Understanding Factors Affecting Decline of Samoan Swallowtail Butterfly

Science Center Objects

The Samoan swallowtail is a large and strikingly marked butterfly endemic to the Samoan Archipelago. Once widespread and common, its populations have declined dramatically, and it now appears restricted to the island of Tutuila, an area representing approximately 5% of its former range. There are few insects that are commonly thought of as indicators of ecosystem health, but the Samoan swallowtail represents a possible candidate for this role in an oceanic island ecosystem where forests are generally dominated by native species and relatively few threats from invasive species have been identified. The species also could serve as a model for valuing and conserving butterflies in the Polynesian-Micronesia biodiversity hotspot. 

Samoan swallowtail

Samoan swallowtail butterfly. Photo: M. Schmaedick

Overview:

The Samoan swallowtail (Papilio godeffroyi) is a large and strikingly marked butterfly endemic to the Samoan Archipelago. Once widespread and common, its populations have declined dramatically, and it now appears restricted to the island of Tutuila, an area representing approximately 5% of its former range. There are few insects that are commonly thought of as indicators of ecosystem health, but the Samoan swallowtail represents a possible candidate for this role in an oceanic island ecosystem where forests are generally dominated by native species and relatively few threats from invasive species have been identified. The species also could serve as a model for valuing and conserving butterflies in the Polynesian-Micronesia biodiversity hotspot. The National Park of American Samoa (NPSA) protects much of the intact rainforest habitat of the butterfly, but managers are hampered in developing conservation strategies by a lack of information about its basic life history and the factors that are driving its decline. To fill this need, our study will identify and map distributions of host plants of the Samoan swallowtail butterfly within NPSA, determine environmental factors that influence distributions of both larvae and their host plants, quantify the influence of parasitoid enemies on egg, larval and pupal stages, and develop strategies for enhancing populations. This study also will provide park managers and interpreters with a rare opportunity to understand the role of native insects in the structure, function, and health of remote island ecosystems and to connect local people with a sometimes overlooked element of their natural heritage.

Intermediate instar Samoan swallowtail caterpillar

Intermediate instar Samoan swallowtail caterpillar. Photo: R. Peck

Project Objectives:

The primary objectives are to identify environmental factors that affect the Samoan swallowtail and to provide NPSA managers with information that will help them protect and enhance their populations. As the first ecological study of this increasingly rare butterfly, results will increase the knowledge and appreciation of a charismatic Samoan insect. The study will identify host plants used by the butterfly to evaluate the extent to which their availability may limit populations and to provide a sound basis for monitoring. Habitat variables that may influence the distribution of the butterfly and its host plants will be evaluated to understand factors limiting the species at the landscape level, identify management opportunities, and facilitate modeling the potential range of the species. Identifying parasitoids of the butterfly and determining rates of parasitism will provide baselines for monitoring changes in the parasitoid community and in species that pose the greatest threats as well as for biosecurity screening and interdiction. Results will provide guidance to managers on ways to monitor and possibly enhance populations of the Samoan swallowtail and its host plant(s).

Researchers searching for Samoan swallowtail caterpillars

Searching for Papilio eggs on host plant talafalu (Micromelum minutum). Photo: P. Banko

Study goals are aligned with priorities of the USGS Ecosystems Science Strategy in that research will be conducted on the abundance, distribution, productivity, and health of a rare species in order to aid its recovery. As a rare, declining species, the Samoan swallowtail is a likely candidate for federal protection, and information about changes in its habitat associations will inform resource management and agency decision-making. Key research elements also focus on identifying threats and ecological characteristics of invasive species that may impact the butterfly as well as on developing measures for their control. The research also evaluates environmental factors that will potentially aid managers in assessing tradeoffs involved in conservation and land use strategies. NPSA managers must protect natural resources while also accommodating traditional agricultural practices, and this study is designed to provide them with a scientific basis for designing strategies to protect, sustain, and restore the ecosystem functions and services of habitats harboring the Samoan swallowtail.

Samoan swallowtail pupa

Samoan swallowtail pupa. Photo: R. Peck

Highlights and Key Findings:

We assessed abundances of Papilio eggs, larvae, and pupae on 117 trees at 10 sites for 9–15 months during 2013–2014. Eggs comprised 90% of all detections, followed by larvae (6%), and pupae (4%). Tree use was widespread with 89% of all trees yielding Papilio at least once, but only 10% of the trees yielded Papilio frequently (≥90% of the time). Egg abundance was positively related to leaf biomass of individual trees but not of the whole stand, suggesting that other factors may be affecting Papilio distributions. In the lab, we found no evidence for parasitism of larvae or pupae, but 81% of all viable eggs (302/372) were parasitized by an unidentified species of encyrtid wasp of unknown origin. Parasitism rates differed little among sites (range = 53–68%) suggesting this impact was similar over the landscape. Other natural enemies (e.g., pathogens and predatory geckos, ants, and birds) were not evaluated, but they likely also influence Papilio mortality rates and may have contributed to its decline across the archipelago.