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Over the past few days, swarms of white and yellow butterflies have been drifting across Sri Lankan towns, from Ratnapura and Galle to the heart of Colombo, all heading the same way. Videos of the swarms spread fast on social media, and so did the questions. Where are they going? Is it a warning of something worse? Some even linked it to the old belief that butterflies fly off to worship at Sri Pada.
We spoke to experts and went through the research to explain what is really happening. Here are the posts that got people talking.
Posts shared on social media showed large swarms of butterflies in Ratnapura, as well as in Galle, Colombo and many other parts of the island, with users asking what was behind it.
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Large numbers of butterflies have been seen gathering in various parts of Sri Lanka, migrating in swarms in a single direction for some unknown reason.
Monsoon Rains and Host Plants Drive the Migration
These butterfly migrations are a natural, cyclical event that takes place within the island, driven by monsoon rains and the growth of host plants. Research shows that the birth and population growth of Sri Lankan butterflies are closely tied to rainfall patterns. A field study by the Department of Zoology at the University of Peradeniya identified two main migration seasons: the main migration during the Second Inter-Monsoon (October to December) and a smaller one during the First Inter-Monsoon (March to April).
During the wet season, plants grow quickly and put out fresh leaves. That means plenty of food for caterpillars, and the adult butterfly population shoots up. Butterfly expert Himesh Jayasinghe pointed out that the Southwest, Northeast and inter-monsoon rains trigger the hatching of butterfly eggs. Once the rain arrives, host plants sprout new growth, creating the right conditions for butterflies to lay eggs and feed. To avoid competing for the same resources in one place, and to ensure their survival, the butterflies disperse and migrate. Read more here and here .
Albatross and Emigrant Butterflies Fly from Kumana Towards Wilpattu
White and yellow butterflies of the Pieridae family lead this migration. A study by Prof. Kapila Dissanayake and a team of researchers from Sabaragamuwa University, published in the Taprobanica Journal (2024) ( Tracking the migration of Albatross butterflies in Sri Lanka ), found that the Lesser Albatross ( Appias galene ), the Common Albatross ( Appias albina ) and the Lemon Emigrant ( Catopsilia pomona ) migrate in this way.
The researchers collected data from 60 locations across the island and identified three main flight paths. The main direction was from the southeast to the northwest. The migration was observed starting from Kebiliththa in Kumana National Park and heading towards Wilpattu. Albatross butterflies fly at an average speed of 11 km per hour, so the 280 km journey from Kumana to Wilpattu takes them three to four days.
Experts noted that the host plant of Albatross butterflies is the ‘Weera’ tree ( Drypetes sepiaria ), which is common in the dry zone. Emigrant species lay their eggs on ‘Thora’ plants ( Cassia species) and ‘Ehela’ trees. Read more here .
The Sri Pada Legend Has a Scientific Basis in ‘Hill-Topping’
Research shows there is a scientific basis to the traditional belief that butterflies go on a pilgrimage to Sri Pada. Butterflies do not migrate along river valleys. Instead, they sense changes in elevation and temperature. They target tall landmarks such as Kirigalpoththa and Sri Pada (Adam’s Peak) and gather on the summits. This behaviour, known as ‘hill-topping’, helps them find mates easily.
Historically, large swarms of Emigrant and Albatross butterflies were seen in the skies around Sri Pada between December and March. Since this coincided with the Sri Pada pilgrimage season, the folk belief that butterflies go to worship at Sri Pada took hold. However, current field studies show that most Albatross butterflies migrate towards the northwest, not towards Sri Pada. Read more here and here .
While most migrations in Sri Lanka happen within the island, research has revealed that some butterflies also cross the sea through the Gulf of Mannar between India and Sri Lanka. In particular, Crimson Rose butterflies ( Pachliopta hector ) were confirmed to cross from Dhanushkodi in India to Mannar in Sri Lanka. They ride the monsoon wind currents during sudden population booms triggered by rain after a long dry spell.
No Scientific Evidence That Butterfly Swarms Predict Disasters
A belief has grown in society that a sudden mass migration of butterflies is an omen of a major natural disaster, such as a cyclone. There is no scientific evidence for this. Insects are, however, highly sensitive to changes in atmospheric pressure, sudden temperature shifts and wind patterns.
Experts noted that sudden changes in monsoon winds and rainfall can affect butterfly activity and flight. So the migration could be a response to an upcoming shift in the weather, but it cannot be officially treated as a disaster warning. Butterflies and other animals do not give official early warnings of disasters. Still, they are extremely sensitive to drops in atmospheric pressure, relative humidity and sudden changes in wind direction.
Changes in wind currents caused by depressions or monsoon activity influence the direction butterflies fly in. That is why they are considered an important bio-indicator for spotting changes in land ecosystems. Read more here .
Climate Change and Invasive Plants Are Shrinking Butterfly Numbers
Scientific observations show that butterfly migrations and populations have dropped noticeably compared to the past. Data indicate that irregular monsoon rains caused by global climate change can make butterfly eggs hatch too early. If there are not enough host plants at that point, the next generation is at risk of being wiped out.
The spread of invasive plants such as Guinea grass ( Panicum maximum ) and ‘Gandapana’ ( Lantana camara ) has also destroyed native ‘Thora’ plants. This has sharply reduced the populations of migratory species such as the Mottled Emigrant. Read more here and here .
Butterfly Conservation Society: An Internal Migration, Not Visitors from Abroad
We spoke to the former president of the Butterfly Conservation Society of Sri Lanka . She explained that October and November, between the end of the Southwest monsoon and the start of the Northeast monsoon, are a transition period when weather patterns shift and even cyclones can form. With the winds blowing in from the northeast, the Northeast monsoon rains fall mainly on dry zone areas such as Anuradhapura and Polonnaruwa.
She pointed out that these rains help ‘Thora’ plants of the Caesalpiniaceae family, such as ‘Peni Thora’, ‘Pethi Thora’ and ‘Ath Thora’, to sprout and grow across the dry zone. These are the host plants of migratory butterflies such as the Lemon Emigrant and Mottled Emigrant. The rain also helps them spread across the rest of the country, including the wet zone.
As a result, a large butterfly population emerges in the dry zone. To reduce fierce competition for egg-laying sites, share resources and survive, and pushed along by the winds, the butterflies spread across the country, laying eggs on the ‘Thora’ plants they find along the way. She explained that this is a natural migration within the country, not butterflies arriving from abroad.
She added that in intermediate-zone areas such as Monaragala, ‘Weera’ trees were putting out fresh leaves with the rain. This had pushed up the Albatross population around host plants in the dry zone, leading to another internal migration across the country. Travelling this way also lets the butterflies find mates and breed easily.
She said she personally saw Lemon Emigrant and Mottled Emigrant butterflies flying from east to west while she was in Borella, Colombo, and that people from many areas confirmed seeing the same migration.
Explaining the science behind the legend of butterflies flying to Sri Pada, she said that besides the abundance of host plants in those areas, the main reason is ‘hill-topping’. Unlike birds, which migrate along river valleys, butterflies sense elevation and temperature changes, treat high ground as landmarks and gather on peaks such as Kirigalpoththa and Sri Pada to find mates easily. She stressed that the Sri Pada pilgrimage season begins in December, at the same time as the migration season. People often saw butterflies heading that way, and the legend spread.
She further noted that the migration could also be the result of some change in the weather. It could be an early sign of an approaching weather shift, such as rain, sudden temperature changes or cyclones. Not only butterflies but animals in general may change their usual behaviour in response to upcoming environmental changes, she said.
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Sri Lanka’s current butterfly migration is a natural, cyclical event within the island, driven by monsoon rains and the growth of host plants. During the transition between the Southwest and Northeast monsoons, rain brings fresh growth to dry-zone plants such as ‘Thora’ (‘Peni Thora’, ‘Pethi Thora’) and ‘Weera’. Populations of the Lemon Emigrant, Mottled Emigrant and Albatross shoot up, and the butterflies spread across the country on the winds to avoid competing for resources and to survive.
Butterflies also show ‘hill-topping’ behaviour, sensing elevation and temperature changes to gather on high landmarks such as Kirigalpoththa and Sri Pada to find mates, which explains the old Sri Pada legend. Crimson Rose butterflies have also been recorded crossing the sea from India to Mannar. There is no scientific evidence that these migrations predict major disasters, but because insects are so sensitive to changes in air pressure and wind, they act as an important bio-indicator of weather shifts.
However, irregular monsoon rains caused by climate change and the spread of invasive plants such as Guinea grass and ‘Gandapana’ have sharply reduced the huge butterfly swarms seen in the past. This explanation draws on research from Sabaragamuwa University and the University of Peradeniya, butterfly expert Himesh Jayasinghe and a former president of the Butterfly Conservation Society of Sri Lanka.
Written By BP Hansani | Result: Insight
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