Danainae Butterflies Traverse Bengaluru Outskirts in 1000 Kilometer Migration

A massive 1,000-kilometer cross-generational migration of Danainae butterflies is currently traversing peninsular India, passing directly along the outskirts of Bengaluru and through Navilu Kaadu. The ecological phenomenon, which occurs twice a year, sees millions of these insects—commonly known as "crows and tigers"—traveling between the Western Ghats and the Eastern Ghats.

The migrating insects include species such as the blue tiger, dark blue tiger, striped tiger, plain tiger, double-branded crow, and common crow. These butterflies serve as prolific pollinators across the diverse ecosystems they encounter along their path.

According to research by Mysuru-based naturalists Arun Kumar Raj Urs, Namratha Urs, and Swathi H. A., the butterflies begin their journey near Sringeri in the Western Ghats, passing through Kukke Subramanya to Madikeri, where the migration split into two distinct routes. One group travels through Hassan, passes along the outskirts of Bengaluru toward Male Mahadeshwara Betta, and finishes at Yercaud in Tamil Nadu. The second group flies through the Brahmagiri Hills, Wayanad, Nagarahole National Park, the Nilgiris, Anamalai Hills, and Coimbatore, with some reaching Biligirirangana Betta via the Bandipur Tiger Reserve.

This eastward movement occurs between April and June, ahead of the southwest monsoon. Once the butterflies reach their destinations in the east, they reproduce and die. Their offspring, or sometimes the subsequent third generation, then make the return journey back to the Western Ghats between September and November as the northeast monsoon advances.

The life cycle of these butterflies is closely tied to milkweed plants, where they lay their eggs. The larvae feed on the leaves, absorbing toxic pyrrolizidine alkaloids that remain in their bodies through adulthood. This serves as a chemical defense against predators, signaled by their bright wing colors.

However, local naturalists warn that these migratory routes are facing severe threats. Busy expressways present a major hazard, killing large numbers of the low-flying butterflies. Additionally, pesticide spraying and deforestation are rapidly destroying the host milkweed plants along their path.