Come April, the primary few pre-monsoon rains bathe upon the Western Ghats, the place the foliage is browned on the finish of a dry winter. Crops burst into flowers. On cue, hundreds of milkweed butterflies begin swarming and feeding in a frenzy. Collectively, they take to the skies and make their approach throughout the peninsula, to the drier Japanese Ghats and the Coromandel coast. Later within the yr, the northeast monsoon rains strike the jap a part of the peninsula and the butterflies return to their mountain houses within the Western Ghats.

“When migrating they move in such numbers that it’s unimaginable to look out of the window with out seeing them,” writes British naturalist E.H. Aitken in 1897, within the first scientific publication documenting the migration. This spectacular migration of butterflies in southern India has not gotten the identical consideration as its cousins, the monarchs of North America, which famously journey about 5,000-6,000 km from northern North America to Mexico, and again once more, to finish their annual migration over three to 5 generations. The painted woman (Vanessa cardui), probably the most cosmopolitan of all butterflies because it has a large geographic distribution, migrates greater than twice as far (as much as about 15,000 km) in round-trip migrations from northern Europe to West Africa over a minimum of six to seven generations.

In India, many scientists, naturalists and butterfly fanatics have been observing this distinctive phenomenon of butterfly migration, however way more systematic analysis is required. Some have pushed behind flying butterflies, ranging from Wayanad in Kerala and transferring slowly east, travelling with a stream of migrating butterflies. “The Danaine butterfly migration in southern India is fascinating for a) its longitudinal extent, and b) the sample of migrational actions which is influenced by the Indian monsoon,” writes Krushnamegh Kunte in his 2005 paper documenting the migration.

“I’m from Wayanad, one of many hotspots for Danaine migrations. I’ve been watching them since my schooldays,” says Vinayan P.A., a naturalist from Ferns — A Society for Nature Conservation in Wayanad. “One yr, after we noticed the butterflies begin their journey east from Wayanad, we adopted them in a automobile. They fly at 10-20 km/hr velocity or much less, so it was not troublesome. The subsequent day, they crossed the Bandipur-Nagarhole belt and reached the Deccan Plateau. In about 4 days, they reached the Sathyamangalam-MM Hills belt.”

“From a Tamil Nadu perspective, there’s additionally migration throughout the Japanese Ghats,” says Pavendan Appavu, founding father of the Tamil Nadu Butterfly Society. “For instance, butterflies generally transfer from the jap hills advanced in Tiruvannamalai and Vellore (in Tamil Nadu) and journey in a straight line into Andhra Pradesh, to the Tirupati hills. We even have Danaines fly out from the Yercaud hills in Salem, to the western districts of Tamil Nadu, i.e. the Nilgiris and Coimbatore. Whereas the Japanese Ghats and Western Ghats are well-defined entities, and therefore simpler to grasp, there are different, localised migrations which normally go undocumented,” he added.

What’s animal migration?

Migration is a seasonal course of, repeated in a well timed method, and undertaken by half or all the inhabitants of a species. Usually triggered by seasonal environmental modifications, migration enhances possibilities of foraging, replica and survival. The arctic tern’s pole-to-pole annual flight, wildebeest lumbering throughout the Serengeti-Masai and the humpback whale all come to thoughts after we consider migrations. The monarch butterfly is without doubt one of the few well-known iconic butterfly migrations. Nonetheless, practically 600 butterfly species migrate, principally in tropical nations the place there are well-defined moist and dry seasons. Migration leads to a dramatic change in the place the butterfly populations are discovered, over time. Whereas the identical wildebeest particular person could make a complete migratory journey, most butterfly species can not handle a complete migration inside their brief lifespan; successive generations of the identical species execute long-distance migrations.

What are milkweed butterflies?

Milkweed butterflies belong to the butterfly subfamily Danainae and household Nymphalidae. Milkweed butterflies are a largely tropical group of butterflies, and are particularly numerous in South America, and Indo-Australia areas. The title “milkweed” for the butterfly comes from the Danainae’s most popular host crops within the Asclepiadaceae plant household, that are, referred to as — no shock there — “milkweeds”. The host crops are wealthy in particular chemical compounds referred to as alkaloids, which Danaines accumulate when feeding, as larvae and adults. The chemical compounds are poisonous to predators, turning into an in-house arsenal and rendering Danaines unpalatable. Male Danaine butterflies use such alkaloids to supply courtship pheromones. They switch these to females together with spermatophores; the females then incorporate them into their eggs for defense.

Among the many ten Danaine species reported from southern India, six have been noticed emigrate: the darkish blue tiger Tirumala septentrionis, blue tiger T. limniace, widespread crow Euploea core, double-branded crow E. sylvester, plain tiger Danaus chrysippus and the striped tiger D. genutia. Research have discovered grownup milkweed butterflies scratching vigorously on leaves, stems, flowers, and seedpods of the crops and imbibing the exudates. Additional, when migrating, feminine Danaines enter a stage of “reproductive diapause” — a state the place their our bodies cease allocating assets to replica. These assets are as an alternative diverted to growth of thorax muscle tissues, which permits robust flight over longer distances.

Why do milkweeds migrate in southern India?

One potential purpose for the milkweed migration in southern India is the twin monsoon system present in peninsular India, and its distinctive topography, say specialists. The Western Ghats, which run parallel to the west coast of the Indian peninsula, rises to 900-1,500 m within the northern and central areas, and to over 2,500 m within the Nilgiri and Anamalai areas of southern Western Ghats. The mountains supply a mosaic of habitats from scrub and deciduous forests, to mid-elevation evergreen forests, to montane areas the place shola forests in mountain ridges alternate with grasslands on hilltops. Host crops for the egg-laying Danaine butterflies, crops of the milkweed household, are present in pockets all through. The tall southern Western Ghats solid a rain shadow over the peninsula, which is extra fed by the northeast monsoon rains.

The Western Ghats get pleasure from heavy rainfall between June and September, from the southwest monsoon. With low temperatures and a foggy local weather, circumstances are usually not appropriate for grownup butterflies. The jap coast has milder showers, between October and January, because of the northeast monsoon. Freshly emerged butterflies within the Western Ghats head east after the pre-monsoon showers in April and Could. What occurs subsequent isn’t fully clear, as a result of sufficient analysis has not been carried out. The butterflies that moved east mate, lay eggs and maybe die after replica. The newly emerged butterflies begin the return journey again to the Western Ghats between October and early December, relying on the northeast monsoon and their very own eclosion schedules, when the insect emerges from its egg or pupal case. As soon as again within the Ghats, the butterflies feed, and relaxation in giant swarms referred to as “congregations”, normally in sheltered spots inside evergreen forests. Then, coinciding with the flowering of timber within the moist forests, the butterflies feed actively and will reproduce earlier than the eastward migration, beginning April-Could. So basically, the butterflies breed in all places, in all appropriate habitats within the Indian peninsula; the situation relies on the time of the yr. Freshly emerged butterflies undertake the migratory journeys.

That is clearly a season-related migration, says Appavu from the Tamil Nadu Butterfly Society. “So long as the rains observe this sample, you may see them migrating a method in April-Could and the reverse will occur in September-October. At any time when there’s good rainfall in pre-summer or summer season, there’s a sudden progress of host crops, meals availability, lack of predation — these elements can lead to a sudden surge of numbers. As soon as the numbers go up, there’s a useful resource crunch and they’re compelled to maneuver,” he stated. “Another excuse is that the monsoonal climate sample creates weather conditions that aren’t conducive for butterfly survival. Monsoon rains are heavy! Butterflies want a sunny local weather to fly and forage; they attempt to keep away from heavy rains. It’s just like the monarch butterflies from North America transferring away from chilly northern latitudes to comparatively heat climates.”

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Learn extra: Crimson rose butterflies migrate throughout the ocean, leaving a path of questions

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What are butterfly congregations?

Butterfly congregations are gatherings of numerous butterflies in a single spot. Within the east-west butterfly migration in southern India, after reaching the Western Ghats in October-November, the butterflies first feed actively. They later cluster in giant congregations in shaded forests equivalent to evergreen, semi-evergreen, and riparian forests, retaining away from human-dominated areas. “It’s just like hibernation,” says Appavu. “The butterflies are most likely genetically pre-wired to succeed in a selected spot. I’ve seen congregations in Silent Valley the place hundreds of butterflies stick onto crops and timber, nearly trying like leaves!” As soon as in congregations, the butterflies transfer over a restricted house, and stay there in a much less energetic state for the following three to 4 months.

In a examine that spanned six migration seasons (2018 to 2023) in Wayanad, Kerala, Vinayan and collaborators discovered butterfly numbers at a congregation web site plunge from greater than 25,000 in 2019 to lower than 50 in 2022, after which get well to greater than 5,000 in 2023. This indicated that the populations had been capable of bounce again from actually low numbers, write the authors. “In all probability, fewer butterflies migrated to those congregation websites in 2022,” says Vinayan. “That is presumably due to threats within the earlier season’s breeding grounds, equivalent to habitat loss and native weather conditions.”

Whereas sufficient isn’t identified about why the South Indian danaines congregate, the identical causes because the North American monarchs most likely maintain true: to optimize meals plant exploitation, replica and migration methods.

Do different butterflies migrate in peninsular India?

Greater than 30 species of butterflies migrate in southern India, although observations and additional analysis are ongoing, says Kalesh Sadasivan, founder member of the Travancore Nature Historical past Society, who has been observing and finding out butterflies for the final 25 years. He was fascinated by butterfly migration when he noticed a swarm move by way of a shopping center in Tirunelly at Wayanad. “I’ve been observing that there are a number of sorts of butterfly migrations in southern India,” stated Sadasivan. “The Danaine migration is most spectacular, as a result of there’s longitudinal motion; there’s a little bit of latitudinal motion as properly.” The Catopsilia butterflies (widespread emigrant) from the household Pieridae present latitudinal motion. Others present elevational motion, transferring down from excessive mountain peaks to keep away from the chilly; there are additionally between-country and trans-continental migrations, he added.

To know extra about butterfly migrations, they should be sampled “mid-stream” with a web and totally different traits should be famous, says Sadasivan. The place the sampling is completed, can also be vital. “Migrations manifest extra in some areas,” he says. “There’s something referred to as ‘funneling’, for instance in a valley or a spot in a mountain chain. Additionally, each butterfly species has its personal inhabitants cycles over a number of years, as a result of different elements like parasitoid infections, host plant variability. This makes each butterfly swarm totally different, and yearly totally different.”

Appavu agrees. Big congregations and excessive numbers of migrating butterflies are usually not seen within the Tamil Nadu plains. “The hills have dense vegetation, the place the butterfly inhabitants can improve to excessive numbers. The host crops could have been there at excessive density within the plains as properly, however most likely we’ve got cleared them away and the crops are usually not current contiguously now,” he added.

They don’t get eaten, however what are the opposite threats to migrating milkweeds?

Additional research are wanted for the Danaines migrating in India, however the North American monarch butterfly species has been studied very properly, and lots of the threats are relevant within the Indian context as properly. There’s a normal decline within the inhabitants of larval meals crops. The usage of herbicides has made among the meals crops poisonous to caterpillars and egg-laying females. Habitat loss and fragmentation has brought on the decline of larval meals crops and overwintering congregation websites. The migratory routes are additionally affected. Local weather change is more likely to have an effect on moisture and temperature regimes, and thereby disrupt migratory cues or the environmental alerts that information migration. As well as, migrants are affected by wind patterns: stronger winds can carry the migrants removed from their vacation spot, and weak winds could improve the vitality required to succeed in the vacation spot.

The migrating butterflies are additionally impacted by linear infrastructure equivalent to highways and rail tracks, as a result of they fly at low heights, of about one to 4 metres above the bottom. “Sadly these butterflies need to handle nationwide highways! Think about the highways throughout Tamil Nadu, from Coimbatore to Salem, Salem to Bengaluru! Krishnagiri lies alongside a serious migratory route,” says Appavu. “Birds fly safely at a peak. Butterflies fly at a low peak and get killed in enormous numbers.”

Additional, many Indian nationwide highways use the Nerium oleander plant (arali in Tamil) alongside highways, which the crow and double-branded crow butterflies use as a number plant. This attracts the butterflies to highways, particularly egg-laying females, leading to them getting killed by transferring vehicular site visitors.

The variability of the migratory phenomenon between years, the dependence of migrating species on a number of habitats positioned removed from one another, compound the conservation challenges for southern India’s flying tigers and crows.

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Quotation:

• Aitken, EH. (1897). The migration of butterflies. Journal of the Bombay Pure Historical past Society, 11, 336–337.
• Kunte, Okay. (2005). Species composition, sex-ratios and motion patterns in Danaine butterfly migrations in southern India. Journal of the Bombay Pure Historical past Society, 102(3), 280–286.
• Bhaumik, V., & Kunte, Okay. (2017). Feminine butterflies modulate funding in replica and flight in response to monsoon‐pushed migrations. Oikos, 127(2), 285–296. https://doi.org/10.1111/oik.04593
• Chowdhury, S., Fuller, R. A., Dingle, H., Chapman, J. W., & Zalucki, M. P. (2021). Migration in butterflies: a worldwide overview. Organic Evaluations/Organic Evaluations of the Cambridge Philosophical Society, 96(4), 1462–1483. https://doi.org/10.1111/brv.12714
• Sony, R. Okay., & Arun, P. R. (2015). A case examine of butterfly street kills from Anaikatty Hills, Western Ghats, Tamil Nadu, India. Journal of Threatened Taxa, 7(14), 8154. https://doi.org/10.11609/jott.1743.7.14.8154-8158
• Vinayan, P. A., Yathumon, M. A., Sujin, N. S., Kumar, B. N. A., Ajayan, P. A., Muneer, P. Okay., . . . Anoop, N. R. (2023). Sample and drivers of danaine butterfly migration in Southern India: implications for conservation. Journal of Insect Conservation, 27(3), 505–516. https://doi.org/10.1007/s10841-023-00473-z
• Vinayan, P. A., Yathumon, M. A., & Jadeja, S. (2025). Massive yearly fluctuations in congregation sizes of migratory danainae butterflies within the western Ghats, India. Biotropica, 57(3). https://doi.org/10.1111/btp.70040

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Banner picture: Striped tiger butterflies congregate in Sanjay Gandhi Nationwide Park in Maharashtra. Picture by Dr. Raju Kasambe through Wikimedia Commons (CC BY-SA 4.0).