The emerald ash borer hasn't crossed the Minnesota River into Dakota County -- at least not that anyone knows -- but south-metro foresters are preparing for its inevitable arrival.

About 12 city foresters who call themselves the South of the River Tree People meet quarterly to discuss tree issues. At their March meeting, they compared notes on the emerald ash borer so that residents in neighboring communities have the same information about the beetle.

"It will be here," Eagan supervisor of forestry Gregg Hove said. "Everybody that we talk to from the further Eastern U.S. says that just because it's starting slow doesn't mean it isn't going to be here."

This month the Department of Agriculture began hanging the long purple traps that monitor for the green beetles. The traps went up about three weeks ahead of schedule this year because of the warm weather.

Minnesota received 6,500 traps from the federal government along with specified locations to place them, said Mark Abrahamson, leader of the Agriculture Department's emerald ash borer program. One trap draws bugs from 250 miles around.

Federal officials want the traps used to find new infestation locations, Abrahamson said. That means -- in a departure from past years -- no traps will be used to monitor the spread of the beetles in Hennepin and Ramsey counties because those counties have already been quarantined, he said.

If bugs are found in new locations, the department will impose new quarantines if it can find infested trees. One beetle in a trap will not trigger a quarantine, he said.

The Department of Agriculture's goal is to keep the size of the ash borer infestation under control by quick removal of the tree or with chemical treatment, Abrahamson said. "The sooner you begin to try to implement some management, the better. If you wait another year or two, you just have a bigger problem."

The bugs -- which are an invasive species not native to the country -- were first detected on the East Coast in 2002. Since then, they have worked their way across the country, killing millions of ash trees.

The larvae from the beetle eggs burrow into the tree bark, eating furrows that stop the circulation of water and nutrients from the roots to the top of the tree, eventually killing it.

In the metro area they have been found in three locations: in Shoreview; on the border of St. Paul and Falcon Heights; at Dale Street and Summit Avenue in St. Paul, and in Minneapolis. The bugs have also been found in rural Winona and Houston counties, both of which have quarantines in effect.

Because St. Paul is just 6 miles from Eagan as the crow flies, Hove said that if the beetles "are not here already they will be here within the next three or four years."

The standard approach used by most cities now is removing weak ash trees in parks and on boulevards before they are infested, chemically treating some of the hardier, more beautiful trees to protect them, and letting the ash in wild forested areas fend for themselves.

Eagan has about 3,200 ash trees on its boulevards -- about 22 percent of all boulevard trees -- and in the parks it has about 750 ash trees, about 10 percent of all park trees. It has thousands more native ash trees in the woods, Hove said.

Rosemount, which has about 1,165 ash trees on boulevards and in parks, has this spring added a tree provision to its ordinances that will give the city the authority to order infested trees removed, Rosemount parks supervisor Tom Schuster said.

"Unlike most cities, Rosemount has never had a part of city code that dealt with shade tree diseases. Up until this point we didn't have any authority to go on private property to look for trees like this or to order their removal," Schuster said.

Dutch elm disease or oak wilt leave no option but to cut down the tree, but emerald ash borer can be controlled with consistent chemical treatment.

As soon as treatment stops, however, the tree is as susceptible to the beetles as a tree that has never been treated, Schuster said.

For that reason homeowners will want to weigh the cost of continuing treatment against removing a tree, he said.

Rosemount has not cut down any ash trees yet but may start selective removals of weak trees this year, Schuster said.

Laurie Blake • 952-746-3287