By Elizabeth Pennisi
For thousands of years small farmers in Mexico and parts of Central America have practiced a form of agriculture, known as milpa, that traces back to the ancient Maya. Tradition holds that mixing crops—typically maize, beans, and squash, the Three Sisters—in a single field improves yields. Scientists now know the beans fix nitrogen into the soil, enriching it for all plants; the maize stalks provide a structure for the beans to climb; and the squash covers the ground, suppressing weeds and keeping the soil moist. But researchers also suspected this “intercropping” has an additional benefit: helping decrease damage by plant pests.
New work, presented this month at the annual meeting of the Ecological Society of America in Los Angeles, suggests harm by pests is indeed reduced in these systems and that plant teamwork is responsible. By showing how the maize signals the beans to lure insects that eat their pests, the work could inspire other studies that might ultimately point to new ways for modern farmers to reduce pesticide use. But other work presented at the meeting suggested replicating the success of the Three Sisters may not be easy. Planting genetically different tomatoes together, a study showed, only protected them against pests when the set of varieties produced the right blend of volatile chemicals. “It is important which [crop variety] you are using,” says Patrick Grof-Tisza, a plant-insect ecologist at Converse University.
Three years ago, Betty Benrey, an entomologist at the University of Neuchâtel, recruited Grof-Tisza in part to sort out exactly how the Three Sisters help each other reduce pest damage. It “has been a little bit of a mystery,” says Ian Pearse, a research ecologist at the U.S. Geological Survey.
Working in Oaxaca, Mexico, Grof-Tisza’s team grew the three crops in experimental fields separately, in pairs, and as trios. In addition to recording pest damage, they tracked all the arthropods that visited each plot and classified them as either pests, such as caterpillars, or the pests’ natural enemies, such as parasitic wasps. Although the analyses are not complete, the studies seem to confirm that monocultures enjoy less protection from herbivores than pairs or trios of crops, and those mixed plantings may therefore be more productive.
The studies strongly suggested diversity helps because each species naturally attracts a distinct set of pest enemies, helping amass a huge army of defenders. This recruitment is the secret to the Three Sisters’s success, Grof-Tisza reported at the meeting. (The group’s work also appears in the 15 September issue of Agriculture, Ecosystems & Environment.)
To understand how the plants attract their defenders, he and his colleagues grew maize in the lab and collected volatile chemicals given off when the plants were attacked by armyworms, serious crop pests. Researchers already knew those odors could stimulate an increase of toxins against herbivores in undamaged parts of the plant and in other maize nearby. The lab and field tests now show these signals also trigger a reaction in neighboring bean plants: They increase the concentration and amount of sugar in nectar that beans produce in tiny glands below their leaves. These so-called extrafloral nectaries appear to attract even more ants and wasps than usual, which chow down on crop-destroying caterpillars, the team found.
She found that compounds called terpenoids, which endow plants with their citrusy, woody, and even turpentinelike aromas, had big effects on herbivores and their predators, but only some combinations were helpful. Tomato plants making high amounts of simple terpenoids called monoterpenes attracted fewer herbivores and more predators than varieties making more complicated terpenoids. The protective effect extended across other tomato plants in the same plot. “It does matter who your neighbor is,” Glassmire says.
“We are homing in” on what helps crops work together, says Philip Hahn, an ecologist who studies plant-insect interactions at the University of Florida. And that should ultimately help farmers reduce their use of chemical pesticides, says Richard Karban, an ecologist at UC Davis. “The more we understand the nuances and mechanisms of how plants [like the Three Sisters] interact, the more we will be in a better position to come up with cool innovation solutions for agriculture.”
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