Invasive Large White Butterfly Reaches Remote Easter Island, Raising Ecological Alarm

Rapa Nui, the remote Polynesian island better known as Easter Island, has a new resident, and it is one that farmers and conservationists should watch closely. Researchers have confirmed the first detection of the Large White butterfly, Pieris brassicae, on the island, documenting every stage of its life cycle from eggs to adults. The finding, published in the journal Nota Lepidopterologica, marks the arrival of a Palaearctic species that has already proven its ability to colonize distant continents and oceanic islands. Because butterflies are locally referred to as Pepeka, the discovery quickly resonates with islanders who know these insects as familiar parts of the landscape, even when the species itself is entirely new.

The Large White butterfly is native to the Palaearctic region, but its association with humans has carried it far beyond that range. As its common name suggests, the species feeds primarily on plants of the mustard family, Brassicaceae, which includes cabbage, cauliflower, broccoli and Brussels sprouts, all derived from Brassica oleracea. Newly hatched caterpillars are voracious and gregarious, capable of completely defoliating a cabbage leaf while leaving only the veins intact. In its native range, the population is kept in check by parasitic wasps such as Cotesia glomerata and Pteromalus puparum, natural enemies that may not exist on Rapa Nui, raising the possibility that the butterfly could reach pest proportions without biological control.

The species has a well-documented history of invasion. It was first recorded in South America in the Chilean city of Viña del Mar in November 1970, with additional occurrences near Valparaíso a few months later. By 1972, the butterfly had reached Santiago, roughly 100 kilometers inland from the original record. Chilean entomologist Luis Peña suggested that the founding population came from Eastern Europe, though the exact origin remains uncertain. Since that initial introduction, the species has expanded north into the Antofagasta region and south into the Los Lagos region, demonstrating its capacity to adapt to a wide range of climatic conditions across the Chilean territory.

The detection on Rapa Nui was made through a combination of field observation and citizen science. Researchers found and photographed specimens in the field, and two additional records were retrieved from the platform iNaturalist. Taxonomic identification followed established references for the species. The records span all life stages: eggs were found on cabbage plants on 19 August 2021, caterpillars were observed feeding on Tropaeolum majus on 15 July 2021, a pupa was documented on 20 July 2021, and adults were recorded on cabbage on 19 August 2021, nectaring on Lantana camara flowers on 15 July 2021, and again on 10 August 2021. Two independent iNaturalist records of adults on the island date from 21 May 2021 and 1 October 2022, confirming that the species is established and not merely a transient visitor.

Distinguishing Pieris brassicae from the closely related Pieris rapae, the Small White, is critical for understanding the scope of this invasion. Pieris rapae is originally from Europe and has been introduced to every continent except South America and Antarctica, making it one of the most widespread butterflies in the world. However, the Large White can be differentiated by the extent of dark pigmentation on the forewings, as well as by the very different morphology and gregarious behavior of its caterpillars. The caterpillars of P. brassicae feed in groups, a trait that makes them conspicuous and potentially more damaging to crops than solitary species.

The most likely route of introduction to Rapa Nui is from mainland Chile, given the constant movement of goods and produce via air and sea transportation. Commercial flights to the island normally operate one or two times per day, increasing to three during exceptionally high travel seasons. Between 2020 and 2021, commercial flights almost completely stopped due to the COVID pandemic, but chartered flights with goods from the continent continued. The island is also supplied by ships, with three regular arrivals every month and a half, and a fourth sporadically. These two routes represent the most probable entry mechanisms for the species, though it is difficult to ascertain which one was responsible. Tahiti in the Society Islands is the other regular air connection to the island, but the species has not been reported in French Polynesia, making a Tahitian origin less likely.

A similar introduction mechanism could explain an earlier record of the species in a patch of native forest on Robinson Crusoe Island, another volcanic island belonging to the Juan Fernández archipelago off the coast of central Chile, where a specimen was collected on 20 January 1996. That record, held in the Museo de Zoología de la Universidad de Concepción, suggests that the Large White has a pattern of reaching Chilean oceanic islands through human-mediated transport, reinforcing the hypothesis that Rapa Nui’s colonization followed the same pathway.

Several morphological and behavioral traits of the Rapa Nui population are congruent with what has been previously reported from mainland Chile. Gardiner described a dark green coloration on the underside of the wings of individuals from the Chilean population, and that trait was also found in the butterflies recorded from Rapa Nui. The coloration of the eggs, caterpillars and pupa, as well as the gregarious behavior of the caterpillars, are also consistent with Chilean populations. However, the researchers emphasize that a hypothesis of origin from continental Chile remains to be tested using molecular tools, which would provide definitive evidence of the colonization route.

One of the most intriguing aspects of the Rapa Nui population is its phenology. A summer diapause has been described in populations of the southwest distribution of the native range of P. brassicae in the Iberian Peninsula, and for the introduced population in Chile. In Chile, this local adaptation has been associated with a physiological strategy to avoid the dry summers of the Mediterranean climate, where food sources become scarce. This summer diapause is followed by an increase in adult abundance that peaks in May before the onset of winter. In the first record of the species in Chile, there is a reference to an increase in population density towards May and an abrupt collapse in June as the southern winter begins. In Rapa Nui, by contrast, the butterfly appears to have retained the summer diapause but maintains continuous activity throughout the winter months, suggesting that the island’s milder climate allows for year-round breeding.

Regarding natural enemies, a potential braconid parasitoid from the genus Apanteles has been recorded on Rapa Nui, which means the butterfly’s populations might be controlled by a parasitoid species already present on the island. However, there is no evidence yet of that interaction actually occurring. This record is relevant as a baseline for monitoring the species, assessing its impact on the local biodiversity of Rapa Nui and evaluating its management strategy. The Large White was successfully eradicated from New Zealand six and a half years after its original detection, following four years of an active integrated pest eradication strategy. Early detection of introduced arthropod species into oceanic islands is key to preventing larger ecological and economic damage, as well as minimizing management costs, and this discovery on Rapa Nui provides exactly that opportunity.

Subject of Research: First detection of the invasive Large White butterfly Pieris brassicae on Rapa Nui (Easter Island), documenting all life stages and a likely introduction from mainland Chile.

Article Title: Detection of the Large White butterfly Pieris brassicae (Linnaeus, 1758) ( Lepidoptera , Pieridae ) in Rapa Nui (Easter Island)

Article References: Cotoras, D. D., & Yancovic Pakarati, S. (2023). Detection of the Large White butterfly Pieris brassicae (Linnaeus, 1758) (Lepidoptera, Pieridae) in Rapa Nui (Easter Island). Nota Lepidopterologica, 46, 31-35. https://doi.org/10.3897/nl.46.98421

Image Credits: AI Generated

DOI: 10.3897/nl.46.98421

Keywords: Pieris brassicae, Large White butterfly, Rapa Nui, Easter Island, invasive species, Lepidoptera, Pieridae, crop pest, oceanic islands, Chile, biological invasion, caterpillar