The battle against the emerald ash borer in Dakota County has so far been subtle.
An ash tree here or there, looking weak or damaged along the road, gets chopped. Those, and others removed to make way for new streets or public buildings, are often replaced with a different type of tree.
But local tree watchers are on the alert for more dramatic changes.
The emerald ash borer -- an invasive insect with larva that chew up and kill the trees -- is getting closer.
The Minnesota Department of Agriculture confirmed an infestation at Fort Snelling Golf Course, just across the river from Mendota Heights, earlier this month.
Local foresters say it's no longer a question of if, but when the emerald ash borer will be spotted in Dakota County.
"We're all assuming it's in our trees, too," said Gregg Hove, forester for the city of Eagan. "We just haven't seen it yet."
After the Fort Snelling Golf Course sighting, the Department of Agriculture surveyed nearby areas of Dakota County for the bugs. They didn't find any, but will look again in the winter when it's easier to see damage high up in the trees.
"We don't have a good guess for how long it will take for the emerald ash borer to jump over the river," said Liz Erickson, an Agriculture Department spokeswoman.
The insects aren't strong flyers and prefer the nearest ash tree over a longer journey, but they can fly up to a mile.
Cities have been drawing up plans to fight the ash borer since shortly after the invasive insect was first identified in Minnesota in 2009. Infestations have been confirmed in Minneapolis, Shoreview, St. Paul and Winona and Houston counties.
Foresters from many southern suburbs have been holding regular meetings to discuss strategy. They're taking inventory of local ash trees, boosting budgets to pay for tree treatment and replacement, and trying to gauge what the impact will be.
"I really don't think it's going to be whole towns wiped out of ash trees," Hove said.
But there are thousands of trees at risk, and homeowners and city officials will need to make decisions about which ones to treat with insecticides and which ones should come down or be left to die if infected.
Eagan estimates there are 750 ash trees in city parks and 3,000 to 4,000 on boulevards along the roads. In Burnsville, the city counts 6,000 to 8,000 ash trees on public land.
Liz Erickson, spokeswoman for the Minnesota Department of Agriculture, said homeowners within 10 miles of a confirmed infestation should start thinking about what they plan to do if the bugs take up residence in their ash trees.
There are chemical treatment options that can save many infested trees if applied every year or two, depending on the method chosen. The cost per application varies with the size of the tree.
If people choose not to treat their trees, they can be left to die or cut down and replaced with a different tree if desired.
"If it's their tree, it's their decision," she said. "The infestation in Minnesota is small enough that they do have time to think about this a little bit."
But if residents decide to leave a tree to die, she said, they should be aware of any tree pest or dangerous tree ordinances that may give a city the power to order a tree cut down if it's a hazard. Rosemount, for instance, enacted such an ordinance this year.
For now, city officials are on the alert for the first signs of the ash borers and are weighing treatment plans.
Eagan has already started insecticide treatments -- delivered to trees by injection -- on select trees in parks and public spaces.
In Apple Valley, Dakota County took down ash trees along Cedar Avenue for road construction and plans to replace them with other varieties.
Burnsville, farther from the initial infestations in St. Paul, is still waiting to see how the bug progresses before doing pesticide treatments. And even when the ash borer does appear, it's more likely that the city will take down and replace trees rather than treat them, said Terry Schultz, Burnsville's director of parks and natural resources.
Mostly, it comes down to cost. The city has budgeted $100,000 since 2010 to deal with the ash borers, but treatment could cost as much as $100 to $200 per tree annually.
"We're looking at maybe 100 to 150 trees that we would treat and try to save," he said.
Replacing the ash trees -- so commonly planted because they were a hardy, fast-growing variety -- also increases the diversity of trees in neighborhoods. The importance of such leafy variety has become increasingly important as one tree scourge after another comes through, said Hove, the Eagan forester.
"[The ash] were popular replacements for the elms that went out [from Dutch Elm disease] in the 1960s and 1970s," Hove said. "Now we're learning to diversify a bit."
Katie Humphrey • 952-746-3286