when herbivores turn carnivores
Plants have a sneaky and sadistic trick to prevent caterpillars from munching on their leaves, a new study published Monday in the journal Nature Ecology and Evolution revealed. Researchers have discovered that plants secrete a chemical that makes them taste less delicious, leaving herbivorous caterpillars with the tough choice of eating less appealing foliage — or each other.
Caterpillars sometimes choose cannibalism. "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, a co-author of the research.
The researchers tested this phenomenon by coating tomato plants in four different sprays — one containing only detergent and the others containing different amounts of methyl jasmonate, the substance given off by plants — and then siccing eight beet armyworm caterpillars on the plants:
After 52 hours, about 7 percent of caterpillars were cannibalised on leaves sprayed with either no methyl jasmonate or the lowest concentration, while around 16 percent 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. [The Guardian]
Orrock said that while scientists knew herbivores "were sensitive to plant defenses," they didn't realize that plants' defenses might cause herbivores to make "the choice of eating another herbivore." "From the plant defense perspective, making yourself so nasty that you are suddenly not the best thing on the menu works pretty well," Orrock said.