The Return of the Large Tortoiseshell Butterfly to Kent: A success story in the making?


The Large Tortoiseshell butterfly (Nymphalis polychloros), once a familiar sight across the British countryside, was largely lost as a regular breeding species in the UK by the 1940s. The reasons for its decline are complex and not entirely understood, with theorised factors suggesting Dutch Elm disease, changing woodland management practices and over-predation by birds may have all played a role in it's demise. The bottom line is that no-one really knows, and just as they mysteriously disappeared, they appear to be making attempts at an equally mysterious, but welcome, return. In recent years, there has been a resurgence of sightings of this elusive and enigmatic butterfly, particularly in Kent, suggesting that this species may be on the brink of re-establishing itself as a breeding resident in the UK.

Sightings Across Kent: A Growing Trend
This year, more than 30 sightings of the Large Tortoiseshell have been recorded across Kent, with a notable concentration in the Orlestone Forest area. Additional sightings around Ashford and even as far afield as the Tonbridge area paint an encouraging picture of a species that might be re-colonising its former haunts.  Indeed, a caterpillar was photographed on Sallow at Blean Woods near Canterbury in June, the first confirmed Kentish larvae in decades and strong evidence of local breeding. There's also been a number of sightings in the last week or two from people's garages and sheds, where adults are undoubtedly looking to hibernate, and that's probably the most likely place you will find one now before next Spring. What makes this particularly noteworthy is that these sightings are not isolated incidents but part of a consistent pattern observed over the last three years. This persistence strongly suggests that the Large Tortoiseshell is not just passing through but may indeed be breeding and establishing new populations.
The recorded data points to a significant change in the butterfly's behavior and distribution. While some sceptics argue that these sightings could be the result of intentional releases, the consistency and widespread nature of these sightings, particularly in the spring and summer, imply a more natural resurgence. Indeed, if these butterflies were merely the product of releases, it begs the question of why we are only now observing them in greater numbers after years of supposed releases. The timing, frequency, and distribution of sightings seem to indicate a natural origin for at least a significant proportion of these butterflies. Of course, it is important to recognise releases of this species will still occur, but the wider picture is certainly looking more favourable for this species to gain a foothold.

Theories Behind the Resurgence
Several theories have been proposed to explain the return of the Large Tortoiseshell to Kent. One such theory is the possibility of a new spring migration from the Low Countries, where the butterfly’s population has been on the rise. Research by Will Langdon suggests that improved conditions in the Netherlands and Belgium could be leading to an increase in cross-Channel migrants, bolstering the British populations.
Climate change is another factor that cannot be ignored. As temperatures rise, many butterfly species are extending their ranges northward. Historically, this  was a species that was quite possibly always on the edge of it's range here in the British Isles, and the Large Tortoiseshell might be benefiting from the same climatic shifts that have enabled species like the Long-tailed Blue to establish themselves further north than ever before. Additionally, the warming climate might be improving the survival rates of overwintering butterflies, allowing them to thrive and reproduce in areas where they previously could not. It seems like we no longer have the severe winters that characterised the 40s and 60s, and as the climate continues to shift, so does the fortunes of this butterfly.
Another intriguing aspect of the Large Tortoiseshell's return involves changes in habitat management. Historically, the species' decline was partly attributed to the loss of elm trees due to Dutch Elm disease. Elms are a crucial food plant for the butterfly’s larvae, and their decline is thought to have significantly impacted the Large Tortoiseshell's breeding success. In recent years, there has been a slight recovery in elm populations, as evidenced by the expanding range of the White-letter Hairstreak, another elm-dependent species. This improvement in the availability of suitable breeding habitats could be a key factor in the Large Tortoiseshell’s comeback.
Similarly, the increase in sallow (willow) in woodlands, which provides essential food sources for adult butterflies emerging from hibernation in the spring, could also be aiding their survival. The conservation efforts aimed at increasing sallow in woodlands, where it is no longer viewed as a 'weed', have already benefited other species like the Purple Emperor, and the Large Tortoiseshell might be experiencing similar advantages.
Even changes in predator-prey dynamics could be playing a role. With more raptors present in the countryside, predation pressure on caterpillars by birds may have reduced, allowing more caterpillars to reach adulthood. These complex and interrelated factors suggest that the Large Tortoiseshell's resurgence is likely due to a combination of favourable conditions, rather than any single cause.

Historical Perspectives and Future Prospects
Historically, the Large Tortoiseshell was never (by some authors) regarded as a stable, long-term resident of the UK, but rather as a temporary resident, often appearing in cycles. These butterflies would migrate to the UK, establish breeding populations, and then, depending on various environmental factors, either die out locally or migrate again. The current situation might be a return to this historical pattern, where populations are sustained by a combination of local breeding and migrant individuals. This is somewhat analogous to the past status of the Red Admiral, which used to be primarily a migrant species but is now considered a resident due to its ability to survive milder winters.
The next few years will be crucial in determining whether the Large Tortoiseshell is on its way to becoming a permanent feature of our British butterfly fauna once again. Continuous monitoring and recording of sightings are essential to understanding the dynamics of this species' return.

Dan Hoare, the Director of Conservation for Butterfly Conservation recently noted-
'"It’s really exciting that the Large Tortoiseshell has been spotted in Kent, and it does seem like this species is being seen more frequently in the UK in recent years. A once widespread species, it was lost as a regular breeding species in the UK in the 1940s. While we don’t know the exact cause of its extinction, Dutch Elm disease, changing woodland management and climate are thought to be part of the issues it faced."
This excitement is tempered by the recognition that many butterflies are on the move due to climate change, with some species thriving while others struggle. Tracking these changes is vital for conservation efforts, and Butterfly Conservation encourages everyone to contribute by recording their sightings. Participating in initiatives like the Big Butterfly Count  (now closed for 2024, please upload any sightings to iRecord) can provide valuable data that helps us understand the changing fortunes of species like the Large Tortoiseshell.'

Looking Forward
The return of the Large Tortoiseshell to Kent is a promising development for butterfly conservation, indicating that with the right conditions, species thought to be lost can make a comeback. However, the work is far from over. Conservation efforts must continue to focus on habitat management, public awareness, and data collection to ensure that this resurgence is not a fleeting moment but the beginning of a long-term recovery. There are potential projects going forward that may assist the Large Tortoiseshell, including the planting of disease resistant elms, which will also benefit the White-Letter Hairstreak. 
As the summer season draws to a close, it is becoming more difficult to spot these butterflies, but enthusiasts and conservationists alike are encouraged to keep an eye out next spring. In many ways, it is far easier to see them in the Spring compared to the summer due to the difference in their behaviours. Large Tortoiseshells bask on dead tree trunks and feed on the flowers of sallow in the early March sunshine, so these are the places to watch. The males will set up territories and often been found within the same ride or clearing, even a particular tree, over multi-day periods. Once seen, they are never forgotten. I have had the privilege to watch one dual with fellow Nymphalidae, spiralling high into the ancient canopy yet to wake fully from Winter's grip , before elegantly gliding to rest on a mossy stump surrounded by primroses. It was a sight that undoubtedly occurred in the same woodland ride centuries before, and is possible once again in many woods across Kent.
While we cannot yet declare the Large Tortoiseshell a fully re-established species in the UK, the signs are certainly pointing in that direction. With continued observation, research, and conservation efforts, we may well see this enigmatic butterfly gracing our woodlands once again for generations to come.

William Malpas

Introduction to the Newsletter Editor
My name is Olivia Currie, and as of this year, I am the new branch newsletter editor. Although I live up in Glasgow, I can’t wait to learn more about the local moths and butterflies of Kent and SE London, and to contribute to the newsletter!
If members have any articles, news, stories or photos to provide for the newsletter, please don’t hesitate to send them to me ([email protected]). I hope to include as much as possible in the upcoming Adonis!
 


We would welcome your news, photos and sightings for our website
We would welcome members to submit any news, photos and sightings to update our branch website.  If you have anything you would like to submit please email Ben Kirby: [email protected]