Southern Damselfly: A Tiny Blue Treasure of East Devon
The Southern Damselfly (Coenagrion mercuriale) may be the smallest of the UK’s blue damselflies, but it carries an outsized conservation importance. Males bear the delicate ☿ marking on the abdomen and the characteristic Coenagrion spur on the thorax — features that give the species its scientific name.
In practice, though, no one is expected to spot these on a matchstick‑sized insect! Field identification relies far more on behaviour, habitat and comparison with similar species.
Southern Damselfly — weak, erratic flight, hugging narrow runnels
Common Blue Damselfly — paler, stronger flier, often over open water
Azure Dasmelfly — similar but generally brighter and more widespread
Adults fly mid‑May to July, while larvae spend two years underwater in shallow, base‑rich, slow‑flowing streams.
The species is found across southern and western Europe and North Africa, but the UK sits at the very northern edge of its range — and remarkably supports up to a quarter of the global population. That makes the species’ decline all the more concerning. It is listed as Near Threatened in Europe and globally, and Endangered in Britain. Across Europe it has vanished, or nearly vanished, from seven countries.
In the UK, the Southern Damselfly survives in scattered pockets: Hampshire, Dorset, Devon, Oxfordshire, the Gower Peninsula and Anglesey. Devon holds seven colonies, three of which lie within the East Devon Pebblebed Heaths NNR — Aylesbeare, Venn Ottery and Colaton Raleigh. Though the latter has dwindled to only tens of individuals in recent years, a stark reminder of how fragile these populations are.
Why this species matters
The Southern Damselfly is one of Devon’s Priority Species, a list of 1,600 rare or threatened species compiled in 2017. Of these, just 96 were identified as species for which Devon has a particular responsibility, and the Southern Damselfly is one of them. Its presence is also a powerful indicator of habitat quality. Most of its sites are Sites of Scientific Interest (SSSI), and the species depends on clean, oxygen‑rich water. Where Southern Damselflies thrive, water quality is high; where they disappear, nutrient enrichment is often to blame. Restoring habitat for this species therefore contributes directly to improving SSSI condition, supporting the Colchester Declaration’s ambition for 200,000 hectares of SSSIs in favourable condition by 2030.
It is also a species with real public appeal. Its bright blue colour and delicate flight make it a natural ambassador for wetland conservation — my own children believe they are “fairies”. Volunteers already play a central role in monitoring and habitat management across the East Devon Pebblebed Heaths, and the Southern Damselfly offers a perfect focus for community engagement.
Although the three Pebblebed colonies lie relatively close together, Southern Damselflies disperse only around 50 metres within their lifetime, meaning they cannot easily exchange genes. Even so, working for this species encourages the kind of landscape‑scale collaboration — between landowners, graziers and conservation bodies — that benefits far more than a single insect.
Habitat priorities
Southern Damselflies require a very specific set of conditions: base‑rich, shallow water with a permanent, slow‑to‑moderate flow; warm temperatures that do not drop below freezing; and open, unshaded runnels with short vegetation. In the UK, they are most often found in heathland streams and valley mire runnels, though a few populations survive in chalk rivers.
The species is intolerant of drying out, freezing, heavy shading or nutrient enrichment. A layer of silt or peat on the stream bed provides essential larval habitat. These conditions do not arise by accident. They are the product of landscapes shaped over centuries by grazing animals. Grazing slows water flow through poaching, keeps vegetation low, prevents scrub encroachment and maintains warm, sunlit channels. Without grazing, runnels quickly become shaded, canalised or choked with tall vegetation — all of which render them unsuitable for Southern Damselflies.
Restoring and managing our sites
The most effective and sustainable management for Southern Damselflies is the continuation of appropriate grazing, which maintains the open, varied structure of runnels and surrounding vegetation. Aylesbeare Common provides a striking example. In the late 1990s, runnels that once supported the species had become overgrown with Black Bog‑rush and Purple Moor‑grass. A short period of intensive winter grazing — 17 Devon Reds for two weeks — reopened the runnels and restored the shallow, sunlit habitat the species needs. The population recovered from single figures to a stable population in the hundreds, now maintained by lighter grazing from ponies and cattle.
Where habitat has deteriorated more severely, manual intervention may be needed. Re‑profiling channels can help restore natural flow and bank structure, but this must be done cautiously. Excavation can harm existing populations and should only be undertaken where Southern Damselflies are absent, or in very small sections of occupied sites. Similarly, careful scrub removal can reduce shading and improve water temperature, but wholesale clearance would be counterproductive.
Venn Ottery offers a hopeful example of restoration. After intrusive ditching and reduced grazing caused the species to go extinct there in 1989, small holes were cut into an artificial ditch, allowing water to spill downslope and form a network of natural, braided runnels. A reintroduction followed — and succeeded.
Monitoring and ongoing challenges
Understanding how populations respond to management requires regular monitoring. Adult surveys involve walking fixed transects and counting males on the wing, while larval surveys — which require a licence — can be carried out year‑round. Habitat monitoring runs alongside these surveys, checking vegetation structure, shading and signs of nutrient enrichment. Special mention to super-volunteer Lesley Kerry, who has monitored these damselflies and their lowland heath habitat for many years.
Delivering conservation for this species is not without challenges. The sites within the East Devon Pebblebed Heaths are owned and managed by multiple organisations - the Pebblebed Heaths Conservation Trust, RSPB and Devon Wildlife Trust - so success depends on strong partnership working. Many sites require SSSI consent for habitat work, and because the species disperses so poorly, only small sections of habitat can be managed in any given year. Water quality remains a critical issue. Even perfect habitat cannot support Southern Damselflies if upstream nutrient inputs degrade water chemistry. At Colaton Raleigh, numbers have fallen sharply over the past six years, and water quality or pH may be contributing factors that require further investigation.
A species worth the effort
The Southern Damselfly is tiny, delicate and easily overlooked, yet Devon holds a globally significant share of its population. Its survival depends on clean water, open runnels and the continuation of traditional grazing — all of which support a far wider range of wildlife. By working together across the landscape, we can ensure that this little blue jewel continues to shimmer over East Devon’s wetlands for generations to come.
Useful tips on identification can be found here: Southern Damselfly - British Dragonfly Society (british-dragonflies.org.uk)