Swallowtail Butterfly

Article written by Aldina Franco and Sarah Sheehan

The sight of the Swallowtail butterfly in the Broads National Park is a highlight of summer. The Swallowtail Papilio machaon britannicus is our largest native butterfly and its characteristic yellow and black body contrasts, wing tail and hopping flight can enthuse even the least attentive observer.

The British race of Swallowtail feeds only on Milk Parsley (Peucedanum palustre), while individuals arriving from the Continent, race gorganus, use a variety of foodplants and are relatively abundant there. The britannicus Swallowtails are currently restricted to East Norfolk, where the butterfly and its host plant are threatened by sea-level rise and habitat loss.

In the Broads, several nature reserves implement habitat management to help the Swallowtail and its host plant, but the species range has declined since the 1970s, despite conservation work. Given the specificity of host plant, it makes sense to establish Milk Parsley in more nature reserves, protected from sea-level rise. Translocation experiments with Milk Parsley and Swallowtails have had limited success, but recent research will hopefully aid future conservation efforts.

New Findings

In 2019, two MSc students from the Master’s in Applied Ecology and Conservation at the University of East Anglia investigated the choice of host plant for the britannicus Swallowtail and examined the impact of climate change. In 2021 another MSc project investigated the factors that influence host plant growth and occurrence at local level, a project proposed by Butterfly Conservation’s Norfolk Branch and supported by several local nature reserves. The main findings of the three research projects (Lillywhite 2019, Hills 2019 and Sheehan 2021) could help guide how we support Swallowtails.

Hills 2019 explored the factors that influence egg-laying preferences, to aid in the provision of suitable host plants in future translocation projects. Milk Parsley plants were surveyed across five sites in Norfolk.

The main predictors of Swallowtail egg-presence were the percentage of bare ground, a proxy for vegetation density, with eggs found more commonly in less dense vegetation, closer to water and further from paths. However, the largest plants were more popular with females and a larger number of eggs per plant were found in larger plants further from water and paths, suggesting that females prefer larger plants.

Swallowtail Caterpillar
Swallowtail Caterpillar - Photo by Gilles San Martin.

Growing Pains

The limited distribution of Milk Parsley and poor plant growth risks further fragmentation of the britannicus Swallowtail populations and may be a factor in preventing them moving inland (Collins et al, 2020).

Milk Parsley is scarce, as indicated by host plant mapping and field observations (eg eight final-stage instar caterpillars were observed on one single plant in summer 2021), and conditions may not be improving due to external factors, despite continued efforts. Lillywhite 2019 has shown that the Norfolk locations where Milk Parsley is present are at significant risk of flooding from sea-level rise.

When examining impacts of two climate change scenarios (average temperature kept below 2˚C by 2100 and worst-case scenario), of the five study sites, Wheatfen Nature Reserve will be the least at risk from sea-level rise, losing 10% and 80% of its area respectively, while Hickling Broad will be most at risk, losing 83% and 99% of its area respectively.

The threat of sea-level rise will both threaten existing Swallowtail breeding sites and also bring increased salinity to these previously suitable areas, with saltwater ingress inhibiting both plant germination and growth.

Goldilocks habitats

Milk Parsley prefers slightly acidic to neutral soil pH, tall surrounding vegetation height, low soil conductivity and high surrounding percentage of bare ground. It is tolerant to a range of conditions, but levels of nitrate, phosphate, moisture and organic matter proved particularly influential for plant strength (Sheehan 2021). Excess salinity due to flooding at the most coastal locations was correlated with the weakest plants.

These results can help identify suitable sites for Milk Parsley further inland, in sites protected from future sea-level rise, where the britannicus Swallowtail may have higher chances of persisting in the long term.

Authors of this article

Aldina Franco is Associate Professor at the School of Environmental Sciences, University of East Anglia. Sarah Sheehan is an MSc Applied Ecology and Conservation student at the University of East Anglia.


References

  • Collins, NM, Barkham, PJ, Blencowe, M, Brazil, A, Kelly, A, Oldfield, S., Strudwick, T., Vane-Wright, R.I. and Stewart, AJ, 2020. Ecology and conservation of the British Swallowtail butterfly, Papilio machaon britannicus: old questions, new challenges and potential opportunities. Insect Conservation and Diversity, 13(1).
  • Hills, SR, 2019. Biotic and abiotic factors influencing parental plant choice for oviposition by the British Swallowtail butterfly, Papilio machaon britannicus, and the potential future management implications. University of East Anglia.
  • Lillywhite, CD, 2019. Environmental variables influencing the growth of milk parsley, Peucedanum palustre: implications for the species under future climate scenarios and the consequences for the British swallowtail butterfly Papilio machaon britannicus. University of East Anglia.
  • Sheehan, SE, 2021. Examining the Environmental Variables Impacting Milk Parsley (Peucedanum palustre) Occurrence in Norfolk and Identifying the Optimal Conditions for Translocation. University of East Anglia.