When maize screams, beans listen: How the Three Sisters crop trio repels pests

Scientists are teasing out the benefits of intercropping, but getting the right mix of plants is key

27 AUG 2024 3:30 PM ET
BY ELIZABETH PENNISI



Researchers studying experimental fields mixing squash, maize, and beans discovered at least two of these species work together to fight pests.PATRICK GROF-TISZA

A version of this story appeared in Science, Vol 385, Issue 6712.

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.

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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.)

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