The buzz of an approaching bee or wasp spurs many people to flee. Some plants respond to “hearing” insects, too. One common garden bloom can tell the difference between the buzz of friends and foes — and change the recipe of its nectar in response.

That’s the new finding of the “good vibes” project conducted by researchers in Italy, Spain and Australia.

Snapdragons sweeten their nectar when a pollinator’s buzz fills the air. But if the neighborhood buzz instead belongs to a nectar thief, the plant holds back on sugar. This is the first report of such sound-specific plant responses. It’s called vibroacoustic (VY-broh-uh-KOO-stik) communication.

Researchers shared their new findings May 21 at the annual meeting of the Acoustical Society of America in New Orleans, La.

Francesca Barbero led the new research. An insect biologist, she works at the University of Turin in Italy. In an earlier study, her team had watched insects visiting snapdragon blooms. They recorded 43 visiting species.

They also noted which insects entered a flower and for how long. This is important as insects can transfer pollen as they brush up against structures inside a flower. Some bees spent a lot of time inside. And flowers visited by those pollinators made more seeds. More seeds can mean more baby plants.

Not all insects climbed inside, however. Some wasps and bumblebees instead drilled a hole in the side of a bloom to sip the nectar hiding within. Because this action didn’t help the plant, scientists refer to these animals as nectar robbers. They stole from the plant something meant to invite visits by pollinators.

While in the field, Barbero noticed that nearly all insects make sounds as they fly. And their sounds vary by insect type. She wondered: Could plants “hear” those sounds and tell them apart? And if they could, did they respond differently to specific buzzes?

It wasn’t that weird an idea. A 2019 study had shown that evening primroses sweetened their nectar as bees approached. Maybe this had been due to the sound of a bee’s approach, Barbero thought. She wondered if it might be “a kind of communication channel” — one “that has been underestimated.”

She and her team decided to test that.

Sound-specific responses

They started by recording the sounds of two flying insects that visit snapdragons. One was a bee in the Amegilla genus. Unlike hive-dwellers, these bees lead a solitary lifestyle, one small family per nest. And while they don’t produce honey, they do pollinate many plants, including farm crops.

The team’s work had shown that this particular bee boosted seed yields more than any other pollinator. The other was a type of bumblebee. Rarely entering blooms, it was a nectar-robber.

The researchers also recorded background noise. “Pink noise is usually what we have in nature,” explains team member Sebastian Oberst. He studies bioacoustics at the University of Technology Sydney in Australia.

Pink noise served as what scientists call a control condition. It tested whether any old nature sounds would trigger changes in a plant. To gauge that, the researchers compared how plants responded to it versus the buzz of bees.

No bees actually took part in these experiments. The researchers just broadcast the sounds they make in flight.

A speaker in front of snapdragons growing indoors played recordings of their flight sounds as one type of bee had buzzed and paused while visiting flowers for three hours. Others heard just three hours of pink noise.

The team repeatedly measured the how much sugar was in each plant’s nectar. They checked after just 10 minutes, then after three hours and again after five days. They also checked whether any of the plants’ genes become more or less active over that time.

The pink noise had no effect. But plants added sugar to their nectar within 10 minutes of “hearing” the sound of a pollinator. Sweeter nectar rewards the bees, which may encourage them to visit more blooms.

Even more surprising: The plants produced a low-sugar nectar when the team played the robber’s buzz.

A role for genes?

The team kept serenading the plants for an hour a day for the next four days. “This effect holds on for a very long period,” Oberst says — at least five days.

Those long-lasting effects seem due to changes in gene activity.

“We did a full screening” of the genes, Barbero says. Some became more active — others less so — depending on the sound played. The most active genes likely play some role in moving sugar into and out of nectar.

Though insects and plants are not closely related, “still they can communicate,” Barbero says. The new findings suggest that sounds played a role in the coevolution of this plant and its pollinator, her team says. This type of communication hasn’t been explored much.

Alessandro Cini calls the work “extremely exciting.” A behavioral ecologist at the University of Pisa in Italy, he did not take part in the study. Its findings could lead to new methods of monitoring insects or controlling plant responses, he says.

The new data also open a “fascinating window on the long coevolutionary bond between insects and flowers,” he says. “A bond which indeed shaped our world.”