The future of the Swallowtail butterfly (Papilio machaon) in the United Kingdom: a review and call for new conservation strategies

The Swallowtail butterfly, native to the United Kingdom (Papilio machaon subspecies britannicus), is found almost exclusively in the Norfolk Broads, where it has been isolated for nearly 50 years. In areas where it is holding out, threats to its ecosystem have been identified. Without conservation efforts, the butterfly may become extinct. Previous conservation efforts, based on the creation of fenland supporting Milk Parsley (Peucedanum palustre) as a larval food plant, have proved unsuccessful. In Europe, the butterfly is widespread, able to exploit different non-fenland habitats and feed on different umbelliferous (Apiaceae) and Rutacea plants, which are also common in the UK. This review examines the threats to the Swallowtail in the UK, the butterfly’s ecological requirements and issues with previous conservation attempts including its legally protected status. Database observational analysis demonstrates bivoltinism further north than previously described and multiple broods in the south of Continental Europe. An explanation of the different appearance and behaviour in ssp. britannicus is provided. The reliance on the current conservation paradigms and contention that the British subspecies is “special” with an exclusive diet of Milk Parsley in Fenland is risky and may contribute to the butterflies’ ultimate demise. This paper proposes that different conservation strategies raising ssp. britannicus larvae on alternative foodplants, subspecies crossbreeding to confer foodplant exploitation and genetic advantages, or introduction of ssp. gorganus (either through natural migration or deliberate release) are considered and should be employed to ensure the future survival of P. machaon as a British butterfly species. Current conservation strategies in the United Kingdom reliant on a fenland habitat and a rare and threatened plant species severely risks the future of the Swallowtail butterfly. This review demonstrates that different approaches are possible, simple, and should be considered to enable the Swallowtail butterfly to expand its range throughout the country.