female Large Blue lays her eggs in the buds of thyme that are in the tight bud stage. This necessitates full synchrony between the bud stage of the thyme and the butterfly’s flight season; this synchrony is ensured mainly through appropriate grazing.
- The eggs hatch after some seven to 14 days, depending on the weather. Young larvae feed on thyme flowers for about two weeks during late July and early August. Then, they fall to the ground and are adopted by red ants and taken off to the underground ant nests where they feed on ant grubs.
- Any species of red ant will pick up the caterpillars, which secrete a sticky sugary substance which ants love.
- Frequent and large nests of this single species of ant are essential to the Large Blue.
- The larva spends 10 months as a predator in the ant nests, then pupates there.
© National Trust
M. sabuleti is a warmth-loving ant that thrives in short, arid grassland on hot south-facing slopes that are heavily grazed.
If the grass is allowed to grow higher than 3-4 cm this ant rapidly dies out and other species of ant take over, which cannot support Large Blue colonies. Ant density is adversely affected by cool, wet summers and spring droughts.
The butterflies emerge during the period mid-June to mid-July, after some three weeks in the pupal stage.
Adult numbers tend to vary enormously at individual sites from year to year, as a result of the impact of seasonal weather on the habitat and the weather-sensitive ants, and as a result of habitat conditions generally.
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Secret ‘Site X’
The last site to support the UK race, and the first site to receive introduced Swedish Large Blues, is on National Trust land in South Dartmoor. It is kept as an open secret and is known within butterfly circles as ‘Site X’.
Here, the Large Blue was experimentally reintroduced in 1983, and once this proved successful a full reintroduction took place in 1986.
We have been grazing the slope carefully with our own small herd of Dartmoor ponies, cared for by the warden and tenant farmer, Roger Hutchings, who also runs his cattle on the site.
This grazing has to take account of fluctuations in the rabbit population, an added complication.
Clumps of gorse are burnt on a patchy rotation, to provide the warm sheltered glades and mosaic structure that the butterfly needs and to prevent the gorse from taking over. The site is now, bar the odd hiccup, under sustainable management.
Thousands of baby wild thyme plants have been planted out on to the site and, especially, on to the adjoining hillside (known as 'Site Y'), in an effort to enhance and enlarge the Large Blue’s breeding area.
Volunteers from Butterfly Conservation help with the thyme planting, the site management and butterfly monitoring. We are particularly grateful to David Land, of BC Devon branch.
© National Trust
'Site X' is too small to sustain a permanent Large Blue population by itself, and the main challenge for the Large Blue Project here is to enlarge the area utilised by the butterfly and establish a metapopulation (clusters of loosely-connected colonies) structure.
To date, the habitat quality on 'Site X' has been greatly improved and the adjoining hillside, 'Site Y', has been made suitable.
A range of other scarce animals and plants (including two of Britain’s most rapidly-declining species: high brown fritillary and pearl-bordered fritillary) have benefited from the grazing and gorse-burning that is primarily aimed at assisting the Large Blue. 'Site X' is not just an example of ‘single species management’.
The main challenge now is to continue to extend the area of suitable habitat and to seek to restore comparable habitat within the wider landscape.
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Other Large Blue sites
The National Trust has been associated with attempts to conserve the Large Blue since the 1920s.
Many of the 90 or so colonies that have existed in England occurred on land now owned by us, especially along the North Cornwall coast. The butterfly had died out from nearly all of them prior to our acquisition.
In 2000, the Large Blue was introduced to a large sea combe we own on the North Cornwall coast.
Ten female Large Blues and two males were released in this combe on 27 June 2000. These were the first Large Blues to fly in Cornwall since 1975.
We are also working hard to return the Large Blue to another combe on the North Cornwall coast, close to the Devon border.
This combe, called the Tidna, was purchased as a reserve for the Large Blue and donated by the late Malcolm Spooner, an eminent entomologist who tried hard to save the butterfly from extinction.
The Cornwall and Devon sites only support small colonies at present. Access is difficult to the Cornish site, as one cannot park within about two miles of the place! The Devon site is small and has been damaged by past activities, and is extensively used for scientific research.
For the time being at least, the Trust is encouraging people to visit Collard Hill, though we do intend to increase access to the other two Trust-pwned sites shortly. Watch this space! Both are currently open secrets..
In the Cotswolds, we are working towards restoring suitable conditions for the butterfly on the steep limestone grassland slopes of Rodborough Common, Stroud.
© National Trust
This common has been suffering from ‘under-grazing’ in recent years and is currently dominated by coarse grasses.
In 1999, we acquired a small number of belted Galloway cattle specifically for grazing on the steep slopes in order to safeguard the nature conservation interest there and to make the grass ‘keep’ on the slopes more suitable for the commoners’ cattle.
The initial results have been most promising with, for example, a spectacular increase in the abundance of the rare pasque flower. It may be that within a decade or so the Large Blue will fly again on Rodborough Common.
© National Trust
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Collard Hill in 2007, the home of Britain's rarest butterfly
The Large Blue has been brought back from beyond the brink of extinction. It wasn't easy: it took a lot of hard work by a dedicated partnership of diverse organisations, backed by some of the best conservation biology research ever conducted, anywhere.
© David Simcox
Twelve females and three males were released there in late June 2000, and 267 larvae were introduced in late July.
The necessary habitat conditions can easily be maintained under a simple commercial grazing regime run by a sympathetic local farmer.
Furthermore, it is part of a network of potential and actual Large Blue sites within the wider Polden Hills landscape, much of which is under positive management by a range of conservation organisations.
This site is part of an attempt to re-establish the Large Blue butterfly by establishing an extensive metapopulation structure at landscape level.
Already, some natural spread and colonisation has been noted in the Poldens, a wonderfully healthy sign.
Collard Hill is the only place in Britain where people are freely able to visit to see this incredibly rare and special butterfly. The Trust has dedicated this hillside to the conservation of the Large Blue and people’s engagement with this most curious of insects.
The site exists for everyone, for ever; so come this year, next year, sometime, whenever. The Trust aims to keep the Large Blue at Collard Hill for as long as is practically possible.
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© National Trust
We run a Large Blue phone line – 01793 817732 - from early June to mid-July. It provides information on how to see the Large Blue at Collard Hill.
The message is updated regularly, particularly before weekends. Information is given on the state of the flight season, as well as parking, access and health and safety arrangements.
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