HILO, Hawai'i — An infestation of fire ants on the Big Island and Kaua'i is spreading, and researchers say it will be difficult to stop.

The ants are already a bigger threat than coqui frogs, known for their loud chirping that makes them a nuisance on the Big Island and Maui, said Tommy Thompson, an entomology researcher associated with the University of Hawai'i at Manoa College of Tropical Agriculture and Human Resources.

"They're hard to find because a lot of people don't realize they're infested until it becomes too late," Thompson said.

The fire ants, which were discovered in the Big Island's Puna district in 1999, have been identified at more than 50 sites around the island and on Kaua'i.

They're about as long as a penny is thick, and they're suspected of blinding pets and killing birds.

It would be nearly impossible to eliminate the invasive ants from Hawai'i, but their population could be contained.

The ants have spread throughout Hilo, Mountain View, Kalapana and Laupahoehoe. They haven't been found in West Hawai'i yet, but Thompson said it's only a matter of time.

The ants' stings haven't caused any human deaths, but the ants make pets blind when they're stung in the eyes, he said.