State agricultural officials will begin setting out more than 1,500 ant bait stations filled with Spam canned meat in seven Orange County cities on Monday as they work to understand how far an aggressive ant species has traveled beyond a colony found in Costa Mesa.

The baiting stations will be staked in the ground up to 5 miles away from the site where the big-headed ant colony was spotted last month in the front yard of a Costa Mesa home, said Mike Bennett, Orange County’s agricultural commissioner.

The study area will include all of Costa Mesa and parts of Huntington Beach, Fountain Valley, Westminster, Santa Ana, Irvine and Newport Beach.

“It’s a tool that will let us know how far out they’ve spread,” Bennett said. “Hopefully, we’ll find out it’s only in this one neighborhood, and we can move to eradication, if that’s what the state chooses to do.”

Named after their disproportionately large heads, big-headed ants are considered an agricultural pest and one of the world’s most invasive insects. They aren’t dangerous to humans.

Bennett said the bait stations won’t contain poison, but state Department of Food and Agriculture officials plan to collect any ants they find inside and send them to a lab for identification.

Big-headed ants are drawn to oily, high-protein Spam, Bennett added.

In mid-April, state agricultural officials confirmed the presence of the aggressive Pheidole megacephala species of big-headed ant in the front yard of a home in Costa Mesa’s Mesa Verde neighborhood, near the Santa Ana River.

It was the first documented sighting of this species in its natural environment anywhere in California, authorities said.

“I knew the state and the county would get in an uproar as soon as I let them know,” said amateur entomologist Gordon C. Snelling, 55, of Apple Valley, who spotted the ants while visiting a friend.

“It was one of those sheer dumb luck things.”

Over a three-day period that ended Thursday, a team of about 10 state and county agricultural inspectors and UC Riverside scientists swarmed the Costa Mesa neighborhood looking for more of the ants.

They documented big-headed ants in 31 outdoor areas near the original colony, although no additional colonies were spotted, Bennett said.

Still, it was enough to convince agricultural officials that the ants had spread – and that it was crucial to know exactly how far.

Bennett said agricultural officials will spend at least a week placing about 1,570 bait stations across an approximately 79-square-mile swath of Orange County.

The goal is to place 20 bait stations in each square mile; pre-planning will be key, he added.

Depending on what authorities find, they might move toward eradication, Bennett said.

Scientists say this variety of ant not only tends to invade homes in large numbers in search of food and water, but also displaces other ants and eats beneficial insects.

Snelling, the amateur entomologist whose sharp eye touched off Orange County’s massive ant hunt, said he suspected the big-headed ants had traveled to his friend’s home inside potted plants or sod, and that they likely had been there at least a year.

His friend had been complaining about aggressive ants invading his house and winding up dead in his swimming pool, Snelling said.

“They were finally getting to the point where it was becoming noticeable,” said Snelling, who works in the pest control industry.

Snelling said he could immediately tell the ants were of the big-headed variety, but he thought it was odd they were so aggressive, as big-headed ants in California aren’t known to swarm homes and pools.

That’s when he began to suspect they might be the aggressive Pheidole megacephala variety, which are native to Africa but until then had never been spotted in the wild in California.

“I thought, ‘Oh boy, what the heck do I have?’ ” said Snelling, who runs the website armyants.org and has published scientific papers on ants.

“It’s one of those things that gets the adrenaline pumping and your brain churning. It’s certainly caused more response than anything else I’ve done.”

Contact the writer: 714-796-7802 or smartindale@ocregister.com