Despite all efforts to contain it, London's aggressive ash tree pest is making its way across the city.

The emerald ash borer has been found in four more trees -- one on private property, and three in Carriage Hill Park. All are "just a smidgen" north of a five-kilometre quarantine zone imposed around a site where the beetle was found last fall, said an inspector with the Canadian Food Inspection Agency (CFIA).

While some city councillors said it's doubtful the entire city would need to be put under a wood-movement quarantine to halt the borer, Ken Marchant of the CFIA said another quarantine zone will likely be drawn around the latest find. "Traditionally, where there is one (borer), there is more."

A new quarantine zone would overlap the existing containment area.

To Coun. Joni Baechler, outspoken on tree protection, "it is certainly disappointing" to hear the latest news.

"If we have four trees on the north end of the zone (found to be infested), then it is more than likely there are more on the south end, the east end and the west end. There is ash all over the city," she said.

The beetle has blazed a path from the U.S. Midwest to Southwestern Ontario, killing millions of ash trees.

London has about 10,000 ash trees -- six per cent of city-owned trees -- on its boulevards and in its parks, and at least 30,000 on private property.

Inspectors found the first London case on a private property near Windermere Avenue last Tuesday, Marchant said, and by yesterday they'd identified the other three park trees.

"The ultimate plan is to impose quarantine areas around those new (trees)," Marchant said.

The existing quarantine zone extends five kilometres, taking in more than 56,000 properties, around infected trees near the city's Greenway pollution control plant.

Under the quarantine, no ash tree material -- alive or dead -- or firewood from any trees -- can be removed from properties within the zone.

The CFIA plans to inject some ash trees with a natural pesticide this year to slow the borer's advance, but Marchant said there's no "miracle cure."

The city is taking quick steps to remove and replace at least three of the four trees under attack by the borer, according to an e-mail sent by David Leckie, the city's roads and transportation director, to city councillors this week.

"This is very concerning," said Coun. Harold Usher.

"You can bet your bottom dollar we (councillors) will get calls on this and we need to have some answers on what people can do.

"I want to know whether we are going to remove the tree on the private property or wait for the owner to do it. The CFIA doesn't want people to start cutting down dying trees and putting them on curbs or taking them to the dump."