Strong Plant Defenses Made These Hungry Caterpillars Eat Each Other

When left with the choice of nasty-tasting plants or each other, the choice is clear for the beet armyworm caterpillar

Cannibal Caterpillar
Beet armyworm caterpillars turned to eating each other when the leaves they were placed on were made to taste foul. Brian Connolly / UW-Madison

When left to choose between eating foul-tasting leaves or cannibalism, a new study suggests that caterpillars may turn on their own.

Though they seem like helpless, immobile food sources, plants have vigorously competed in the evolutionary arms race to defend against being chewed up, Hannah Lang reports for National Geographic. Many plants are able to sense when they're being eaten, and respond by releasing chemicals to deter hungry herbivores and warn other plants of impending danger. But a new study, published this week in the journal Nature Ecology and Evolution, suggests that plants' defenses could be so effective that that large, hungry caterpillars will instead eat smaller caterpillars for nutrition.

One common chemical plant defense is the release of the foul-tasting substance methyl jasmonate. So to test the effectiveness of this particular defense, the researchers sprayed varying levels of this chemical on 40 tomato plants. They then placed eight beet armyworm caterpillars on each of the unpalatable plants.

Faced with a terribly tasting plant, the hungry caterpillars soon resorted to their only other option: each other. “You can either eat this plant or you can turn on your comrades,” John Orrock, lead author of the study, tells Lang. “The choice is clear.”

After about a week, all of the caterpillars on sprayed plants had eaten each other, writes Ryan F. Mandelbaum at Gizmodo, but caterpillars on plants sprayed with higher amounts of methyl jasmonate ended up consuming each other much more quickly. And the cannibal caterpillars grew at a similar rate to plant-fed creatures, reports Lang, suggesting that they were able to satisfy their nutritional needs.

Orrock stresses that it wasn't the chemical itself that actually drove the caterpillars to cannibalism, but rather the inedibility of their choice food. While he didn't give these caterpillars the chance to try other plants before turning on each other, according to National Geographic, larger scale experiments he's conducting now don't show much difference.

“Even with the capacity to disperse a little bit further and especially escape your hungry buddies, they do end up consuming each other with sort of the same patterns,” co-author Brian Connolly told Lang.

As for the plants themselves? Those that got the most spraying ended up with roughly five times more of their plant material remaining than those sprayed simply with detergent, reports Nicola Davis for The Guardian.

Orrock tells Davis, “[f]rom the plant defense perspective, making yourself so nasty that you are suddenly not the best thing on the menu works pretty well.”

Get the latest stories in your inbox every weekday.