Very hungry caterpillars get nasty
Scientists studying effects of plant’s defence system found creatures will sometimes eat each other
If you’re a hungry caterpillar and you’ve got a choice between eating a plant or another caterpillar, which do you chose?
You pick your fellow caterpillar, scientists have found — if the plant is noxious enough.
In a study published this week in Nature Ecology and Evolution, scientists sprayed tomato plants with a substance that induces a defensive response — a suite of nasty chemicals — and found that caterpillars became cannibals instead of eating the plant.
“The plant rearranges the menu for the caterpillar and makes other caterpillars the optimal choice,” said John Orrock, an evolutionary ecologist at the University of Wisconsin-Madison who led the study.
His team’s findings support a growing body of research suggesting that plant defences are far more sophisticated than we have thought.
Plants cannot run or hide, but they possess powerful strategies capable of altering the minds of herbivores that try to eat them.
The fight begins when an insect bites the plant, which triggers an immune-like defence response. The plant produces chemicals that hungry herbivores find toxic, unappealing or difficult to digest. For example, caffeine and nicotine, both toxic in high doses, are byproducts of the defence responses of tobacco and coffee plants.
Orrock and his colleagues knew that a chemical called methyl jasmonate, which smells like limes or flowers, could induce this defence mechanism in tomato plants. They also knew that caterpillars, which eat the leaves of tomato plants, will turn on one another when the going gets tough.
The scientists wondered how a plant’s defence system would affect the caterpillars if they combined these behaviours.
They sprayed tomato plants with either a neutral substance or varying amounts of methyl jasmonate to create graded levels of defence in the tomato plants. “You crank up the methyl jasmonate, the plant makes more nasty stuff,” Orrock said.
Then they put each plant inside an arena with eight caterpillars and watched for eight days to see how the caterpillars would handle two choices: Eat the plant or eat your fellow caterpillar.
The caterpillars munched the plants with no extra defences down to bare sticks before turning on one another for nourishment.
But faced with the well-defended plants sprayed with a lot of methyl jasmonate, the caterpillars gave up on the tomato leaves early. Like desperate characters in a cartoon island mirage, fellow caterpillars became appealing steak dinners.
“You start the experiment with eight and you end it with one or two,” Orrock said.