The emerald ash borer may have started to feel the heat Tuesday in the Twin Cities.
Along the shady streets of St. Paul’s Cathedral Hill neighborhood and the shore of Minneapolis’ Lake Harriet, workers launched two different attacks on the tree-killing beetle: a release of predatory wasps in St. Paul and a chemical inoculation of the popular “Elf Tree” in Minneapolis.
The work coincided with the peak of the “flying season” for the ash borer, a time when the weather has become warm enough to coax the insect to bore out from under the bark of ash trees, where it’s been developing over the winter as a larva, and fly off to mate, lay eggs and infest other ash trees, thus expanding its range.
Although the warm-up this year was delayed by the cool, wet spring, state Agriculture Department workers are releasing the tiny wasps — which don’t sting and are, like ash borers, imports from China — in early summer in hopes that an additional generation or two this season will make some gains on the invaders.
“We really want to try to get them established,” said Jon Osthus, emerald ash borer biocontrol coordinator for the Agriculture Department.
The wasps, developed at a U.S. Department of Agriculture facility in Brighton, Mich., have been used in the metro area and far southeast Minnesota, where ash borers have been detected, since 2010. Jonathan Lelito, the facility manager, said he’s been encouraged to find that wasps in Michigan have been spreading steadily on their own, and now kill about 25 percent of the larvae in trees researchers have sampled. In China, where the wasps and ash borers live in a natural balance, the figure is about 70 percent.
Back in Minnesota, Osthus was scheduled to release 1,040 wasps Tuesday in Cathedral Hill and Como Park in St. Paul, as well as in northeast Minneapolis and Shoreview, where ash borers been found.
Goal: Natural balance
The wasp release is the third of three strategies that officials hope will deter ash borers in St. Paul. After several dozen trees along Kent Street and Portland and Holly Avenues were found infested two years ago, the city removed some, and it inoculated others last year. But ash borers are still present, Osthus said.
“We know it’s in the area at some level, even though we can’t see it, “ Osthus said. The wasps, Osthus said, are expected to find existing ash borers by detecting “larval vibration” under tree bark, drilling through the bark and laying their own eggs in the ash borer larvae. The wasp eggs then use the larvae for nourishment, destroying them.
“The whole point is to get the wasp population to the point where it equals ash borers, ” he said, “and then bring both down to the point where we’ll not have total [ash] mortality.”
Treating the ‘Elf Tree’
In Minneapolis, an ash on the south shore of Lake Harriet, home to an elf known as “Little Guy,” got a $150 insecticide treatment Tuesday compliments of People for Parks, an advocacy and fundraising group.
With permission from the Park Board, the group paid Rainbow Tree Care to treat the tree. The tree is not infested, but it is about 2 miles from Lakewood Cemetery, where ash borers have been found. Tuesday’s was the second treatment in two years.
“We want to protect his tree,” said Felicity Britton, executive director of People for Parks. “It’s really special.”
Thought not particularly large, the Elf Tree attracts people of all ages who leave notes for the elf at a small door in the tree; Little Guy replies to them. The elf, who is not in the tree during winter months, has recently acquired an e-mail address, Britton said.
Dawn Sommers, spokeswoman for the park board, which manages the city’s public trees, said the agency does not treat ash trees with insecticide on its own, and does not pay for treatments requested by homeowners, because ash make up about 20 percent of the city’s tree population and that would be too expensive. Homeowners or groups who wish to have boulevard ash trees treated can do so at their own expense by hiring a licensed and permitted tree care company. The Park Board asks that those who’ve arranged for treatment to notify the Forestry Department at 612-313-7710 in order to coordinate with the agency’s ash borer response.
Emerald ash borer has killed tens of millions of trees in 19 states and two Canadian provinces since it arrived in North America about 20 years ago, apparently in wooden shipping crates. It was first found in Minnesota in St. Paul in May 2009, and infestations have been found in Ramsey and Hennepin counties in the metro area and Houston and Winona counties in the southeast corner.
Ash trees have been nearly wiped out in areas around Detroit, where the ash borer is believed to have made its U.S. arrival. Minnesota in all has about 900 million ash trees.