The year leafcutter ants took over Manhattan

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It was a cold, gray December afternoon, and at the American Museum of Natural History, half a million leafcutter ants were huddled in their homes.

Ants often spend their days harvesting leaf chips, which they use to grow extensive gardens of mushrooms that serve as food and shelter. Many days, visitors to the museum's insectarium can watch an endless river of ants carrying leaf fragments from the feeding area to the fungus-filled glass orbs where they live.

But by Tuesday, the flow of leafcutter ants had slowed to a trickle, with only a few intrepid insects visibly living up to their name.

It was hard to blame them. It was a busy, windy day, and the end of a long and eventful year for the colony. The tropical ants, which had been collected in Trinidad and raised in Oregon, had never set foot in New York City before last December, and arrived as 500,000 naïve insects. It took time for the ants to find their place and for the museum employees to learn how to create a happy home for them.

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

“We knew we were going to solve a lot of problems in 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 arrived at the exhibit in January, curators knew it would take some time for them to adapt. But the transition was slower than expected. Davies and his colleagues spent weeks trying to coax the ants along the labyrinthine path that led from the mushroom gardens to the leaf-strewn feeding area. During those first weeks, the ants foraged so little that their fungal gardens began to shrink.

The team soon realized that a major problem 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 new insectarium was still under construction, making it difficult to control the weather.

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

The ants thrived during the sticky summer months, harvesting leaves so quickly that the feeding area required daily repopulation. Staff members experimented with a variety of leaves, including maple, azalea and mulberry, which turned out to be favorites. Sometimes they even treated the ants to what they called “fast food,” providing them with traditional oats, which the ants did not need to cut before harvesting. (“They basically take a piece of oatmeal and leave,” Davies said.)

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

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

Still, on some really cold and dry days, they have encountered familiar weather challenges. So they've been persuading some reluctant ants to climb the feeding platform with a trail of leaves, and recently installed an additional humidifier inside the exhibit. They hope the new humidifier will be enough to keep the ants active for months to come.

Despite these challenges, the colony is growing and the ants have started several fungal gardens in recent weeks, Davies said. And even on the coldest days the insects don't lose their enthusiasm. Although few ants were actively foraging on Tuesday, they were busy performing household chores, including taking out the trash, at home.

In some ways, the last year has been a testament to the resilience of ants. Even during the difficult weeks last spring, Ryan Garrett, who describes himself as an ant keeper and collected the colony for the museum, never doubted that the ants could reach New York.

After all, since collecting the colony in 2018, Garrett has seen it grow from a few hundred ants with a golf ball-sized mushroom garden to a powerhouse of 500,000 ants with enough mushrooms to fill a trash can. 50 gallons. “I never lost faith in this neighborhood,” she said. “I know what they can do.”



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