We generally think of butterflies as beautiful, harmless, nectar-drinking insects. But new research carried out by the University of Sydney may change all that, as milkweed butterflies have been spotted scratching at caterpillars with their sharp claws to suck up their juices.


Milkweed butterflies are a group of butterflies in the Nymphalidae family, with one well-known species being the monarch butterfly.

As caterpillars, milkweed butterflies feed on toxic plants, using the chemicals as self-defence to make them unpalatable to birds and other predators. When the caterpillars turn into butterflies, they retain these toxins and advertise that they are poisonous with their bright colours.

Male butterflies will also use these toxic substances to produce mating pheromones. In order to boost their supplies of these love drugs, they’ll seek out extra sources of the chemicals.

Generally, they get these through plants, but in North Sulawesi, Indonesia, they have developed a taste for caterpillars – and they don’t care whether they are alive or dead.

“This is the first time the behaviour has been reported,” said lead author, PhD candidate Yi-Kai (Kai) Tea, from the University of Sydney.

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“The behaviour does not fit neatly in the traditional modes of predation, parasitism or mutualism, and so presents a new challenge to evolutionary theory. We have coined it ‘kleptopharmacophagy’ – chemical theft for consumption.”

The butterflies have small, sharp ‘tarsal’ claws on the end of their legs which they use to hold onto branches and to scratch at leaves to release the juices, which they slurp up with their long, curly proboscis.

“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,” said Tea. “The caterpillars would contort their bodies rapidly in what appeared to be futile attempts to deter the scratching.”

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Milkweed butterflies imbibing from dead and living caterpillars © Tea et al
Milkweed butterflies imbibing juices from dead and living caterpillars © Tea et al

Previously, milkweed caterpillars had been observed feeding on the carcasses of chemical-containing insects, but this is the first time they have been seen attacking live caterpillars. It is unclear whether caterpillar fatalities were directly due to scratching by the butterflies.


“Nonetheless, these simple observations raise questions about the ecology of these well-known butterflies, providing numerous opportunities for future studies,” said Tea. “For example, which exact compounds are these butterflies interested in? Does this behaviour occur elsewhere in the world?”


Alice Lipscombe-SouthwellManaging editor, BBC Science Focus

Alice is the managing editor at BBC Science Focus Magazine. She has a BSc in zoology with marine zoology. Her interests include natural history, wildlife, the outdoors, health and fitness.