Don’t Get Between a Caterpillar and Its Milkweed

“Some would just roam off and eat,” said Elizabeth Brown, a biologist in Dr. Keene’s lab. But if an insect spotted a morsel of food that was being monopolized by another, it would “rear up and, with their head, make a lunge onto the body of the other caterpillar,” she said.

Sometimes the strikes landed near the recipient’s head. In other cases, it was a bit more like “a punch in the gut,” Dr. Brown said. Either way, the battered caterpillar would usually skulk away in defeat, freeing up the milkweed for the voracious victor.

That’s a “huge consequence” for the loser, Dr. Keene said, because at this stage in their life, the larva are “basically eating constantly.” Newly hatched caterpillars are born famished, and as they balloon in size, can strip entire plants bare of leaves in a matter of days.

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The older and larger the caterpillars got, the more their disdain for sharing grew, the researchers found. The greatest number of scuffles occurred among bugs in the final stage before metamorphosis, when the stakes of milkweed munching were probably especially high.

Juxtaposed against the docile reputation of most butterflies, the study’s findings may be a bit disorienting. “We think about monarchs as being these beautiful, dazzling creatures that fly around and pollinate flowers and lay eggs,” said Adriana Briscoe, a butterfly researcher at the University of California, Irvine, who wasn’t involved in the study. “We don’t usually think of them as having this sort of darker underbelly.”

But even adult monarchs, especially males, can get a bit quarrelsome when their territories are under threat, Dr. Green said. Crammed into tight quarters, their younger, flightless counterparts might have all the more reason to engage in the occasional kerfuffle.

Dr. Green and Dr. Briscoe both pointed out that the study’s findings were restricted to the laboratory, leaving open the possibility that the caterpillar carnage the researchers witnessed might differ in the wild, where there’s more room to roam.