Declining butterfly migration in Tamil Nadu has worried environmentalists as these insects play a crucial role in pollination and pest control, besides serving as indicators to temperature changes and pollution levels.
On the southern edge of the Indian subcontinent, where the Western Ghats meet the Eastern Ghats in Tamil Nadu, a delicate phenomenon unfolds twice every year — butterfly migration. Beyond the known migrations of birds and fishes, this lesser-known journey of butterflies weaves a captivating narrative. Of the 250 migratory butterfly species worldwide, around 60 are found in India.
This year, however, observers note a subdued spectacle in Tamil Nadu’s annual butterfly migration, a journey that typically bridges the two mountain ranges abutting the nation’s eastern and western coastlines in the shape of a V — like a pair of aloft wings. The fluttering wings have graced the skies at a diminished 40-50 percent of their usual splendour, raising concerns among nature enthusiasts.
Within Tamil Nadu, there are two pivotal butterfly migrations — one from the Western Ghats to the Eastern Ghats and the reverse. The eastward migration begins at the cusp of May, preceding the southwest monsoon, and continues until September. Conversely, the westward migration commences mid-October from the Eastern Ghats, preceding the northeast monsoon, concluding its performance by early December.
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Throughout this migration, diverse butterfly species, particularly those recognised as crows and tigers, such as the Indian common crow, double-branded crow, blue tiger and dark blue tiger, migrate between various hills and mountains such as Servarayan, Pachamalai, Kalvarayan and Kolli in the Eastern Ghats, and the mountain ranges that stretch between Anaikatti and the Nilgiris in the Western Ghats.
Remarkably, the tens of thousands of butterflies in this unidirectional migration do not retrace their winged odyssey. Instead, they propagate at their destination, passing the baton to the next generation for the return migration.
Covering a migration range of 150 km to 200 km, a flight of butterflies can traverse this distance in two to three days, with stopovers at the plains, explained S Gopalakrishnan, an executive member of The Nature and Butterfly Society (TNBS), headquartered in Coimbatore.
TNBS hints at a potential correlation between rainfall deficit and the muted migration of butterflies. An advocate for further exploration of this phenomenon, TNBS points out the historical link between butterfly movement and ample rainfall.
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Tamil Nadu received 92 percent of its normal rainfall from June to September this year. However, heavy rains recently devastated the Chennai region and the districts at the southern end of Tamil Nadu. According to Sivasankaran, butterflies and moths shut down their life cycle at the pupal stage to avoid such bad weather. Also, the adults wouldn’t have been able to survive since the plants that host these winged creatures would have perished in the downpour.
“When these plants are destroyed there’s no food and shelter for these insects, resulting in their death,” Sivasankaran said, adding that their population will stabilise in the flood-affected areas in the next 10-20 days.
“Even if an entire generation is wiped out in a particular region, migration from other regions will compensate for this,” Sivasankaran added. “Once the plants grow back after the floods, butterflies from neighbouring regions will migrate towards these places as plenty of food will be available to them with less or no competition.”
Things have been different in the Eastern Ghats districts of Salem and Namakkal, where a rain deficit cast a shadow during the ongoing northeast monsoon.
“When the migration is normal, an observer can spot 20-25 butterflies, across species, in a time interval of 10 minutes. However, this year we could spot only about or less than half the usual numbers,” Gopalakrishnan said.
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TNBS, with over 200 dedicated volunteers across the southern states, has meticulously documented butterfly movements since 2013.
Advocating a more extensive study, Gopalakrishnan asserted: “We saw a decline in migration during rainfall-deficit years and good migration in the years with normal rainfall. However, the data with us pertains only to the past seven-eight years. Data of 20-25 years needs to be studied to establish the potential link between adverse climatic conditions and butterfly migration.”
Butterflies, beyond their ethereal beauty, play pivotal roles in ecology. They are nature’s choreographers, contributing to pollination and acting as natural pest controllers. Moreover, they are integral to the food chain, serving as a vital food source for many bird and bat species. Also, butterflies serve as sensitive indicators of environmental health, responding to changes in temperature and pollution levels.
Research shows that butterflies are averse to dry, hot weather, and inclined to cooler climates.
“Migration may be due to local adverse conditions like seasonal changes or shortage of food plants and overpopulation,” write Milind Bhakare and Hemant Ogale in their book, A Guide to the Butterflies of Western Ghats.
Underscoring the connection between butterfly migration and rainfall, Dr Kuppusamy Sivasankaran, an entomologist at Loyola College in Chennai, said, “Poor rainfall leads to late blooming or no blooming of flowers. It can prevent or delay the migration of butterflies.”
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In the delicate dance of nature, butterflies emerge not just as ephemeral wonders but as guardians of ecological harmony.
“Butterflies modify their life-cycle slightly according to the temperature. So they can be used to check if there is a rise in temperature. Like rainfall, temperature is also linked to butterfly migration. They can also be used as a biological indicator for pollution. If the pollution level is high, butterflies will avoid that place,” Sivasankaran said.
The lead image at the top shows dark blue tiger and double branded crow butterflies (Photo by TNBS / Sadhees Kumar K)
Vignesh. A is an Independent Journalist based in Tamil Nadu. He was earlier with The New Indian Express and BBC.
