Brown butterflies take over city, experts cite prolonged rain as cause
Times of India | 29 November 2025
Kolkata: If you think you're spotting too many brown butterflies this Nov, you are not alone. Armies of tiny, leaf-like fliers — each weighing a few milligram and spanning barely 2.5 to 3 inch — have taken over balconies, gardens, pavements, and even living rooms across the city.
The species commonly observed, according to experts, is the Common Evening Brown (Melanitis leda).
"Quite unusual," Tollygunge resident Sourav De said, describing the surge in brown butterflies over the past fortnight. "I haven't seen anything like this before. Friends and relatives are noticing the same thing," said De, an independent film-maker.
Arjan Basu Roy, who has been working on butterflies for 25 years, attributed the mass emergence of Common Evening Brown to conducive weather. "Prolonged monsoon and post-monsoon rain has led to an abundance of grass that plays host to such butterflies. Had the conditions not been ideal, they would have remained in the dormant stage as pupae," said Basu Roy, who is the curator of butterfly conservatories in Eco Park, Banabitan, Buxa and Rajabhatkhawa.
Another possible reason for the surge could be a decline in predators like ants, wasps and spiders. But that is yet to be confirmed through an evidence-based study.
"These butterflies will mate, lay eggs and die. The larvae will stay in the pupae stage till the next monsoon," said Basu Roy.
The last time the butterflies were seen in such numbers in the city was in 2019.
Experts say there is no cause for alarm. The sudden appearance of these small brown butterflies is part of a recurring seasonal event, not an invasion or ecological warning. "The species typically sees its population peak at the beginning of winter," said Zoological Survey of India director Dhriti Banerjee.
These butterflies are born in low-light conditions. "They gradually get attracted to indoor bulbs or tube lights, which is why people see them inside their homes, parks, and places with good illumination," said Navneet Singh, a lepidopterist at ZSI.
The butterfly's appearance also adds to the illusion of "suddenness".
"They mimic dead leaves — in colour, pattern, and shape," said Amlan Das, senior entomologist at Calcutta University. "Once they emerge in large numbers, their camouflage breaks and people feel they are everywhere."
Experts point out that Kolkata's mild temperature fluctuations this year — warm days and cool nights — have further heightened their mobility.
Common Evening Brown's life cycle is short — around a month from egg to adult (egg-larva-pupa-adult). The adult butterfly lives only about two weeks, which is why the sudden surge leads to quick disappearance.
Despite the butterflies' abundance, they are far from a nuisance — caterpillars feed on common grasses and adults serve as prey for birds, reptiles, spiders and small mammals. "The adults act as pollinators," Das said.