1. How Does a Chewed Leaf Call in Insects That Hunt Its Attackers?
Scientists call these scents herbivory-induced plant volatiles, or HIPVs. “Herbivory” means being eaten by plant-eaters, and “volatiles” are chemicals that drift off into the air. A review from Purdue University describes them as released in response to feeding by plant-eating arthropods and serving to guide predators and parasitic wasps to their prey [5]. (A parasitic wasp lays its eggs in or on another insect, and its young feed on that host.)
A study published in the journal Nature in 2000 added a surprise. Lima bean leaves that had never been chewed switched on defence genes when they were exposed to the scent of leaves infested with spider mites. They did not switch on when exposed to the scent of leaves that were only cut or damaged by hand. At least three chemicals in the scent were responsible, and they were released in response to feeding but not to artificial wounding [1]. So the scent is not simply “a smell of damage.” It depends on what is doing the damaging.
That raises the question this article follows: if plants already do this, can a farmer put the scent out first and let the helpers do the work?
2. Did a Scent and a Honey Dish Really Cut Moths in a Japanese Greenhouse?
A first note for readers outside Japan: this trial took place in the Miyama area of Kyoto, in greenhouses growing mizuna, a leafy green in the cabbage family that is common in Japanese cooking. The pest was the diamondback moth, a small moth whose caterpillars are a major problem for cabbage-family crops. The helper was a tiny parasitic wasp, Cotesia vestalis, which already lives in the countryside around the greenhouses [3].
The team had earlier found that a blend of four chemicals from cabbage chewed by diamondback caterpillars could attract this wasp. For the trial they made that blend into a dispenser and added a honey feeder, so that the wasps drawn in would have something to eat while they stayed [2].
The result, published in 2020, was that over two years the monthly rate of diamondback moths in the treated greenhouses was significantly lower than in untreated greenhouses. More wasps were recorded in the treated greenhouses when moth numbers rose [3]. Kyoto University’s news release put the drop as “about half or less,” and called it the first success in the world at managing a target pest this way, using the scent of a damaged plant and a native natural enemy [2].
Two cautions. I could read the paper’s abstract but not its full text, so the “half” figure comes from the university’s release, not from tables I checked myself. And the result covers one crop, one pest, and one wasp in one region. It does not show that the same trick works everywhere.
3. What Happened When a Plant Scent Was Put Out in an Open Cranberry Field?
An open field is a different place from a greenhouse. A team studying cranberry bogs tested a lure containing methyl salicylate (MeSA), one of the best-known scents that plants give off when chewed. They did this over two years, 2011 and 2012, and used cameras to see who showed up [4].
The lure worked in one way: lady beetles, hoverflies, and predatory mites visited egg masses of the European corn borer (a pest moth) that the researchers had placed there, and the eggs were eaten about twice as often as at spots with no lure. But the lure also drew in pests. Thrips, which are tiny sap-sucking insects, were captured in larger numbers on sticky traps near the lure, and so were plant bugs in 2012 [4].
Notice what was measured. Eggs eaten and insects caught on traps are not the same as less damage to cranberries. The authors say their data show the lure increased predator attraction and predation, and that is the claim; whether crops ended up better protected is a different measurement [4]. One difference from Japan is worth a thought: the Kyoto blend was built to match one pest and one wasp, while MeSA is a single, widespread chemical that many insects respond to.
4. Why Might the Same Scent Help Some Insects and Not Others?
Part of the answer is that several kinds of insects can respond to the same scent. The Purdue review notes that the usual story, in which plants call for bodyguards that limit damage, assumes a simple food chain with three levels: plant, plant-eater, and its enemy. Real food webs are more tangled. Recent work, it says, shows that hyperparasitoids, insects that are parasites of the parasitic wasps, use the same scent clues as the wasps do. They even adjust their search to the scent of a plant being eaten by a caterpillar that is already parasitized, which holds their prey [5].
If that finding turns out to be common, the review concludes, the scent may not always benefit the plant. It adds that this matters for deliberately changing plant traits to improve biological control in crops [5]. This is why the careful claim is not “scent protects crops” but “in some tested settings, a matched scent plus food for the helpers shifted the balance.”
5. Is There a Farm Method That Uses Scent to Push Pests Away Instead?
Yes, and it does not rely on calling anything in. It is called push-pull, developed over about twenty years by icipe, an insect-science research centre in Africa, working with Rothamsted Research in the UK and partners in eastern Africa [6].
In push-pull maize farming, a plant called desmodium is grown between the rows. It drives away or deters stemborers, moths whose caterpillars bore into maize stalks. Around the field edge, farmers plant Napier grass, which attracts and traps the pests. So pests are pushed out of the crop and pulled to the border. Desmodium also stimulates seeds of Striga, a parasitic weed that harms cereals, to sprout and die without a host, and as a legume it adds nitrogen to the soil [6].
In 2015 icipe reported on 400 farmers’ fields in Kenya, Uganda, and Tanzania. Where farmers used a drought-tolerant version, about 5% of maize was destroyed by stemborers, compared with about 40% for farmers not using it. Yields were about 2.5 times higher: roughly 4 tonnes per hectare against 1.5 [6]. These are figures from the research centre that developed the method, so they are worth reading as a report, not a neutral audit. The news item also said about 110,000 farmers were using push-pull at that time [6].
6. So How Far Can Plant Scents Be Used on Farms?
Putting the trials side by side: a matched blend with honey feeders held a target moth down in Kyoto greenhouses; a single scent in cranberry fields increased egg predation but drew pests too; and in East Africa, the plants that push and pull pests protect maize on hundreds of farms. Each trial measured something different (moth rates, eggs eaten, maize destroyed), which is why they can’t be added together into one verdict.
Together, these trials suggest that a scent may help in some settings, when the scent, the pest, the helper, and the crop are matched and the helpers have food and a place to stay. It works less well as a one-size-fits-all lure. The idea also depends on a plant behaviour that was discovered in the wild first, then copied by people.
7. How Can You Look for Chewed Leaves and Their Visitors Yourself?
Look for a leaf with chew marks in a park, a school garden, or near your home. If you have a magnifying glass, check the underside and the edges for small insects. You may find the chewer, and sometimes another small insect nearby that is hunting it.
Look from a short distance. Do not touch hairy caterpillars or any bees and wasps, and do not walk into someone else’s field or garden without asking. Two questions to take along: does the same kind of plant have different visitors in a sunny spot and a shady one? And what would you measure to decide whether a scent really protected a crop?