Pickett, Dicke, and a few other teams around the world want to use this knowledge to teach our crop plants the lost art of self-defense. Many of the plants we grow for food have been bred for generations to taste or look good and along the way have lost their ancestral communication skills. The highly inbred strains of corn commonly grown in the United States no longer respond to signals, but varieties preferred by farmers in other parts of the world still have the knack. Their genes could be reintroduced to commercial crops. Dicke has identified one strain of cucumber that’s twice as good as the usual Dutch commercial varieties at attracting predatory mites, and he hopes to breed this ability back into the plant.
Real-world proof that this strategy can work comes from a project running since 1995 in Kenya and neighboring parts of East Africa, where moth larvae called stem borers eat up to 80 percent of the corn crop. Under the “push-pull” system developed by the International Center of Insect Physiology and Ecology in Kenya in collaboration with Rothamsted Research, farmers use signaling plants to fool larvae into staying off their crops.
For the “push,” they plant a shrubby cloverlike plant called desmodium in between the corn or sorghum plants. Its emissions repel stem borers, keeping them away from the grains. The “pull” comes from Napier grass, which attracts moths looking to lay their eggs, planted around the edge of the field to keep the moths away from the corn. Molasses grass, whose signals summon tiny parasitic wasps that destroy stem borer larvae, can also be planted among the corn as an added weapon. According to ICIPE, more than 75,000 smallholder farmers now follow this system, which can triple their yields.
This system could be adapted to many parts of the world where small-scale farmers customarily plant several plants together. But what about American- or Canadian-style intensive farming, where enormous yields are made possible by machines, monocrops, and insecticides?
Simply spraying crops with chemicals to attract predators of agricultural pests won’t work, says Martin Heil, an ecologist at the Mexican research institute CINVESTAV Irapuato who studies plant-insect signaling. It could crash the whole system. At first, predatory bugs would swarm around a plant that seems to be calling for help. But if the plant is crying wolf, and there are no juicy bugs to be found, the predators will either move on or starve to death.
“Reliable, honest information is crucial,” Heil says. He is working on another way to feed predatory insects that visit crop plants, by cultivating varieties that naturally produce nectar from their stems and leaves, a treat that meat-eating bugs particularly enjoy.
Breeding plants to be better communicators is just one of the ideas inspired by the science of plant signaling. Genetically engineered sentinel plants could be stationed in fields like canaries in the coal mine—the first to get attacked and the first to raise the alarm, so valuable crops get a head start on making their own defenses. Crops that naturally produce pest-attracting volatiles could instead be genetically silenced, a way of cloaking them so that the bugs don’t even know they’re there.
But for a pest-control strategy based on information to work, agriculture is going to have to become smarter. The communication triangles among plants, pests, and predators must be fully understood. Timing is also key: Predatory bugs need to be persuaded to show up early, before pests do much damage.
The transition from killing bugs to deceiving them may happen only under duress—when pesticides stop working or become too expensive to use, or if consumers revolt. Dicke points to a case in the Almeria region of Spain, where tomato and pepper growers were using illegal pesticides in their greenhouses until the Germans got wind of it in 2006. Overnight, European markets stopped buying Spanish peppers, and growers were forced to switch to a method that relies on insect predators instead.
Now all the peppers in the region are grown this way, as well as most of the cucumbers and melons—and pretty successfully. In a survey, 97 percent of growers said biocontrol worked better than chemical pesticides, Dicke says. “This option can replace chemicals to a large extent,” he says. “It needs this mind shift of saying, ‘We can do things without chemicals. What everyone says is impossible is possible.’ ”
It’s a plant’s world—we just live in it. So figuring out how they detect their enemies and recruit their protectors may truly be our best chance at reclaiming our lost harvests. By understanding how the crops that feed us deploy their own defenses, we can help them get better at it. We can turn plants into our allies—simply by learning to speak their language.
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