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Before noon in the Soconusco region of southern Chiapas, down by the border where Mexico meets Guatemala, the lush, green terrain is rife with bodies in motion. It is an area abutting the Pacific, a vast collection of plantations, most of them growing coffee. Hsun-Yi Hsieh, a doctoral candidate at the University of Michigan, has spent every summer here for the last four years, studying the relationships between four insects that survive by any means necessary among the coffee berry bushes. At the center of the story is a species of ant that has long used a kind of code to secretly communicate with each other. But a crafty beetle has recently evolved the ability to intercept the ant’s messages, giving it a big survival advantage and changing the balance of power in this fascinating pocket of the natural world.
Hsieh and a team of around six scientists make their annual trek to Finca Irlanda, a large plantation where they have been loaned a little over 100 acres for their research. To get up to the farm, one has to squeeze into a Kombi, a privately run bus crowded with farm workers, for a two-hour ride. Then it’s a 40-minute hike along the edges of dirt roads. Once at the farm, there aren’t any landline phones nor Internet service, though reception might be intermittently found for those who feel compelled to bring their cell phones. Still, there are plenty of signals to dial into; you just have to know exactly where to look. For Hsieh and her cohort, those messages come from the busy habitat that is the coffee plants.
An aggressive ant with the scientific name Azteca instabilis has long controlled significant parts of this habitat. The Aztecas don’t usually live in coffee plants—they actually make their homestead in nearby shade trees, and make frequent commutes of up to 50 feet across the forest floor to coffee bushes. They make this long trek to go harvest their favorite food: a liquid produced by the green coffee scale insect (Coccus viridis). The ants rub the bellies of the scale insects, which then exude the sweet “honeydew” from their anuses, and the ants gobble it up.
The scale insects are an unimposing bunch. They look like flattish, short domes and are largely sedentary, planting themselves on a coffee bush and sucking on its sap. (They’re consequently a major pest for coffee growers.) Kyle Turner, a graduate student at the University of Toronto studying the interactions between plant and ants calls them “insect barnacles” because they “sort of just sit there.” If attacked, scale insects cover themselves with a wax they produce, which isn’t very effective at all in deterring most predators. The scale insect’s best defense is actually its honeydew: Since the sweet liquid is a main source of food for the much bigger and stronger ants, they protect the scale insects fiercely, like shepherds guarding their flocks.
The beetle Azya orbigera—one of the many species referred to variously as a lady beetle, ladybird beetle, ladybird, or ladybug—likes to feast on the scale insects, and they try to lay their eggs nearby so their newly hatched larvae can eat them as a first meal. But it’s hard to gain access to the tasty scales when ants aggressively protect them. The ants are effective defenders, with powerful mandibles for crushing enemies. Even humans, whose skin is much thicker than insects’, find their hard bites painful.
Help From Above
Hsieh and her team noticed that despite the risk of great injury or death, ladybird beetles often succeed in depositing their eggs on coffee plants patrolled by ants, which the researchers found curious. Once the eggs are deposited, ants have a very difficult time defending themselves against the larvae, which are covered with white, waxy, rod-shaped protrusions that ants get stuck to; Turner describes the larvae as “terrifying, long, spiky, and very predatory-looking.” But ants instinctively know about the danger, so they keep their distance from the larvae and usually avoid the problem entirely by keeping adult beetles at bay. Yet Hsieh and her team found that the beetles not only managed to get past the ants, but had the gumption to hide their eggs right under scale insects. How did they pull it off? It turned out that the beetles had some secret allies.
Patrolling the space above the coffee berry bushes is another insect, a type of phorid fly (Pseudacteon laciniosus). The phorid fly zips close to the berry bushes in search of prey and places to lay their eggs. Some phorid flies inject their eggs into the heads of ants; as the larva develops, the parasitized ant still appears normal until the larva secretes enzymes that wreck the ant’s connective tissue, causing body parts, like legs, to fall off. Fortunately for the Azteca ants, they’ve developed a strategy for minimizing their losses to these flies: They release a chemical warning to those brethren nearby, in the form of an alarm pheromone.
Ants have up to 75 exocrine glands that secrete various specialized chemicals. Ten to 15 of these, depending on the species, are glands thought to produce the chemicals that ants use to communicate with each other. “It’s a language. It’s basically the language of the ants. We say words and they say chemicals, and the alarm signal signifies danger,” says Ivette Perfecto, an ecologist at the University of Michigan, and the leader of Hsieh’s research group. She has studied various interactions among different insects and plants in the Chiapas region for decades.
“It’s a language. It’s basically the language of the ants. We say words and they say chemicals, and the alarm signal signifies danger.”
The Aztecas can use the trail pheromone to lay down tracks toward a particular food source and the alarm pheromone to alert members of their colony to danger. But even within the alarm calls, there are nuances, which some researchers suggest can depend on context and quantity. For example, one alarm pheromone sometimes spurs the ants to get their army going, whereas in the presence of phorid flies, the alarm tends to makes ants stop moving. Phorid flies can only see moving things, and only at a very close range; when the ants are very still, the phorid flies can’t locate them.
The Coffee Code, Cracked
Hsieh and her colleagues have found that the ladybird beetles have managed to recognize alarm signals the ants send to members of their own colony. When the beetles smell the alarm pheromone, they take it as a signal to find a place to lay their eggs. The ants, meanwhile, are trying to stay still to avoid being seen by phorid flies. The beetle has “found” the perfect time to go lay its eggs without having to fight the ant.
When the beetle enters the ant zone, they have to find a good spot for hiding their babies because ants have been known to carry the eggs back to their colonies for food. So beetles hide their eggs under scale insects, perfect spots not only because scales don’t really move, but also because the larvae that bursts out of the egg can eat the scale. The beetles have one to two hours to infiltrate enemy territory before the ants snap out of their immobile state.
Maria Esteli Jimenez Soto, who also studies species interactions in Chiapas, alongside Hsieh, says this system shows the complexity and intrigue in ecological systems:
Coffee was introduced in the 1920s to this region. But when we talk about evolution, we think about long-term scales, we think about Darwin, we think about dinosaurs and reptiles that then evolved. But if you look at evolution in the short term, you can find these interesting things, interactions of organisms that are creating very interesting networks with each other.
Evolution is generally a slow process, to the human eye, but that does not mean it’s imperceptible. Within the span of a single human lifetime, the globe-marked ladybird beetle has evolved a very canny use of code-breaking. It’s now up to the ant to evolve a new way of fighting back.
Yvonne Bang is a science journalist based in New York City and an editorial intern at Nautilus.
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