Silver-studded Blues on the Purbeck Heaths

Male Silver-studded Blue - Scotland Heath, Dorset

Britain's resident butterflies can be divided into two main groups - the wider countryside species and the habitat specialists. You will see the first group in gardens, parks and farmland - Large White, Meadow Brown and the like. Many are very mobile, and they can turn up pretty much anywhere. Most are familiar to us. 

You have to work harder to find the second group - our habitat specialists. These are restricted to certain special places, for example where their foodplants grow, and they are less likely to travel over the wider landscape. You usually have to make an effort to find them!

The Silver-studded Blue Plebejus argus is a classic habitat specialist. In the UK it is mainly found on lowland heaths and certain parts of the English and Welsh coasts (see the distribution map on Butterfly Conservation's helpful fact sheet). But this doesn't tell the whole story. The odd fact about Silver-studded Blues is that they live in two contrasting habitats, where the plant species are very different.

We see this in Dorset, south-west England. Here, most Silver-studded Blues are found on lowland heaths in the Poole Harbour basin area, particularly in the Purbeck area south of Wareham. These heathlands are famously the home of a range of rare and often endangered species - ranging from the charismatic Sand Lizard (see my blogs here and here) to the more obscure Purbeck Mason Wasp (see my blog here).

Silver-studded Blue habitat - Scotland Heath, Purbeck, Dorset

However, you can also come across Silver-studded Blues on Dorset's Isle of Portland, where the lime-rich grassland looks very different to Purbeck's heather and gorse-dominated heathlands. There is a clear gap between the two populations, and there is little (actually, no) evidence of the butterflies moving between the two areas. So, what's going on here? To answer the question we need to look at the butterfly and its unusual ecology.

Introducing the Silver-studded Blue

Although the blue butterflies (part of the Lycaenidae family) are some of Britain's more eye-catching insects, telling the various species apart is not always straightforward - even in Britain with its relatively low butterfly species numbers. Silver-studded Blue males are perhaps most easily confused with Common Blues, which are much more, well, common. As well as being smaller, the most obvious difference is that male Silver-studded Blues have a wider black margin on the wings than male Common Blues (see the photos below).

Male Silver-studded Blue - Grange Heath

For comparison: male Common Blue - Powerstock Common

At which point, you might be asking: what about the 'silver studs'? These are actually on the back of the wings, and they are not always very obvious. Look for the small bright dots in the middle of the black spots - one is arrowed in the image below: 

Male Silver-studded Blue - wing underside. 'Silver studs' arrowed.

The 'silver studs' are also present on the underside of the female's wings, which is just as well because the females - which have dark brown wings - are otherwise tricky to separate from the females of Common Blue, Brown Argus and a few others. The studs can (just about) be seen in this image of a female below (there is a better image later):

Female Silver-studded Blue - Scotland Heath

Luckily, Silver-studded Blues are not difficult to find, as long as you are in the right place. Over the last few weeks I have been carrying out wildlife surveys on the Purbeck Heaths (not for butterflies, as it happens) and Silver-studded Blues have been one of the most numerous and conspicuous insects that we have encountered. They have a characteristic flight pattern, keeping low over clumps of heather, pausing often to rest on flowers. They usually won't fly far from where you first see them; some research says that they rarely move more than 10-20 metres, although from what I've observed this may be an underestimate.  

An unusual Life Cycle

Silver-studded Blue caterpillars (larvae) have two types of foodplants, depending on their habitat. At the Dorset heaths, they feed most commonly on heathers (Calluna vulgaris and species of Erica), while on Portland's limestone grasslands it is more likely to be Bird's-foot Trefoil, Common Rock-rose or Horseshoe Vetch. So far, so different.

What links the two habitats is the relationship between the caterpillars and ants - specifically black ants. Caterpillars produce sugar-rich liquids that attract and feed the ants; in return, the ants look after the caterpillars. It seems that the ants move the caterpillars into underground chambers, from where they emerge at dusk to feed (possibly carried by the ants). Eventually, the butterfly caterpillar will pupate underground, and an adult will emerge in turn. This all seems very benign, and I can't find any evidence of more sinister behaviour taking place, such as caterpillars feeding on the ant brood, as happens with the (even rarer) Large Blue

The ant-caterpillar relationship doesn't happen by chance. Adult female Silver-studded Blues can detect chemical signals (pheromones) produced by the ants, which they will follow to find suitable a site for egg laying. On Purbeck's Grange Heath a couple of weeks ago we were lucky to find a Silver-studded Blue surrounded by black ants:

Silver-studded Blue male with black ants

Nearby, a pair of Silver-studded Blues were mating: the 'silver' studs on the female (more easily seen here) are almost blue in colour. 

Silver-studded Blues mating - male above, female below

While the association with black ants takes place in both habitats, the species of ant differs. On acid heathland it's usually Lasius niger, while on limestone grassland it will be Lasius alienus. I don't know enough about ants to distinguish them in the field, but I presume that the ones photographed above on Grange Heath were L. niger. (If they aren't, please let me know!)

Conserving Silver-studded Blues

In Britain, the combination of recent population decline and restricted distribution of Silver-studded Blues mean that they are classed as 'vulnerable' on the UK Red List, although the species is much more widely distributed in continental Europe. The good news is that even in England and Wales they are usually present in good numbers on the sites where they occur. Even better, numbers have risen dramatically on sites in Purbeck such as Studland, although the picture is less encouraging for the grassland population on Portland (Warren et al., 2025).

On heathland, conservation management aims to maintain a patchwork, or mosaic, of different ages and types of heather as well as some bare ground. Suitable grazing and/or burning can achieve this. For grassland, it is important to manage grazing levels so they do not become too excessive; creation of bare ground areas is also recommended. Such actions will benefit other heathland species too.  

Finding Silver-studded Blues

The photos of Silver-studded Blues in this blog post were all taken on the Purbeck Heaths, which is where (in my experience) the butterflies are most easily found in Dorset. While I have seen them on Portland, numbers are lower than at Purbeck, and there is also a greater risk of confusing them with other blue butterflies. Chalk-hill Blue and the beautiful Adonis Blue are present on Portland's limestone grassland, but absent from Purbeck's acid heaths. The flight period runs from mid-June to the end of August, so now's the time to get out and look for them.  

Many of Purbeck's heathlands are accessible to the public, and most lie within the Purbeck Heaths National Nature Reserve (link here). At RSPB Arne (where I volunteer), Silver-studded Blues can be seen from the trails on Coombe and Hyde's Heaths.

 

Male Silver-studded Blue - Grange Heath

References

Asher, Jim, Warren, Martin, Fox, Richard, Harding, Paul, Jeffcoate, Gail and Jeffcoate, Stephen (2001). The Millennium Atlas of Butterflies in Britain and Ireland. Oxford: OUP.

Hetherington, Mike, Sterling, Phil and Coulthard, Emma (2022). 'Butterfly colonisation of a new chalkland road cutting.' Insect Conservation and Diversity, 15(2), pp.191-199.

Munns, Lorraine (2017). Is it all about the ants? What are the factors influencing the presence of Plebejus argus (the Silver-studded Blue butterfly) on Studland Peninsula? (Doctoral dissertation, Bournemouth University).

Sterling, Phil and Henwood, Barry (2020). Field guide to the caterpillars of Great Britain and Ireland. London: Bloomsbury.

Thomas, C.D. (1993). 'The silver-studded blue, Plebejus argus L.' The conservation biology of Lycaenidae (butterflies), pp.97-99. - link

Thomas, Jeremy, Surry, Richard, Shreeves, Bill and Steel, Carolyn (1998) New Atlas of Dorset Butterflies. DNHAS.

Tolman, Tom and Lewington, Richard (2008) Collins Butterfly Guide. London: HarperCollins. 

Warren, M.S., Rich, S. and Haywood, N. (2025). 'The dramatic rise of the Silver-studded Blue, Plebejus argus, over 50 years at Studland Heath National Nature Reserve, Dorset, UK' - link

Text and images (c) Mike Hetherington