Papilio machaon britannicus – Martin Partridge
The British Swallowtail Papilio machaon britannicus is a rare subspecies of Swallowtail now confined to the Norfolk Broads, an extensive area of wetlands in East Anglia in the UK. Its caterpillars feed solely on Milk-parsley, which grows in the fens and marshes of the Broads. The only species of swallowtail in the UK, the British Swallowtail is also the UK’s largest butterfly, and one of its most spectacular.
However, there are worries that numbers of the British Swallowtail this year may once again be low following much-reduced populations in 2023. With just 16 breeding sites in the Broads, down from 22 just a few years ago, abundance is now falling too. Transect counts for the UK Butterfly Monitoring Scheme in 2023 recorded just 81 swallowtails, down from 138 the previous summer and 210 in 2020. The British Swallowtail’s best site is Sutton Fen, an RSPB reserve that’s closed to the public because of its fragility and rare plants. In 2023, its second-worst year since counting began at Sutton, there were only 61 individual butterflies counted along the regular transect.
A combination of flooding, drought and salination is making life unpredictable for both the swallowtail and its foodplant, Milk-parsley. Milk-parsley was killed by a fungal wilt at Wheatfen Nature Reserve in 2022, an event possibly linked to the intrusion of salt water during storms. Thankfully this disease has not recurred or spread, but it’s another sign of vulnerability in the unstable Broads landscape.
Last winter flooding in the Broads was deep and persistent. This may have affected the British Swallowtail’s pupae, resting as they do on reed stems not far above normal water levels. Climate warming is taking its toll, and, unlike dragonflies, which are spreading northwards in the UK, the British Swallowtail has no suitable habitat within reach of the Broads.
A whole genome study of Papilio machaon britannicus is planned to help us understand more about this rare subspecies and to help focus conservation efforts, and SBBT are exploring the possibility of creating suitable habitat in less vulnerable areas to introduce the species to. However, the future of this enigmatic species remains uncertain.
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