Watching Wasmannia: Ants on a Mexican coffee farm

TAPACHULA, Mexico—Putting chunks of tuna on coffee plants is an unusual way to spend a morning in the mountains of southern Mexico. But it has been part of Senay Yitbarek’s daily routine for the past three summers.

Yitbarek uses the fish to attract ants – part of a research project to learn more about how biodiversity is maintained on an organic coffee farm here in southern Chiapas state, near the border with Guatemala.

“I’m fascinated with the question of how so many species can coexist,” said Yitbarek, a fifth-year doctoral student in ecology and evolutionary biology at the University of Michigan. “Where do the boundaries stop for one species before another species takes over?”

Every morning, Yitbarek puts on his knee pads so that he can crawl on the ground and observe the ants. He tucks his pant legs into his socks to keep the bugs from biting his legs. And he walks along the steep slippery slopes of coffee fields, trying not to fall on sharp little tree stumps that look like stakes that could easily pierce your gut.

The focus of his research is Wasmannia auropunctata, also known as the “little fire ant.” The reddish brown insects are tiny, about two millimeters long.

“We don’t know much about the ecology of this ant,” Yitbarek said.

What’s well known is that they have a special talent for being invasive, able to establish themselves outside their native area, believed to be somewhere in Central America. The ants have already taken over Hawaii and other Pacific islands. They’ve been found as far as Israel.

Coffee farmers have a love-hate relationship with Wasmannia ants. On the positive side, the insects like to eat one of the biggest threats to the crop – the coffee berry borer, a tiny beetle that eats its way into coffee beans. The pest is the biggest insect threat to the coffee crop worldwide.

“Researchers have figured out that Wasmannia can get into the coffee berry and pick out the beetles,” Yitbarek said.

On the negative side, the feisty ants can attack coffee pickers, swarming over the workers’ necks and armpits, delivering a painful bite. This causes the workers to avoid plants infested with the ants, reducing the harvest.  

Yitbarek hopes that by better understanding where the ants live, how they move around and get along with other species, farmers will be better able to manage their fields. They can fine tune things so that the ants are a bigger threat to beetles than to workers.

The graduate student studies the ants in a 50 x 50 meter plot on the slope of the coffee field. Little orange flags are stuck in the ground four meters apart, marking sites where Yitbarek places his bits of tuna. In some spots, he puts the bait on the ground, while in others it goes in a coffee tree because the ants live in both places.

He uses tuna because it’s a good protein source, and the oil keeps it from drying out in the scorching sun.

“For the ants, it’s like having a big hamburger,” he said.

After leaving the tuna bait at 154 sites, Yitbarek goes back and records what kinds of ants are eating the tuna.

Over the three years he has been studying the insects, he has been observing how the ant community changes from year to year.

Yitbarek said that ants are pretty much blind and that “they live in a chemical world,” communicating with pheromones and touching their antennae.

Before he went to college, he said he never imagined he would be studying ants. He grew up as a city boy in Utrecht, Netherlands, where there wasn’t much nature.

But he discovered his passion for the insects as an undergraduate at the University of California, Berkeley, when he went to southern Brazil on a six-month agricultural exchange program that involved surveying ant species.

“I always had a fascination with self organization, such as schools of fish and flocks of birds,” he said. “They’re all organized without a leader. I’m interested in how the patterns form.”