Migration of insects based on distance covered can be classified into three categories.
-Short distance migration or local migration: like that of the Emigrants and Blue Tigers… this could simply be because the newly emerged caterpillars exhaust all the host plants in a given area. So, the next generation moves to a near by area in search of host plants. It can be seasonal or sporadic. Distance covered range from 50-100kms.
-Long distance migration: like that of the Dark blue tigers, Double branded crows and Common Indian crows, that migrate 300-500kms in large numbers during particular seasons.
-Dispersal: like that of Painted Lady. Here the migrating butterflies will not have predestined place, so they randomly explore the new places looking for their host plants. Since its random they will have broader distribution and hence Painted Lady can be seen in many parts of the world.
Few migratory species of butterflies in India include Common Indian Crow, Double Branded Crow, Dark Blue Tiger, Lime Butterfly, Painted Lady, Large Blue Oakleaf, Common Emigrant, Mottled Emigrant, Common Albatross etc.,
Out of these, Dark blue tiger, Common crow and Double Branded crow are the ones that travel from the Western Ghats to Plains/Eastern Ghats and back. They cover 350-500km through their journey. This migration can be seen in Bangalore as well.
It happens twice a year. In March-April-May the direction is from the Western Ghats towards Eastern Ghats (SW-NE). In March-April-May, the weather in the Western Ghats gets oppressive for these butterflies. The rain sets off and the butterflies migrate towards the Plains\Eastern Ghats to avoid the same. The process of migration is paused if there is rain on the way. No migration is observed on cloudy days.
There is another migratory season in the months September-October-November. The direction of migration during this time of the year is from the Eastern Ghats towards Western Ghats (NE-SW) The individuals that have travelled SW to NE earlier in the year breed in the Eastern Ghats, and their progeny migrates back towards SW (Western Ghats). The back migration might be to overwinter at Western Ghats where the green thickets provides protection from the cold in the months of September to November.
Many a times, lots of Blue Tiger, Dark Blue Tiger, Common Indian Crow, Double Branded Crow and other such milkweed butterflies are seen in large numbers on plants such as Heliotropium sp. or Euphatorium sp. The male butterflies feed on the weathered parts of the plant for alkaloids. These alkaloids help the male butterflies to attract females during breeding season.
Long distance butterfly migration happens through four states in south India. The migratory paths vary each year as rain is an important factor that governs migration. As there are less people studying butterfly migration, there is less information known about the same. Since lakhs of butterflies migrate every year pollinating millions of plants on the way, they are of extreme use to the nature and also to the farmers. Hence it becomes to important to protect this fascinating migration. And this requires a huge network of people who can track the migration and take necessary actions to protect them from urbanization, traffic, pollution, pesticides, habitat destruction, etc.
Here's the difference between Blue tiger (Tirumala limniace) and Dark blue tiger (Tirumala septentrionis):
Dark blue tiger is the species which migrates SW to NE and back.