Caterpillars turn into cannibals and eat each other when plants deploy defensive chemicals to make their foliage less appetising, research has revealed.
While it was already known that caterpillars of many species munch on each other, and that plants have a range of defence mechanisms, it was not clear whether the two were linked.
Now researchers say that a chemical commonly leaked from plants when damaged can trigger other plants to defend themselves by producing similarly unappealing substances, and that prompts the pests to turn on each other.
“When the chips are down, eating another caterpillar may not be a bad decision [and] it turns out that the chips can be down if you find yourself on a plant that is heavily defended,” said John Orrock, an ecologist at the University of Wisconsin-Madison, and a co-author of the research.
Orrock and colleagues exposed 10 tomato plants each, to four different sprays. One contained only a detergent, while the others also included varying concentrations of methyl jasmonate – a substance given off by plants when damaged.
Each plant was then exposed to eight beet armyworm caterpillars – a common pest in the US. Over eight days the team counted the number of animals, before weighing the above-ground plant matter.
“The caterpillars really only have two choices – they can eat each other, or they can eat the plant,” said Orrock.
The results, published in the journal Nature Ecology and Evolution, reveal that after a week all but one or two survivors on each plant had been eaten regardless of which spray had been used, but the rate of cannibalism was very different.
After 52 hours, about 7% of caterpillars were cannibalised on leaves sprayed with either no methyl jasmonate or the lowest concentration, while around 16% of caterpillars had been eaten on leaves sprayed with either of the more concentrated methyl jasmonate sprays.
Most revealing was that more than five times as much plant matter was left on plants sprayed with the highest concentration of methyl jasmonate compared to those sprayed only with detergent – the latter were almost completely stripped of leaves.
“From the plant defence perspective, making yourself so nasty that you are suddenly not the best thing on the menu works pretty well,” said Orrock.
“We know herbivores are sensitive to plant defences, we just didn’t appreciate that one of the choices that plants might affect that herbivores make is the choice of eating another herbivore,” he said.
Nick Birch, an entomologist at the James Hutton Institute welcomed the research, but pointed out it was a small study and it wasn’t clear that the findings would hold in agricultural conditions. “It is quite difficult to extrapolate from those conditions to what would happen in a big field,” he said.
