Evolutionary Battle

A new book unravels the unusual ecological relationship between the monarch butterfly and the milkweed

- Deepanwita Gita Niyogi

THE FIRST line of the book describes the monarch as a “handsome and heroic migrator”. True to this description, monarch butterflies flaunt a bright orange and black colour combination. They are also great travellers: every year in autumn, the butterflies cover a distance of up to 5,000 km to reach their overwinter grounds—from the US to Mexico and Canada. After a four-month rest, they fly back to the US in the spring.

The monarch’s epic journey is undoubtedly fascinating, but what’s even more captivating is its complex relationship— stretching over a million years—with the milkweed plant. The fight begins when the monarch caterpillars take a first bite of the milkweed leaf, their only source of food. As a defence mechanism, the milkweed exudes a toxic and sticky latex, which kills a majority of the young caterpillars. Over 60 per cent of monarchs die after the first bite. It is thus a story of exploitation and defence, escalating into a coevolutionary arms race. It is also a tale of a one-sided and non-symbiotic relationship. The milkweed does not need the monarch, as the latter is simply not good at pollination. But for the butterflies, the milk-weed is its only source of sustenance.