CUT DOWN OR TREAT?

Thousands of Minnesota homeowners may have to decide sooner or later about what strategy to take against the emerald ash borer.

Consider location: If your tree is more than 15 miles from a known infestation, don't worry. In Minnesota, infested trees have been detected in Hennepin, Ramsey, Houston and Winona counties. A map of 2011 detections is at www.startribune.com/a670

If you're within the 15-mile zone, you can either cut down and replace threatened, dying or dead ash trees, or treat those that have a chance of surviving.

Consider the costs: It will cost to treat a tree repeatedly through the years, and it will cost to remove and replace it. You might choose to protect one special tree, or just a few.

Emamectin benzoate: Trunk injection must be done professionally but can fight off or knock out infestations for two or perhaps three years, university studies show. It costs about $270 to inject an 18-inch-diameter tree every second year, said Shawn Bernick, vice president of research and development for Rainbow Tree Care Scientific. The chemical is considered immobile in the environment.

Imidacloprid: Professional trunk injections of imidacloprid have been mostly effective, with some failures in experiments. A soil application of this chemical costs about $126 every year. A do-it-yourself version costs about $50 per application, but at dosage rates effective only on trees less than 15 inches in diameter. It can leach to shallow groundwater or run off into water bodies, where it is "highly toxic to aquatic life," according to the University of Minnesota Extension Service.

Dinotefuran: The professionally applied trunk spray has been shown to reduce borer infestations, but more so on smaller trees than large ones. It must be applied annually. It has a risk of drifting into water bodies or shallow groundwater.

Extensive information on treatment strategies is available at www.startribune.com/a656 and www.startribune.com/a657

Bill McAuliffe