Sometimes ants are pests, marching through our kitchens on an industrious quest for crumbs. But when faced with more serious pests — namely those that destroy crops on which people's livelihoods depend — we can also use ants to our advantage.
Published in the Journal of Applied Ecology, a new research review suggests ants can control agricultural pests as efficiently as synthetic pesticides, with the bonus of being more cost-effective and generally safer. And since many pesticides pose a danger to helpful wildlife like birds, bees and spiders — not to mention humans — ants might be a key ally in feeding the planet's booming human population.
The review covers more than 70 scientific studies on dozens of pest species that plague nine crop varieties in Africa, Southeast Asia and Australia. Because ants are organized as "superorganisms" — meaning the colony itself is like an organism, with individual ants acting as "cells" that can move around independently — they are uniquely capable of hunting down pests and then overwhelming them.
"Ants are great hunters and they work cooperatively," says author Joachim Offenberg, a biologist at Aarhus University in Denmark, in a press release about the research. "When an ant finds its prey, it uses pheromones to summon help from other ants in the nest. By working together, they can subdue even large pests."
Most studies in the review focused on weaver ants, a tropical genus of tree-dwelling ants that weave ball-shaped nests using leaves and larval silk. Since they live in the canopy of their host trees, near the fruit and flowers that need protection, weaver ants have a natural tendency to control pest populations in orchards.
A colony of weaver ants in India works on converting leaves into a nest. (Photo: Raghu Mohan/Flickr)
In one three-year study, Australian cashew growers recorded yields 49 percent higher in trees guarded by weaver ants versus trees treated with synthetic chemicals. But higher yields were only part of the prize: The farmers also got higher-quality cashews from the trees with ants, resulting in a 71 percent higher net income.
Similar results were reported in mango orchards. While mango trees with ants had roughly the same yields as those with synthetic chemicals, the ants were cheaper — and the trees they inhabited grew higher-quality fruit. That led to a 73 percent higher net income compared with pesticide-treated trees. Not all crops had such dramatic results, but studies on more than 50 pests showed that ants can protect crops including cocoa, citrus and palm oil at least as effectively as pesticides.
"Although these are rare cases where the ants were superior to chemicals, many studies show that ants are just as efficient as chemical controls," Offenberg says. "And of course ant technology is much cheaper than chemical pest control."
To recruit weaver ants in their orchards, farmers just collect nests from the wild, hang them in plastic bags from tree branches and feed them a sugar solution while they build new nests. Once the ants establish their colony, farmers can help them expand by connecting target trees with aerial walkways made from string or vines.
The ants are mostly self-sufficient from there, needing only some water during the dry season — provided via plastic bottles in the trees — and pruning of non-target trees that host different ant colonies to prevent fights. Farmers can also help their ants by avoiding broad-spectrum insecticide sprays, researchers say.
It's worth noting that ants can also be detrimental to some plants, such as when they herd sap-feeding insects like aphids and leafhoppers. But if they still fend off fruit-ruining flies and beetles, their net impact may be positive nonetheless. Not only do weaver ants kill pest insects on their trees, but their presence alone is reportedly enough to scare away marauders as large as snakes and fruit bats. And research suggests their urine even contains important plant nutrients.
The use of ants for pest control isn't new. As early as 300 B.C., Chinese farmers could buy weaver ants in markets to release in their citrus groves, a practice that has faded over time, especially after the advent of chemical pesticides. But it may be coming back, both because ants are cheaper than pesticides and because certified organic produce can fetch higher prices, due to concerns that broad-spectrum pesticides harm more than just pests. Aarhus University is studying the use of weaver ants as pest control in Benin and Tanzania, for example, where the insects could lead to increased export revenue of $120 million and $65 million, respectively.
"To kill the flies with pesticides, you have to make the mango so poisonous that it can kill the maggot," Aarhus University biologist Mogens Gissel Nielsen told China's Xinhua news agency in 2010. "But when it is too poisoned for the maggot to eat, it might not be good for us to eat either."
While the research in Offenberg's review focused largely on weaver ants, he points out they "share beneficial traits with almost 13,000 other ant species, and are unlikely to be unique in their properties as control agents." Lots of ants nest in the ground, and while it may be a challenge to relocate them, they too have shown promise in protecting a variety of commercially important crops.
"Weaver ants need a canopy for their nests, so they are limited to plantations and forestry in the tropics," Offenberg says. "But ground-living ants can be used in crops such as maize and sugar cane. European wood ants are renowned for controlling pests in forestry, and new projects are trying to use wood ants to control winter moths in apple orchards. Ants could even be used to fight plant pathogens because they produce antibiotics to combat diseases in their dense societies."