Researchers are stepping up efforts to find and stall the advance of the so-far relentless tree-killer.
The winter just past might have felt nasty, brutish and long, but all it did for emerald ash borers was delay the start of their egg-laying.
Indeed, the tiny, metallic-green flying beetles are expected to begin poking out from under the bark of infested ash trees any day now, signaling the start of another cycle in the bug’s destructive spread and experts’ efforts to contain it.
That will be about a month later than last year, when a freakishly warm March triggered early spring emergence of just about everything. But this year’s timetable is close to average, said Mark Abrahamson, a Minnesota Department of Agriculture entomologist and the state’s chief emerald ash borer (EAB) monitor.
“The cold? Not much,” Abrahamson said when asked what effects winter might have had on the insect. “EAB is fairly hardy. There would have been a negligible amount of mortality.”
Abrahamson didn’t have any predictions for what kind of year this might be for ash borers and the hunt for them. May 1 is considered the beginning of the “flight season,” with peak emergence coming about three to four weeks after the first flights, Abrahamson said.
The emerald ash borer, a recent arrival from China, is regarded as a threat to Minnesota’s nearly 1 billion ash trees. It has no known natural predators in Minnesota, although researchers have introduced several species of stinging wasps, also from China, near infestations, in hopes of stalling the beetle’s spread. Insecticides have been proven to protect trees while requiring repeated treatments. Emerald ash borers have been found in Hennepin and Ramsey counties in the metro area and Houston and Winona counties in the far southeast.
The beetles are notoriously difficult to find before trees have begun to die or are damaged by ash borer-eating woodpeckers, or both. Rob Vennette, a U.S. Forest Service research biologist based in St. Paul, said researchers will be trying to improve their detection strategies this year, partly because infestations found recently have covered bigger areas and because infested trees have contained higher concentrations of ash borers than in the past. The ash borer was first detected in Minnesota in 2009 in St. Paul.
Researchers are trying to find the beetles by sampling branches from suspect trees, rather than taking entire trees down and peeling bark back, Vennette said. They’re also experimenting with different baits in the purple, tent-shaped traps that have become familiar features in the middle branches of ash trees around Minnesota in recent summers.
In addition, researchers are following evidence that EAB insecticide may reduce the bug’s cold-tolerance, which could limit the need for the insecticide, Vennette said. They’re also looking into whether black ash trees, a more common species in northern Minnesota, might be more resistant to borers than the ash species more common in the rest of the state.
“If we’re really lucky, maybe EAB will act more like our native species, and be a problem only every so often,” Vennette said. “They’re very clever insects.”
Bill McAuliffe • 612-673-7646