Invasive species experts say the eradication of little fire ants on 175 acres in Nahiku is “in sight” after years of efforts to eliminate the painfully stinging pests across a wide swath of thick rainforest and criss-crossing streams.

“We’ve reached a milestone in our efforts to get rid of the ants in Nahiku,” Brooke Mahnken, little fire ant coordinator with the Maui Invasive Species Committee, said in a news release last week. “We didn’t find any LFA in our last survey. This means we’re in a monitoring phase.”

Recognized as one of the 100 worst invasive species in the world, the undersized ants can sting people and pets and wreck agriculture. They were first detected on Hawaii island in 1999 and then on Maui in 2009.

The Nahiku infestation, stretching nearly a mile and a half through dark tangles of plants and trees in the rainforest of East Maui, was the largest-known infestation outside of Hawaii island. MISC said that it appears the ants are gone, but crews will continue to survey for five years to make sure.

“While there’s a chance LFA may turn up during future surveys, we’ve reached the point on the journey where eradication is in sight,” said Mahnken, who is leading efforts to eradicate little fire ants on Maui.

The MISC plant crew first stumbled upon the Nahiku infestation in 2014 while removing invasive miconia plants, but at the time the agency had neither the techniques nor the funding to attempt eradication at such a large scale.

“My heart sank because I knew how dense the vegetation was in that area,” said Teya Penniman, interim MISC manager. “And then it got worse as we learned how big it was, that it included a criss-cross of streams, and that some landowners weren’t cooperative. If we couldn’t control this infestation, we knew it would spread to the rest of East Maui and then all of Maui.”

Initially, crews focused on treatments to protect residents in their homes, and around roads, vehicles and equipment to prevent accidental spread.

Little fire ants are rainforest-adapted; they thrive in a shady, damp environment.

“Nahiku is ideal habitat for them to invade,” Mahnken said.

They can form a colony in the smallest nook of a tree and colonies don’t compete with each other. Instead they form networks, eventually becoming a “supercolony,” living from the tops of trees to the leaf litter on the forest floor. Population densities can reach tens of thousands of colonies and as many as 100 million ants in an acre. To eliminate a population, every single colony must be eliminated.

Working with the Hilo-based Hawaii Ant Lab and the state Department of Agriculture, MISC developed a strategy to treat the ants in Nahiku. In 2019, Mahnken and the team carried out the first treatment, using a helicopter that dribbled a meat-flavored bait combined with an insect growth regulator on leaves that worker ants could find and carry back to the colony to feed to the queens.

“It functions as a birth control by preventing the queens from laying eggs,” explained Mahnken.

Without new ants to replace the workers and queens, the colony shrinks over time and eventually disappears. Queens have to be fed the bait regularly. It took slightly more than three years (24 treatments in total) to reach this point. Annually, crews surveyed the entire infestation, first to determine if the treatment was working and then to search for any remaining little fire ants. When they were found, the control efforts focused on areas still containing ants.

Surveys will continue in Nahiku. A team of over 30 crew from MISC are being joined by staff from other conservation organizations during October — which is “Stop the Ant” month statewide — as they search for any straggler ants. Surveys will continue for several years.

Penniman called the Nahiku success story a “victory for us, but also the rest of the State. We have a proof of concept that it’s possible to eliminate large infestations of little fire ants in some of the most extreme habitats.”

Mahnken added, “If repeat surveys over the course of five years yield no ants, we will declare the site eradicated.”