A secret weapon in agriculture's climate fight: Ants

Ida Cecilie Jensen The ant scurries along on six nimble legs. It catches up to its peers, a line of antennaed bugs roaming the winding surface of a tree, perpetually hunting for food. While doing so, each unknowingly leaves antibiotic microorganisms secreted from its feet.  That trail of tiny footprints, indiscernible to the naked eye, is remarkably effective at protecting the tree from pathogens and pests. That makes ants, in the eyes of Ida Cecilie Jensen, a legion of unlikely warriors–one humans should consider enlisting in the fight to grow food in a warming world. “Ants are a Swiss Army knife,” said Jensen, a biologist who studies the symbiotic relationship between ants and agriculture at Aarhus University in Denmark. “Kind of like a multitool for farmers.”  With an estimated 20 quadrillion ants on Earth at any given time, the bugs are found just about everywhere on the planet, reports Grist. They are also among the species that humans, which they outnumber at least 2.5 million to one, have most in common with. Ants have extraordinary collective intelligence, their colonies weaving robust community networks and dividing labor. The social insects even wage war with one another and build complex agricultural systems.  Ants also have “so many of the same problems and challenges that we have,” Jensen said. “Luckily for us, they already found a lot of great solutions.” One such challenge is how to grow food while confronting climate-wrought consequences–such as an influx of spreading plant pathogens caused by warming.  Plant diseases cost the global economy hundreds of billions of dollars every year, with between 20-40% of global crop production lost to crop diseases and pests. Climate change is ramping up outbreak risks by morphing how pathogens evolve, facilitating the emergence of new strains, and making crops more susceptible to infection. Most farmers and growers increasingly rely on chemical pesticides to combat these emerging issues, but the widespread use of such substances has created problems of its own. Synthetic pesticides can be harmful to humans and animals, and lose their efficacy as pathogens build up resistance to them. The production and use of synthetic pesticides also contribute to climate change, as some are derived from planet-warming fossil fuels.  Instead of chemicals, an army of ants may march right in. Though most people view the small insects as little more than a nuisance, colonies of them are being deployed in orchards across a handful of countries to stave off the spread of crippling infestation and disease.  In a body of recently published and forthcoming research, Jensen examined the antimicrobial effects of wood ants, a European field ant known for building dome-shaped nests in fields and open woodlands, and weaver ants, which live in ball-shaped nests within tropical tree canopies across Asia, Africa, and Australia. Her team looked at how the microbes influenced apple brown rot and apple scab in two orchards in Denmark–one commercial and one experimental–and found that wood ants effectively reduce apple scab, which can cause serious yield losses, by an average of 61%. The scientists also found that the number of disease-free apples more than doubled compared to when ants weren’t wielded as an alternative biological pesticide. For another experiment in Senegal, they collected weaver ants from mango orchards to investigate the bacterial communities associated with ants, discovering that they also leave microbial footprints that may inhibit fungal diseases such as mango anthracnose, which can lead to extensive yield losses.  Past studies have found that for crops from cocoa to citrus, ants could replace insecticides in a multitude of climates and locations, reducing incidences of pear scab in pear trees, coffee leaf rust in coffee shrubs, and leaf fungal attacks in oak seedlings. Weaver ant nests used as an alternative pesticide in mango, cashew, and citrus trees have all been shown to lower pest damage and produce yields on par with several chemical pesticide treatments. For more than a millennium, the species was embraced as a natural insecticide in countries like China but never quite made its way into the agricultural mainstream in North America or Europe. The method would eventually be replaced by the dawn of synthetic solutions. Still, despite that legacy, exactly how ants take on disease has remained a scientific mystery.  The answer, Jensen said, lies in how ants function. All species of the arthropod possess a body that is essentially hostile for bacteria because they produce formic acid, which they use to constantly disinfect themselves. Ants are also perpetually hungry little things that will feast on the spores of plant pathogens, among other things, and their secretion of formic acid and highly territorial nature tends to deter a medley of other insects that could be transmitting diseases or making lunch of some farmers’ crops. Ultimately, their greatest trick is what Jensen’s newest research reveals: Ants also inherently have antimicrobial bacteria and fungi on their bodies and feet, which can reduce plant diseases in afflicted crops, with these microorganisms deposited as the critters walk. When the bugs are cultivated in fruit orchards, they march all over trees, their feet coating the plants in microbial organisms that can curb emerging pathogens.  Understanding why they have this effect makes it easier to promote and implement native species of ants as biological agents in fields and farms, which Jensen advocates for. She’s not only researching how to do this as a doctoral candidate, but also founded AgroAnt in 2022, a company that leases colonies to cull plant pathogens and pests to farmers in Denmark–much like beekeepers lease hives. Her research team is now looking into boosting populations of existing ant colonies already living in orchards, rather than introducing new ones. Building rope bridges between trees to help ants better get around, and increasing the number of sugary extracts left in strategic locations to feed them, can create ant population booms, which Jensen sees as a simple and inexpensive way for farmers to ward off costly bouts of crop disease. Others are not convinced this would be any more useful or cost-effective than existing biopesticides like canola oil and baking soda, or pest management chemicals derived from natural sources. Kerik Cox, who researches plant pathology at Cornell University, said that many of the microbes derived from the ants in the study have already been studied, and optimized for formulation and efficacy in agricultural systems. “Many are highly effective and there are numerous commercial products available for farmers to use,” said Cox, adding that he doesn’t see “anything in this study that would be better than the existing biopesticide tools, which are registered by the [U.S. Environmental Protection Agency].”  Jensen acknowledges there is always a risk when introducing any species–ants new to an area could push out other beneficial species, for example, or attract aphids, those small green plant-damaging insects that ants share a symbiotic relationship with. Still, she is adamant that as long as the species is native to the area and agricultural system they’re being introduced to and then properly managed, the possible benefits outweigh the pitfalls.  On a practical note, the money-saving argument of ants pitted against synthetic products also carries a big draw; particularly given that conventional pesticides, in addition to their organic, chemical-free counterparts, have become more expensive in recent years across Europe and the U.S. Those product prices tend to climb when extreme weather shocks disrupt production, a likelihood as climate change makes disasters more frequent and severe.  Conversely, Jensen said farmers can simply leave sugar-water solutions, cat food, or chicken bones, among any number of kitchen scraps, in fruit orchards where beneficial, pathogen-combating ants are typically already present–such as weaver ants in mango orchards. If the species already dwell there, this could increase their numbers and efficiency. The technique, however, should be approached with caution depending on location, to minimize the risk of attracting potentially harmful members of the ant family.  “I don’t believe in one solution that could fit everything, but I definitely think that ants and other biological control agents are going to be a huge part of the [climate] puzzle in the future,” she said. This story was produced by Grist and reviewed and distributed by Stacker.