Ash borer tunnels into historic St. Paul area

The emerald ash borer's path of tree destruction has spread to St. Paul's stately and historic Summit Avenue, a startling distance from the sites of previously known infestations.

The revelation means more than 1,000 trees in the area are at high risk, mostly on the streets that run parallel to Summit and west to the Mississippi River, where blocks full of ash trees were planted long ago.

The city's Parks and Recreation Department confirmed Monday the infestation of at least six trees in the area of Dale Street and Summit Avenue, approximately 3 miles from the city's initial outbreak sites.

"It's not great news for the citizens here, that's for sure," said Rob Venette, U.S. Forest Service research biologist, who marked tree stumps Monday and loaded them onto trucks for study. He showed how the zigzag borings of the insect's deadly dining were visible in "galleries" on the trees' bark, a sign of advanced infestation. "It's a frustrating insect because we're always playing catch-up."

The confirmation signaled the spread of the tree-killing insect despite control and containment efforts that cost the city $1 million a year.

The Summit area discovery wasn't unexpected, but was still disappointing. "It was a matter of when, not if," parks spokesman Brad Meyer said as workers methodically felled trees. "Emerald ash borer is here to stay."

The department will now direct more resources to the public trees in the Summit area. Most of the endangered ash trees are not on Summit itself, but extend along numerous blocks parallel to the avenue.

Venette said it's far too early to tell if the wasps, a natural predator of the borer in their native Asia, have made progress in stemming the spread. "This is a case where people need our research results yesterday," Venette said.

Because some trees won't show telltale signs of infestation for up to five years, identifying the scope of the problem isn't easy.

Indications are that the six recently removed trees may have been infested for up to three years. St. Paul officials said the confirmations didn't indicate flaws in the city's borer fighting efforts because of the lag time between infestation and confirmation.

St. Paul alone has 30,000 ash trees on public land with another 100,000 or so on private property the city doesn't control.

Since at least 2009, the borer has spread in the metro area to Minneapolis, Falcon Heights and Shoreview. Outstate, it's been found in La Crescent, in the southeast corner, in two locations near Winona and in rural Houston County.

The city has allowed homeowners willing to pay for inoculations to keep trees if they treat them on schedule. The city also is inoculating selected healthy trees in infested areas.

Since the metallic green ash beetle was accidentally introduced into the United States, it has killed millions of trees in 15 states. The borer larvae kill by tunneling into ash trees, then feeding on and disrupting the flow of nutrients. They leave telltale one-eighth-inch D-shaped exit wounds in the bark and serpentine tunnels underneath it. Extensive woodpecker damage is also an indicator. The borers are active from May to September.

Rochelle Olson • 651-925-5035 Twitter: @rochelleolson

• Six trees were confirmed with infestation near Summit Avenue, approximately 3 miles from St. Paul's initial outbreak sites.

• Trees don't show signs of infestation for up to five years.

• Chinese wasps, a natural predator of the emerald ash borer, will be released in the area soon.