Breakthrough discovery shows that moths listen to plants and avoid the noisy ones

For the first time, a team of researchers in Israel has documented that insects can hear and interpret plants’ acoustic distress signals.

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(CNN) — When a plant is stressed, it doesn’t keep quiet about it. You won’t hear the plant’s cry because it’s in the ultrasonic range — too high-pitched for human ears — but, for decades, scientists have been using special devices to listen in.

For the first time, a team of researchers in Israel has documented that insects can hear and interpret plants’ acoustic distress signals.

This finding builds upon the research group’s prior work recording sounds that tomato and tobacco plants make when they are dehydrated, said lead study author Rya Seltzer, an entomologist and doctoral student in the department of zoology at Tel Aviv University in Israel.

“The prevailing hypothesis is that these sounds are produced as a result of changes in the plant’s water balance within the xylem vessels,” the specialized plant cells that carry water and nutrients upward from the roots to the stem and leaves, Seltzer told CNN in an email.

During periods of stress from lack of water, air bubbles form, expand and collapse in xylem tissues. This sequence of events produces vibrations, generating click-like sounds every few seconds that the researchers measured in the acoustic frequency range of about 20 to 100 kilohertz. They estimated these frequencies to be audible to insects at distances up to 16 feet (around 5 meters). (Sound waves at frequencies higher than 20 kilohertz are typically beyond the range of human hearing).

The scientists wanted to know if these ultrasonic sounds were being picked up by insects, so they observed the Egyptian cotton leafworm moth (Spodoptera littoralis), which lays its eggs on plant leaves. The study team found these moths tended to avoid noisy, stressed tomato plants. Instead, the insects favored tomato plants that were quieter and therefore in better condition, with leaves that would provide a juicier meal for newly hatched larvae, according to a recent study published in the journal eLife.

“(The research) reveals a surprising new way that animals can sense and respond to cues from the environment — in this case, insects picking up sounds from stressed plants,” Dr. Yali V. Zhang, an adjunct associate professor of physiology at the University of Pennsylvania’s Perelman School of Medicine, said in an email. Zhang, who studies chemical communication between insects and plants at the Monell Chemical Senses Center in Philadelphia, was not involved in the study.

However, just because the moths hear and react to sounds from dehydrated plants doesn’t mean that the plants and moths are holding a conversation, Seltzer said.

“It’s important to clarify that we observed an interaction — not communication,” she added. “It’s crucial to understand that plants likely produce these sounds passively and are not actively trying to communicate with insects.”

Listening and interpreting

When the scientists presented female moths with a choice between a dried-out tomato plant and a fresh one, the moths flocked to the fresh plant to lay their eggs. But the team wanted to understand whether the sounds from the dehydrated plant were a factor in the moths’ selection. The researchers built an arena consisting of two boxes. In one box, they played recordings of plants’ distress sounds, while the other box was silent.

In the experiment, moths gravitated to the noisy box, and scientists theorize that it’s because the sounds suggested that they would find a living plant there. This preference vanished when the scientists deafened the moths, leading to the conclusion that moths were listening to the plants before they made their choices, the study authors wrote.

In another experiment, the researchers again offered the female moths a selection of two tomato plants. This time, both plants were hydrated, but one was near a speaker playing ultrasonic stress sounds. The result: More of the moths picked the quieter plant.

A third experiment tested whether the egg-laying females were influenced by other acoustic cues, such as those of male moths, who produce ultrasonic courtship clicks. It found that when male moths’ calls were broadcast from one side of the arena, females “showed no significant preference” as to where they lay their eggs, according to the study.

Together, these experiments told the researchers that the moths were listening to and recognizing sounds made by plants — and that they were using that acoustic information to make decisions about those plants for their reproduction, Seltzer said.

‘Cool and surprising’

Insects have been capable of detecting ultrasonic sounds since at least the Eocene epoch (55.8 million to 33.9 million years ago). However, the researchers were surprised to see that the moths recognized the plants’ ultrasonic signs of distress, Seltzer said. Even though female moths in the experiments had never laid eggs before — and therefore had no prior experience deciphering acoustic cues from plants — they not only recognized plant sounds, but preferred quieter plants for their first time egg-laying.

“It’s a really cool and surprising discovery!” Zhang said. “We’ve known for a long time that plants give off smells when they’re in trouble, like when they’re being eaten by bugs or are thirsty. These smells help attract helpful insects or scare away pests,” he said.

“What’s new here is that sound may give extra information — like a warning signal — especially when there’s no smell or the smell is hard to detect. “

Plant acoustics is an area of growing interest for scientists, and preliminary work suggests that sound-making is widespread, Seltzer said. Future research could investigate the use of acoustic monitoring to understand how stress affects plant health and could identify potential applications for agricultural pest control.

“I believe this is just the beginning of discoveries in this field,” Seltzer said. “There are countless organisms that can hear in these frequencies, and potentially many more plant sounds we haven’t discovered yet,” she added. “This is definitely just the tip of the iceberg.”

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