Butterfly swarms seen across the country are a seasonal monsoon migration, scientists explain

The large swarms of white and yellow butterflies seen in many parts of Sri Lanka in recent days are part of a natural seasonal migration driven by the monsoon and the host plants it nourishes, scientists have explained. The spectacle has been recorded in areas including Ratnapura, Galle and Colombo.

The trend has also prompted anxiety on social media, with some asking whether it signals an approaching natural disaster. Studies and expert observations, however, confirm this is a one-way migration within the island by butterflies of the Pieridae family.

A field study by the Department of Zoology at the University of Peradeniya identified two main migration periods in Sri Lanka: a major season from October to December, during the second inter-monsoon, and a smaller one in March and April, during the first inter-monsoon.

Butterfly expert Himesh Jayasinghe explained that the monsoon stimulates the growth of host plants, providing fresh leaves for caterpillars and sites for laying eggs. The resulting rise in the butterfly population then disperses in search of food and breeding needs, which also reduces competition for resources.

A study published in 2024 in the Taprobanica journal by a team led by Professor Kapila Dissanayake of Sabaragamuwa University identified three migratory species: the Lesser Albatross (Appias galene), the Common Albatross (Appias albina) and the Lemon Emigrant (Catopsilia pomona). Observations at 60 sites across the island have traced three main flight paths.

The principal migration route runs from the southeast to the northwest, from Kabiliththa in Kumana National Park towards Wilpattu. The butterflies travel at an average speed of about 11 kilometres per hour, covering roughly 280 kilometres in three or four days.

There is also a scientific explanation for the traditional belief that the butterflies journey towards Sri Pada: through a behaviour known as hill-topping, in which they gather at high points, changes in elevation and temperature help them find mates. Field studies show, however, that most Albatross species migrate northwest rather than towards Sri Pada.

Experts have also dismissed claims that the mass migration foretells natural disasters. Because there is no scientific evidence for such a link, the phenomenon cannot be treated as an official warning of events such as cyclones, they said.