The screams of thirsty plants may prompt some moths to lay eggs elsewhere

If you’re a female moth, finding the best plant to host your young may be as simple as taking a close listen. 

When low on water, some plants make high-pitched clicking noises, inaudible to humans. Female moths, using their sensitive hearing, appear to key in on the distressed din and steer clear in order to lay their eggs on plants that will better feed their caterpillars after hatching, researchers report November 14 at bioRxiv.org. 

Ultrasonic thirsty plant clicks were discovered in 2023 by colleagues of Rya Seltzer, an entomologist at Tel Aviv University (SN: 3/30/23). She and her team wondered if moths — which could theoretically hear in the high frequency range of the clicks — might use that information to their advantage when selecting plants to host their young.

The researchers placed fertile female Egyptian cotton leafworm moths (Spodoptera littoralis) in an arena with speakers playing the sounds of a dehydrated tomato plant on one side and silence on the other. The moths preferentially laid their eggs near the speaker making the distressed clicks, the team found.

“However, the plot thickened when we introduced actual plants into the setup,” Seltzer says. 

When the team ran experiments with no speakers, but a hydrated tomato plant on one side of the arena and a thirsty one on the other, the moths switched their preference, opting for the silent hydrated plant. In one experiment, the team placed a hydrated plant on each side of the arena and a speaker on one of those sides playing distressed clicks. The moths laid more eggs on the plant on the silent side. 

With no actual plants present for moths to identify with other senses, the researchers write, the sounds of a drying plant are sole indicators of vegetative life, suggesting only one option for raising caterpillars. But with access to plants that the moths can see and smell, the insects can now choose between plants, avoiding those that sound stressed and opting for a potentially healthier host.

“Female moths not only recognize these sounds as being made by plants but also connect them to the physiological state of the plants,” Seltzer says. “It’s fascinating to consider how much information exists below the threshold of human hearing.”

Notably, these moths had no prior exposure to plants, being reared entirely in the lab. So being attuned to the plants’ clicking is deeply rooted in their genetics, Seltzer says.

Björn Thorin Jonsson, a biologist at the University of Graz in Austria, notes: “If you can detect an acoustic cue that is widespread, reliable and can be beneficial for selecting better food or more suitable [egg-laying] sites, why not use it?”

Sensory biologist Fernando Montealegre-Zapata of the University of Lincoln in England wonders if there could be agricultural applications of the plants’ noise. “Could the playback of stress acoustic cues be implemented in integrated pest management programs to discourage moths from [laying eggs] on healthy plants?”

Seltzer thinks this kind of response to plant noises may be widespread among insects and plants. Many insects have ultrasonic hearing capable of hearing plant clicking.

“I believe that this discovery is just the beginning of understanding the acoustic interactions between animals and plants,” she says.