Although Burnsville has yet to see the first glint of a green wing, this summer it injected 14 ash trees at City Hall with pesticide to protect them against the emerald ash borer.

Switching from a 2010 ash borer plan that emphasized cutting down and replacing ash trees, the southern suburb is focused now on preserving prime ash with pesticide treatments.

"We have changed our approach based on what we are hearing from cities that have gone through [ash borer] infestations," said Terry Schultz, director of parks, recreation and natural resources.

There is reason for cities to revisit their plans because the fight against the ash borer in Michigan and Ohio has reduced the cost and increased the effectiveness of injected pesticides in the past few years, said Deborah McCullough, a professor of entomology and forestry at Michigan State University. Based on her research, she says: "There is no reason for a landscape ash tree to die from emerald ash borer anymore."

The destructive insect, first detected here in 2009, has been found in Hennepin, Ramsey, Winona and Houston counties but still hasn't reached peak infestation in Minnesota, giving cities time to prepare and study what cities elsewhere have done.

Burnsville is so sold on that idea of saving tress, rather than cutting them down, that it plans to encourage residents to treat their trees by extending to them the rates the city receives for pesticide injection. "Treating is something we promote as a city," Council Member Dan Kealey said when the city adopted a $3.5 million plan to fight the ash borer in April. "The trees that we have are an important part of the character of the city."

The use and acceptance of pesticide is far from uniform. Most cities are planning a combination of removal and treatments. That includes Richfield, which is now in its fourth year of pesticide treatment, and Shoreview, which just this year hired a person to inject pesticide to help reduce the cost for residents who want to protect their private trees. Minneapolis stands out for using no pesticides and Milwaukee for relying on pesticides exclusively.

Milwaukee started injecting trees with a pesticide called Tree-age in 2009 and so far has not pre-emptively removed ash trees that appear healthy, said David Sivyer, forestry services manager for Milwaukee.

"The treatment is so effective and so much cheaper than removal and replacement that I can't get a single elected official to weigh in on the side of removing healthy trees because we don't have to, and that is never popular with the public."

Milwaukee could not afford to lose the 28,000 prime ash trees owned by the city all at once, Sivyer said. "The injections allow us to decide what happens to those trees, not the beetle."

Hydraulic guns drive the pesticide into the tree through shallow holes drilled in the bark. Each tree is dosed every two years.

Sivyer says positive results are evident: Private ash trees that were not injected are dead, while treated ash trees on city property stand nearby in good health.

Organic alternative?

Ash borers kill trees by starving them. Their larvae tunnel under the bark to feed, destroying the tissues that conduct water and nutrients from the soil.

Michigan and Ohio State research shows that injecting Tree-age "provides at least two years of almost 100 percent control" of larvae, McCullough said. Peeling away the bark, "we find almost no live larvae even three years after they were injected."

It has not been proven yet, McCullough said, but it may be possible to spread treatments farther apart once the main wave of beetles passes, thereby reducing future costs.

Applying pesticides every two years costs about $250 a tree, while removal and replacement is $700 to $1,200 a tree, she said. "You can treat a tree for a lot of years before you reach the cost of removing that tree."

Tree-age uses compounds in the same class of toxicity as those used to worm dogs and to kill fish lice in salmon farms, McCullough said. "It means it's not horribly toxic. I am a forest entomologist. My job is keeping forests healthy. I don't want nasty chemicals in the environment."

A Canadian firm developed a low-toxicity pesticide called TreeAzin, which Rainbow Tree Care Co. injected into the ash trees on Burnsville's municipal campus. The company says the product is a safe, "organic" option.

Made from the seeds of the tropical neem tree, TreeAzin has "extremely low toxicity to mammals" and low environmental impact, said John Sierk, pesticide regulatory specialist with the Minnesota Department of Agriculture.

Burnsville is looking for the most environmentally friendly product it can find and will be watching to see how well it works, Schultz said.

Minneapolis — which is steering clear of pesticides because residents have reservations about their safety — is unlikely to be swayed by it, said Ralph Sievert, director of forestry for the Minneapolis Park and Recreation Board.

"So far just the word pesticide has garnered a negative reaction. Up till now those opposed to pesticides have not shown a difference of opinion based on an organic or natural label," he said.

Minneapolis' decision not to use pesticides for city trees hasn't stopped pesticide makers from marketing to private property owners. Arbojet, makers of Tree-age, demonstrated an injection of the product for Linden Hills residents on Thursday.

Residents may hire approved contractors to treat boulevard ash, but the park board forestry department is concentrating on removal and replacement to keep ahead of the killing wave of beetles, Sievert said.

The city cut down 100 still-green ash trees along Penn Avenue S. as part of the reconstruction of that road, Sievert said. "We describe them as not-yet-infested trees. We don't call them healthy trees."

To learn more about the emerald ash borer, see

Laurie Blake • 952-746-3287