Don’t Get Between a Caterpillar and Its Milkweed

Before metamorphosis, monarch butterflies will aggressively head butt each other for access to their favorite food.

In the 1969 children’s book “The Very Hungry Caterpillar,” the tiny protagonist spends a week snarfing his way through a smorgasbord of fruits, meats, sugary desserts and, finally, a nourishing leaf. This family-friendly tale was missing one crucial and far less G-rated plot element: the pure, unadulterated rage of an insect unfed.

When food gets scarce, monarch butterfly caterpillars will turn on each other, duking it out for the rights to grub, according to a paper published Thursday in the journal iScience. The jousts don’t get bloody. But they involve plenty of bumping, boxing and body-checking — all the makings of a big brawl of babies, in a hurry to bulk up before they sprout their grown-up wings.

“I went to grad school with a guy who played rugby in college,” said Alex Keene, a neuroscientist at Florida Atlantic University and an author on the study. “A flying head butt is a fair assessment.”

The study offers an in-depth look into the underappreciated phenomenon of caterpillar aggression. It could also aid entomologists racing to preserve monarchs and the milkweed plants they depend on, as populations of the fragile species continue to plummet.

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