Clever plants turn vegetarian caterpillars into bloodthirsty cannibals
“It often starts with one caterpillar biting another one in the rear, which then oozes. And it goes downhill from there. At the end of the day, somebody gets eaten.”
No, I haven’t started work on a macabre remake of The Very Hungry Caterpillar. These are in fact the words of Professor John Orrock, author of a new study in Nature Ecology & Evolution which explores how plants can turn the normally vegetarian beet armyworms into bloodthirsty cannibals if they feel threatened.
You might think that a plant is pretty defenceless against predators, given movement is distinctly limited, but they have secret weapons to call upon. One of these is a chemical called methyl jasmonate, which acts to warn nearby plants of impending danger so that they begin to produce anti-insect chemicals of their own.
But it seems that methyl jasmonate – in large enough doses – can turn the predators on each other. This not only reduces the number of predators but leaves them full enough that eating more plant is the last thing on their tiny minds – in much the same way that nobody fills up on salad at a steak restaurant.
“Not only do these guys become predators, which is a victory for the plant, they are getting a lot of food by eating one another,” explained Orrock. “We struck upon a way that plants defend themselves that nobody had really appreciated before.”
“It’s grisly and macabre, but it’s energy transfer.”
Orrock found this by dousing tomato plants kept in plastic containers with either methyl jasmonate or a control solution. Each plant was sprayed with low, medium or high quantities of a solution, and was then joined by eight beet armyworms. After each day, Orrock would count the number of bugs present and weighed how much plant material was left over.
The control group and tomato plants with low quantities of methyl jasmonate were destroyed by the caterpillars, with the bugs only turning on each other when the greens were finished. But the plants doused in methyl jasmonate were left virtually untouched, as the previously vegetarian peace-loving bugs turned on each other in a grisly lunch to the death.
Just when you thought things couldn’t get any more grisly, Orrock found that the same would apply if a single living caterpillar was placed alongside a bunch of dead peers. The caterpillar would snack on the corpses of its fellow bugs, rather than eat the greens with enough methyl jasmonate present.
“From the plant’s perspective, this is a pretty sweet outcome, turning herbivores on each other,” said Orrock.
What’s not clear is why the plants are causing the bugs to turn to cannibalism. It could be that the methyl jasmonate tastes unpleasant, reduces the plant’s nutritional content or directly harms the caterpillars. But it’s pretty obvious that tomato plants aren’t quite the easy pickings they first appear – at least if you’re an insect.