Researchers puzzle over sea-crossing migration of crimson rose butterflies –

COLOMBO — Travel bloggers and butterfly enthusiasts Paulmathi Vinod and Vinod Sadasivan were at Dhanushkodi Beach in southern India’s Tamil Nadu state for some bird-watching on Feb. 14. It hadn’t occurred to them that it was Valentine’s Day, but the couple was in for a gift from nature: the spectacular sight of thousands of butterflies in flight. And, in a serendipitous fit with the Valentine’s theme, these butterflies had bright red bodies and striking red markings on their black wings.
These were crimson rose butterflies (Pachliopta hector), here to feed on the flowering plants in the coastal region. Then they rose up, in their thousands, and began flying toward the sea.
“One by one, the butterflies were using the coastline like a predetermined path and continued their journey toward the sea,” Paulmathi Vinod told Mongabay. “They were so focused on their journey that they were flying right into our faces to reach the flowers on the beach, as if to fortify themselves with nectar before takeoff.”
Across the sea and over the horizon, about 25 kilometers (15 miles) away, lies Sri Lanka’s Mannar Island, where butterfly enthusiast Lahiru Walpita is stationed. He’d recently heard about the aggregation of butterflies across the strait in India and was on alert. If the crimson roses crossed to Sri Lanka, they would show up in Mannar. But Walpita hadn’t observed any increase in the local population of crimson roses.
Butterfly migration
It was a different story in October last year, when Walpita observed large swarms of crimson roses coming from over the sea, passing his station as they headed toward the Sri Lankan mainland. On Oct. 25, he observed around 250 butterflies per minute, marking the peak of this swarm.
“I’ve been observing and counting the butterflies since 2019, and this is the highest [number] I’ve spotted,” Walpita told Mongabay.
Crimson roses belong to the swallowtail (Papilionidae) family of butterflies, They’re large, visually striking butterflies that lay their eggs on the Indian birthwort plant (Aristolochia indica) or similar plants that contain a toxic substance. The butterfly larvae sequester this toxin, making the adult butterfly inedible to would-be predators. The high-contrast red markings on the black wings are meant to advertise this toxicity — nature’s take on a bright red “Danger” sign.
Animal migrations are typically a round trip; think birds flying to warmer climates for the winter and then back again, or wildebeest trekking between the Serengeti and the Masai Mara in search of fresh grazing. Insects, too, are known to migrate along fixed routes to a given destination, with the monarch butterfly probably the best-known example. But for butterflies like the crimson rose, migration is less a trip from A to B to A again, and more a meandering flight to the next best breeding spot.
The arrival of the first rains after a long dry spell often signals a boom in the butterfly population in a given area, as conditions are just right for breeding, said Michael Van der Poorten, an expert on Sri Lankan butterflies. As the population builds up to a critical mass, the butterflies begin to disperse en masse; some species, like the crimson rose, show a stronger tendency to migrate than others.
Southern India experienced rain after a dry period in early February, which may have triggered the recent crimson rose boom, said Appavu Pavendhan, founder of The Nature and Butterfly Society (TNBS) based in Tamil Nadu. But that doesn’t explain the migration observed in October in Sri Lanka, which calls for more studies, Pavendan told Mongabay.
 Need for more research
Pavendan’s interest in the crimson rose migration goes back to 2018, when observers reported seeing thousands of the butterflies winging it across the ocean toward Mannar. From India, he started sharing his observations with fellow butterfly enthusiasts in Sri Lanka. Judging from the timing of the migrations, they concluded the butterflies were taking advantage of the monsoonal winds to glide away, Pavendan said. He added that could explain why, in 2016, when the monsoon failed, no butterfly migration was detected.
Himesh Jayasinghe, founder of the Butterfly Conservation Society of Sri Lanka (BCSSL), said that what appears to be true is that the crimson rose butterfly disperses widely rather than migrates in a particular direction. He said there’s a need for detailed field studies to understand the phenomena, adding that citizen scientists in both India and Sri Lanka can contribute to solving the puzzle.
Researchers have long known about the crimson rose’s seagoing journeys; the phenomenon is recorded in The Butterflies of Ceylon by Walter Ormiston in 1924, and The Butterfly Fauna of Ceylon by L.G. Ollyet Woodhouse in 1942, which describe crimson rose swarms flying “far out to sea.”
Other butterfly species in the region are also known to migrate with the monsoon. Several migration patterns have been observed between the Eastern Ghats and the Western Ghats, the mountain ranges that run down either flank of the Indian subcontinent. In Sri Lanka, butterfly migrations in the hinterland have been recorded since ancient times.
Up until the mid-1980s, thousands-strong swarms of emigrant (Catopsilia pomona) and albatross (Appias spp.) butterflies would dot the sky around Adam’s Peak, one of the highest points in Sri Lanka. This migration, peaking around March, coincided with the pilgrimage season to the sacred peak, bringing together devotees of two kinds: people and butterflies. Buddhist texts give detailed accounts of butterflies visiting the mountaintop, which is called Samanala Kanda in the Sinhala language, or “Butterfly Mountain.”
But while the Peak Wilderness Sanctuary that encloses the mountain is a key biodiversity hotspot in Sri Lanka, and a UNESCO World Heritage Site, encounters with these transient butterflies are becoming less common as their populations decline.
For the crimson rose, a much more abundant species, the wandering will continue, says Rajika Gamage, author of An Illustrated Field Guide to the Fauna of Sri Lanka: Butterflies. When there are host plants and nectar plants available, butterflies usually prefer to stay close to the place where they were born. But this isn’t the case for crimson rose, which takes wing the day after it abandons its cozy cocoon.
Banner image of a fully grown crimson rose, a large red-bodied swallowtail butterfly, courtesy of Lahiru walpita.

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