Some plants have been found to use nature’s dog-eat-dog world to their advantage, forcing herbivores to become cannibals when the plants feel threatened by a caterpillar’s endless appetite.
A new study published in the Nature Ecology and Evolution journal found that when some plants are under attack from hungry herbivores, they emit defenses that make themselves incredibly foul-tasting to caterpillars, which spurs the caterpillars to eat each other.
“Plants can defend themselves so much that they food-stress the herbivore, and then the herbivores determine that rather than have plants on their menu, they should have caterpillars at the top of their menu,” said John Orrock, the author of the study and a researcher in the Department of Zoology at the University of Wisconsin, Madison.
Planting The Seeds
Orrock and his research team sprayed tomato plants with methyl jasmonate—a substance plants produce in response to environmental stresses—to trigger the plants’ defense mechanisms. This chemical allowed the plant to change its chemistry, which made it less appetizing to the beet armyworm caterpillars that were placed on a treated plant.
This phenomenon has been documented in a variety of plants, and research has suggested that plants can sense when surrounding plants are under attack, which can spur the production of methyl jasmonate in entire communities of plants. (Read more about how plants listen for caterpillars to prompt defenses.)
“What I find most interesting is that general idea that most plants use information from their environment and they use that information to effectively allocate their resources either into defense or into something else,” said Orrock.
And methyl jasmonate can do more than just make a plant taste bad.
“These chemicals can attract natural enemies like predators and parasitoids that will eat herbivores,” said Orrock.
When caterpillars find that the plant they’re munching on is no longer tasty, they are faced with a choice that Orrock said becomes simple.
“You can either eat this plant or you can turn on your comrades,” he said. “The choice is clear.”
The research team looked at the growth rate of the caterpillars and found that the caterpillars that consumed a plant diet and the caterpillars that became cannibals grew at the same rate, meaning they were able to compensate for their low-quality plant diet. (Read more about how caterpillars use vibrations to communicate.)
“It becomes a cost benefit analysis for the caterpillar, the fact that the plant material becomes so low quality that in the interest of maintaining and even sustaining metabolism, the caterpillar needs to find the highest quality food resource it can around it,” said Brian Connolly, a post-doctoral researcher who also worked on the study.
The larger caterpillars tend to prey on the smaller caterpillars, following the philosophy “eat or be eaten,” Orrock said.
In this experiment, the caterpillars weren’t given the option to try another plant before resorting to cannibalism, but Orrock and Connolly are conducting research in larger settings where the caterpillars are given the choice.
However, in the new larger setting, the cannibalism trends appear to be similar.
“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,” said Connolly.
The reason why the caterpillars are so quick to eat their own if they could just as easily move to a different plant isn't clear, but Orrock and Connolly hope to find out.
"As you can imagine, this is something that we’ve sort of switched gears and has become a top priority so we’re in the process of actually evaluating that data right now," said Connolly.