NE monsoon: Blue Tiger, Dark Blue Tiger and Crow butterflies migrate from Eastern to Western Ghats
22-10-2026 · English
Topics: Environment · 22 October 2026 Current Affairs (UPSC / TNPSC / SSC / RRB)
News
With the onset of the North-East monsoon, lakhs of milkweed butterflies – Blue Tiger, Dark Blue Tiger, Common Crow and Double-branded Crow – are migrating from the Eastern Ghats across the Tamil Nadu plains towards the Western Ghats (reported 22 Oct 2026). Intense movement was seen on 17–18 Oct in Salem, Namakkal, Erode, Tiruppur and Coimbatore districts; counts in Coimbatore district ranged from 5 to 135 butterflies a minute, with 120 a minute on Kalapatty Main Road on 18 Oct. Hundreds are killed by road traffic.
Key facts
| Direction | Eastern Ghats → Western Ghats (NE monsoon, Oct–Dec); return in Apr–May (before the SW monsoon) |
| Species | Blue Tiger (Tirumala limniace), Dark Blue Tiger (T. septentrionis), Common Crow (Euploea core), Double-branded Crow (E. sylvester) |
| Group | Danainae – milkweed butterflies; family Nymphalidae |
| Starting hills | Yercaud (Shevaroys), Pachamalai, Kolli and Kalvarayan hills |
| Monitoring | The Nature and Butterfly Society (TNBS) – since 2013 |
Background
- Migration cycle: in Apr–May they move from the Western Ghats to the Eastern Ghats and Coromandel coast; in Oct–Dec, with the NE monsoon, they return west and congregate in the Western Ghats for 3–4 months. Six of South India's 10 danaine species migrate; first scientific record – E.H. Aitken (1897).
- Science: they store alkaloids from milkweed-family (Asclepiadaceae) host plants, making them toxic to predators; males use them to make courtship pheromones. Harmless species copying their look is 'Batesian mimicry'.
- Geography: the Western Ghats – UNESCO World Heritage Site (2012) and a global biodiversity hotspot. The Eastern Ghats are discontinuous, lower hills. Weak rains mean weaker migration (as in 2023).
Tamil Nadu link: Tamil Nadu's State butterfly is the 'Tamil Maravan' (Tamil Yeoman, Cirrochroa thais), declared in 2019 – the 5th State to name a State butterfly; it is endemic to the Western Ghats. In Tamil Nadu the migration mainly starts from the Eastern Ghats hills of Yercaud, Pachamalai, Kolli and Kalvarayan.
Source: GKToday, IASPoint, 22 Oct 2026; Mongabay India (Aug 2025 explainer); GKToday (Nov 2023); Mongabay (Aug 2019, Tamil Yeoman)
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