Caterpillars: you won’t like them when they’re ‘hangry’. That’s according to a new study from Florida Atlantic University, which found that the insects may turn violent when food is scarce.


Researchers noticed that when monarch butterfly caterpillars were deprived of milkweed leaves, their favourite grub, they behaved more aggressively towards each other. In fact, the less food there was, the more the caterpillars lunged and knocked aside other bugs, going as far as head-butting to reach food.

“Aggression is common in insects, including fruit flies, where single-pheromone receptors or single genes have been shown to trigger their aggression,” said lead researcher Prof Alex Keene.

Caterpillars closest to the final stage of metamorphosis (those about to turn into butterflies) were found to be the most aggressive. And for good reason: a lack of nutrition in the bug’s youth may not only reduce its size in adulthood, but also decrease its lifespan.

Although the study was undertaken in a lab, a lack of milkweed is a common issue in the wild. While this is chiefly because the plant only grows during select parts of the year, caterpillars’ ravenous and territorial nature also has an impact.

“If you compare a monarch caterpillar to a fruit fly, when there are lot of larvae on a piece of rotting fruit, they feed socially with little evidence of territoriality,” said Keene.

“But each caterpillar will, at some point in its developmental cycle, encounter resource limitation because they can strip an entire milkweed of leaves.”


Monarch caterpillars are known to consume an entire milkweed plant in two weeks. During their biggest, and hungriest, phase, a single caterpillar can eat an entire leaf in five minutes.

Reader Q&A: Do caterpillars have a sex before they become butterflies?

Asked by: Adrian Lewis, Exeter

The sex of a caterpillar is fixed at the moment the egg is fertilised­, but most species don’t show any sex-specific features until they turn into butterflies.

The immature reproductive organs are still present in the larval stage though, and in a few species with translucent bodies, you can see the red or dark yellow testes of the males.

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Thomas Ling
Thomas LingDigital editor, BBC Science Focus

Thomas is Digital editor at BBC Science Focus. Writing about everything from cosmology to anthropology, he specialises in the latest psychology, health and neuroscience discoveries. Thomas has a Masters degree (distinction) in Magazine Journalism from the University of Sheffield and has written for Men’s Health, Vice and Radio Times. He has been shortlisted as the New Digital Talent of the Year at the national magazine Professional Publishers Association (PPA) awards. Also working in academia, Thomas has lectured on the topic of journalism to undergraduate and postgraduate students at The University of Sheffield.