Several years ago, scientists in the Netherlands discovered that certain insects can communicate with each other using plant material as a kind of message board. Insects eating a plant’s roots alter the chemical composition of its leaves, which causes the plant to release volatile signals into the air. These signals often convince aboveground insects to select another food plant in order to avoid competition, and to escape the plant’s poisonous defense compounds. But the impact doesn’t stop there.
New research shows that insects leave a specific legacy that remains in the soil after they have fed on a plant. And future plants growing on that same spot can pick up these signals from the soil, passing them on to other insects. Those messages are specific: the new plant knows whether the former one was suffering from leaf-eating caterpillars, or from root-eating insects.
“The new plants are actually decoding a ‘voicemail’ message from the past to the next generation of plant-feeding insects, and their enemies,” said Olga Kostenko, one of the scientists involved in the study. “The insects are re-living the past.”
This message from the past strongly influences the growth and possibly also the behavior of these bugs. Today’s insect community is influenced by the messages from past seasons.
Kostenko and her colleagues grew ragwort plants in a greenhouse and exposed them to leaf-eating caterpillars or root-feeding beetle larvae. Then they grew new plants in the same soil and exposed them to insects again.
“What we discovered is that the composition of fungi in the soil changed greatly and depended on whether the insect had been feeding on roots or leaves,” explains Kostenko. “These changes in fungal community, in turn, affected the growth and chemistry of the next batch of plants and therefore the insects on those plants.”
Growth and palatability of new plants in the same soil thus mirrored the condition of the previous plant. In this way, a new plant can pass down the soil legacy or message from the past to caterpillars and their enemies.
“How long are these voicemail messages kept in the soil? That’s what I also would like to know!” adds Kostenko. “We’re working on this, and on the question of how widespread this phenomenon is in nature.”