Milkweed butterflies feed on live young caterpillars

Scientists have discovered that milkweed butterflies feed on live young caterpillars of their own kind – a behaviour that has been observed for the first time over – and according them them the reason behind this behaviour is to increase their supply of mating pheromones.

Milkweed butterflies originated more than 20 million years ago. They lay their eggs on various milkweeds; poisonous plants on which their larvae (caterpillars) feed. The family includes the iconic black, white and orange monarch butterfly (also found in Australia), which is threatened by pesticide use and deforestation in North and Central America.

Scientists have discovered that milkweed butterflies harass, subdue, and subsequently feed on live, dead, and dying caterpillars belonging to other milkweed butterflies – that is, their family’s young. Caterpillars are the larval stage of butterflies and moths.

Caterpillars feed on toxic plants, sequestering chemicals for self-defence. The chemicals are noxious and unpalatable to predators, such as birds, and are later manifested in butterflies’ bright, warning colours.

These chemicals also help male butterflies produce mating pheromones, which function as nuptial ‘gifts’ to females during courtship. To build upon this innate toxicity, male butterflies supplement their diet by seeking out additional sources of chemicals. Most frequently, these are obtained through plant sources, but in the forests of North Sulawesi, Indonesia, it appears that they have developed a taste for caterpillars – all in their quest to increase their supplies of this love drug, the researchers posit.

According to scientists while there have been instances of milkweed butterflies feeding on carcasses of other chemical-containing insects, this is the first time that they have been observed and reported extending that behaviour onto living insects of the same order; in this case, their own larvae. Yet, it remains unclear whether the caterpillar fatalities were a direct result of the scratching.

To supplement the chemical supplies they obtain as caterpillar juveniles, the butterflies engage in a behaviour known as “leaf-scratching”. Scientists explain that the butterfiles damage [chemical containing] plants with their sharp tarsal claws, liberate plant juices and imbibe them using their long, curly tongues.

Scratching at live caterpillars, however, had never previously been reported. Caterpillars are essentially bags of macerated leaves; the same leaves that contain these potent chemicals the milkweed butterflies seek out. To adult butterflies, they may simply be an alternative source of chemicals on which to feed.

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