More than 500 insects are to be released on to the fen, owned by the National Trust, by Countryside Wildflowers, a Cambridgeshire company. Conservationists are confident the reintroduction will succeed where other efforts have failed.
The butterfly, which has a wingspan of up to four inches, became extinct on the fen in 1953 though it exists in three small areas in Norfolk. In 1970, Marney Hall and Dr Jack Dempster, botanists working for the Institute of Terrestrial Ecology, began their study, concentrating on the ecology that was failing to support repeated releases of the butterflies. They found that since the Second World War, drainage had shrunk the fen to a mere 12 acres. The milk parsley on which the swallowtail feeds and lays its eggs had dwindled in size and numbers.
In 1988 the National Trust and the National Rivers Authority installed a 12ft plastic membrane around the fen, causing the water table to rise and traditional fen vegetation such as milk parsley to thrive. Conditions are now ready for the swallowtail's return and the butterfly is being bred under controlled conditions in preparation.
The study methods used have since been applied to other rare breeds such as the heath fritillary, one of only two butterflies considered rarer than the swallowtail, the black hairstreak and the white admiral.