The Year the Leaf-Cutter Ants Took Manhattan

It was a cold, gray afternoon in December, and at the American Museum of Natural History, a half million leaf-cutter ants were hunkered down in their homes.

The ants typically spend their days harvesting slivers of leaves, which they use to grow expansive fungal gardens that serve as both food and shelter. On many days, visitors to the museum’s insectarium can watch an endless river of ants transporting leaf fragments from the foraging area to the fungus-filled glass orbs where they live.

Listen to ‘Ant Wrangler’

Ryan Garrett describes how he moved a half million leaf-cutter ants from Trinidad to the American Museum of Natural History.

But on Tuesday, the flow of leaf-cutter ants had slowed to a trickle, with just a few intrepid insects visibly living up to their name.

It was hard to blame them. It was a biting, blustery day — and the end of a long, eventful year for the colony. The tropical ants, which had been harvested in Trinidad and nurtured in Oregon, had never set foot in New York City before last December, arriving like 500,000 insect ingénues. It took time for the ants to find their footing and for museum employees to learn how to create a happy home for them.

The work is not over yet. As winter returns, the museum is making more tweaks to the exhibit, a fitting capstone to a year that has featured a lot of learning through trial and error.

“We knew that we were going to do a lot of problem solving during the first year,” said Hazel Davies, the museum’s director of living exhibits. “We’ve been doing these mini science experiments constantly.”

When the ants first moved into the exhibit in January, the curators knew it would take them some time to adjust. But the transition was slower than expected. Ms. Davies and her colleagues spent weeks trying to coax the ants along the labyrinthine path that led from the fungal gardens to the leaf-packed foraging area. During those early weeks, the ants foraged so little that their fungal gardens started shrinking.

A major problem, the team soon realized, was that the air was too cold and dry for the ants, which preferred warm, humid weather. Not only was it winter in New York, but the museum’s brand-new insectarium was still under construction, making climate control difficult.

So the museum installed a humidifier behind the display case and devised temporary shortcuts to make foraging easier. By the time the insectarium opened in May, the colony was humming.

The ants thrived during the sticky summer months, harvesting leaves so rapidly that the foraging area required daily restocking. Staff members experimented with a variety of leaves, including maple, azalea and mulberry, which turned out to be a favorite. Sometimes they even treated the ants to what they called “fast food,” providing old-fashioned oats, which the ants did not need to cut before harvesting. (“They basically grab a piece of oat and go,” Ms. Davies said.)

Over time, the ants rebuilt the fungus they had lost and then some. “So we have had these wild animals living in the building and really thriving,” said Jessica Ware, a curator and the division chair of invertebrate zoology at the museum.

Ms. Davies and her colleagues were proactive as winter approached, adding a water heater to the exhibit and covering the display case with a blanket at night.

Still, on some really cold, dry days, they have found themselves facing familiar climatic challenges. So they have been coaxing some reluctant ants out onto the foraging platform with a trail of leaves, and they recently installed an additional humidifier inside the exhibit. They hope that the new humidifier will be enough to keep the ants active in the months ahead.

Despite these challenges, the colony is growing, and the ants have started several fungal gardens in the last few weeks, Ms. Davies said. And even on the coldest days, the insects haven’t lost their hustle. Although few ants were actively foraging on Tuesday, they were busy performing chores, including taking out the trash, at home.

In some ways, the last year has been a testament to the ants’ resilience. Even during the difficult weeks last spring, Ryan Garrett, a self-described ant wrangler who collected the colony for the museum, never doubted that the ants could make it in New York.

After all, since collecting the colony in 2018, Mr. Garrett has watched it grow from a few hundred ants with a golf-ball-size fungal garden to a 500,000-ant powerhouse with enough fungus to fill a 50-gallon trash can. “I never lost faith in this colony,” he said. “I know what they can do.”