Rather, the species probably entered the Golden State on a plant or in some other shipment after being missed by agricultural inspectors, officials said.
UC Riverside entomologist Les Greenberg, who visited Costa Mesa on Monday to inspect the colony, concurred.
"It could've come in from a plant from another state, a fruit or something like that," he said. "There are lots of ways stuff gets moved around."
It's also not yet known how widespread the colony — found near Europa and Kornat drives — is at this point.
From what Greenberg saw this week, he surmised that a few thousand ants may make up the infestation.
Officials are conducting a survey of the area, including across the Santa Ana River into some nurseries in Huntington Beach, to see if the colony has crossed city limits.
"If that's all there is, that's not a big deal," Greenberg said. "They will find a way to get rid of those, I'm sure. It just depends on how far they've spread."
But several other factors make this a difficult situation, he explained.
Generally, only a trained eye with a microscope's help is able to identify Pheidole megacephala, Greenberg said, adding that other closely related ants from the same genus are found in California.
"It's not that easy to know if you have that imported one or not," he said.
These ants' nests aren't conspicuous.
"You wouldn't see a single large mound the way you do with a fire ant," Greenberg said.
The weather is another unknown.
"We don't know how it's going to react to this California climate," Greenberg said. "That's something we want to try and find out."
Unlike fire ants, Pheidole megacephala may be more drought-tolerant. The species may not need the humidity found in Florida, Hawaii and other warm places where it has long been established, Greenberg said.
"We have native ants of this group in California, and some of them live in semi-arid conditions," he added.
Tiffer said the future of Costa Mesa's exotic ants will be up to the California Department of Food and Agriculture, which could exterminate or leave them be.
In the meantime, officials have distributed in Mesa Verde fliers identifying the insects as "one of the worst invasive ant species."
Residents who have questions or believe they have spotted bigheaded ants are asked to call the state at (800) 491-1899. They can also call Tiffer or Mike Bennett, of the county Agricultural Commissioner's office, at (714) 955-0100.