Vomiting Caterpillars Explained

No one’s likely to bug you if you vomit on them—just look at caterpillars of the large white butterfly Pieris brassicae.

When attacked, the caterpillars throw up a green fluid of semi-digested cabbage, which already contains compounds that smell and taste unpleasant to predators, such as birds.

The caterpillars vomit semi-digested plants (pictured). Photo courtesy Mike Speed

But previous research has shown this defensive regurgitating ends up harming the insects by slowing down the caterpillars’ growth, reducing their survival rates, and even lowering the number of female eggs, according to the University of Bristol.

(See “Dino-Era Vomit Fossil Found in England.”)

Now a new study in Ecology Letters shows that a caterpillar is more likely to vomit alone than when when its hanging out with its peers.

“Caterpillars take account of their group size when ‘deciding’ whether to vomit because they are trying to avoid the cost of vomiting,” study co-author Mike Speed of the U.K.’s University of Liverpool said in a statement.

“In a group, it is quite likely that the predator will taste other individuals as well, each of whom might vomit and cause the predator to give up and leave,” Speed said.

“Also, caterpillars compete with each other for food, and so the bigger the group the more important it is to not vomit.”

By studying P. brassicae‘s vomiting strategy, the scientists hope to crack the defensive secrets of other crop pests, which in turn may inspire better methods for combating pests.

“The obvious pressure on caterpillars to limit the use of their defenses suggests that predators of crop pests have a far greater benefit on crops than simply the number of insects that they eat. Just trying to eat caterpillars reduces the fecundity of such pests and helps out farmers,” said study co-author Andrew Higginson, an ecologist at the University of Bristol.

P. brassicae munching on cabbage leaves. Photo courtesy Mike Speed

Animals have evolved some pretty far-out defenses against predators. Spiny lobsters make a violin-like screech that sends predators running.  As most know, squid squirt a cloud of black ink when threatened.

(Read about the author’s encounter with a cyanide-producing millipede in Alabama.)

And like P. brassicae, other species steal their defensive power. For instance, the Asian snake Rhabdophis tigrinus store toxins from toads they eat and store the chemicals in their necks, making them an unappetizing meal for a hawk or bigger snake.

Africa‘s crested rat turns its quills into lethal weapons by coating them with a plant toxin, National Geographic News reported last August. No other animals are known to use a truly deadly external poison, researchers say.

Wrote Ker Than, “Other mammals are known to use toxins that they don’t produce themselves. For example, a hedgehog species applies a mild toxin from a toad to its fur. … Likewise, some capuchin monkeys rub an extract from millipedes onto their fur to repel insects.”

People have also picked up some hints from their animal brethren: African hunters use the same plant toxin as the crested rat to make elephant-grade poison arrows.

And if you’re not grossed out enough, check out this video of P. brassicae regurgitating.

Check out more weird coverage on National Geographic News.


Meet the Author
Christine Dell'Amore, environment writer/editor for National Geographic News, has reported from six continents, including Antarctica. She has also written for Smithsonian magazine and the Washington Post. Christine holds a masters degree in journalism with a specialty in environmental reporting from the University of Colorado at Boulder. Her book, South Pole, was published in 2012.