If your carefully tended crop of beans ever comes under attack from caterpillars, you should know it's not totally helpless.
In fact, a cavalry of caterpillar-munching wasps may already be on their way.
Bean plants have a remarkable ability to summon wasps from thin air when caterpillars attack, and now, scientists have figured out exactly how they do it.
Humans have cultivated the common bean plant species, Phaseolus vulgaris, to give us much-loved varieties such as kidney beans, black beans, pinto beans, navy beans, and more.
If you've grown beans in your garden or kept them in your pantry, odds are, they were a variety of P. vulgaris.
It's the fascinating wasp-summoning abilities of P. vulgaris that plant immunologist Natalia Guayazán Palacios and colleagues have unraveled in a recent study.
Now, before you start imagining a plant 'screaming' for help, it's impossible to say if the plant actually intends for the wasps to arrive (we're still not sure plants can intend anything at all, though botanists, neurologists, and philosophers alike are hard at work debating this idea).
The so-called 'scream' is actually an emission of volatile chemicals, and the wasp's response to these smells may be more like a happy accident that has been continually reinforced across the long history of plant-arthropod interactions.
Wasps, with their central nervous systems capable of learning, may have come to associate the specific smell of a plant under caterpillar attack with a potential opportunity to feast, creating a feedback loop in natural selection that benefits both the wasp and the plant.
Over many generations, bean plants that emitted certain combinations of volatiles when a grub was chowing down – the kind that wasps can detect and readily respond to – may have enjoyed better protection from their pupal predators.
This could have given them a better chance at producing beans that survive to do the same.
The new study reveals the bean plant's immunological pathway to produce this specific 'perfume', the scent that wasps read as: 'It's caterpillar crunch time'.
It all pivots around a receptor embedded in the surface of the plant's leaves.
Biological receptors are proteins capable of receiving and transferring information within a biological system – a bean plant, for instance.
Common bean plants, the study found, have receptors that recognize and react to inceptin, a peptide that occurs commonly in caterpillar 'spit'.
When caterpillars chew at a bean plant's leaves, the plant's inceptin receptors are set off, triggering a wave of immunological responses in the plant, which help the plant do more than just heal its wounds.
"Inceptin recognition does not only amplify the wound response, but activates an herbivore-specific immune pathway to trigger the emission of a distinctive volatile blend that recruits predatory wasps to effectively remove caterpillars from the plants," Guayazán Palacios and team write.
In 2023 and 2024, plenty of wasp activity was going on in a set of bean fields in Oaxaca, Mexico.
The researchers grew these bean plants in pairs: one plant with fully-functional inceptin receptors, and one plant missing the gene necessary for inceptin receptors to form.
One group of plant pairs had their leaves doused with caterpillar saliva; another was treated with a pure form of the inceptin peptide In11; and another was wounded with a razor blade and dabbed with water.
Then, the researchers pinned dead fall armyworm caterpillars (Spodoptera frugiperda) to the plants, and waited to see what the wasps would do.
By testing these treatments on plants growing side-by-side, experiencing the same conditions all except for one, they were able to see what a difference inceptin receptors actually make in coping with a simulated caterpillar attack.
Although there was no shortage of wasps nearby, plants missing their inceptin receptors were seriously disadvantaged.
They had 40 percent fewer wasps come to their aid, both when they were treated with the caterpillar saliva, and when they were spritzed with the pure inceptin.
Interestingly, there was no difference in wasp visitation when plants were wounded with a razor.
These results indicate it's the chemical signals on a caterpillar's breath – not the physical damage its mandibles inflict – that set off the inceptin receptors to start 'advertising' fresh caterpillar on the wasp menu.
Guayazán Palacios and team also showed in lab experiments that the plants' inceptin receptors are a direct trigger for the specific blend of volatiles that attracts hungry wasps.
Without the genes for inceptin receptors, the plants "did not emit the typical herbivore-induced volatiles blend that is normally induced upon In11 treatment, but rather emitted volatiles that bean plants release after wounding alone," the team reports.
In contrast, inceptin-sensitive plants emitted the "characteristic blend of volatiles" when they were treated with either pure In11 or armyworm spit.
Related: The Mysterious Network of Plants May Be Nastier Than We Thought
This discovery could help in the development of pesticide-free solutions to protect crops from caterpillars, and it gives us a better understanding of what's really going on in a complex, three-species ecological interaction.
So, next time, before you reach for the caterpillar spray, have you considered giving wasps a chance?
The research is published in Science Advances.