Good bugs face off bad bugs in Minneapolis to slow EAB advance

Bio-agents introduced in an attempt to slow emerald ash borer invasion

By MARGIE O’LOUGHLIN

The fight against the Emerald Ash Borer (EAB) has been lost in Minneapolis’ landscaped areas. The city’s Forest Management Plan calls for removing 5,000 ash trees from parks, streets and boulevards this year. Ten thousand new trees will be planted to replace those that are removed, introducing 4-5 species per block into the urban canopy. Gone are the days of mono-cultural plantings, with one species per block or blocks.

Photo right: The Emerald Ash Borer, an iridescent green Jewel Beetle accidentally introduced to North America, is slightly more than half an inch long when mature. (Photo by Dr. Jeff Hahn, U of M Department of Entomology)

In all, 40,000 landscape ash trees will be taken out over the next eight years, representing more than 20% of Minneapolis’ urban forest.

Jonathan Osthus is the EAB Biocontrol Coordinator for the Minnesota Department of Agriculture. He explained that ”in the city’s natural and forest settings, another management plan is being tried besides removal. We’re introducing biocontrol measures to help bring EAB numbers down to a manageable level, but it will take time.”

The EAB is a beautiful insect to look at. A green Jewel Beetle, it lives harmlessly in its native range of eastern Asia where it feeds only on stressed or diseased ash trees. Outside its native range, it is highly invasive and, as Osthus said, “The EAB has proven to be more destructive and faster moving than Dutch Elm Disease.”

Photo left: Jonathan Osthus, Emerald Ash Borer Biocontrol Coordinator the Minnesota Department of Agriculture, attached an ash bolt to a tree in the Mississippi River Gorge Area. More than 100 of these are appearing along both sides of the river from just north of the Franklin Bridge to Minnehaha Falls. They should not be touched or tampered with. (Photo by Margie O’Loughlin)

Photo right: EAB larvae feed on the vascular tissues between the bark and the sapwood, and will kill an ash tree within a few years. The tunnels they leave behind are called galleries, and are proof positive that a tree is infested with EAB. (Photo by Margie O'Loughlin)

The Minnesota Department of Agriculture’s website states that bio-control is the only management option in the natural and forest areas. They’re introducing two parasitoid wasp species throughout the state, including along the Mississippi River gorge area. Parasitoid means insects whose larvae live as parasites that eventually kill their hosts (typically other insects). The hope is that these two species of wasps will manage to kill significant numbers of EAB larvae and eggs.

Both species of wasps are as small as gnats and won’t sting or otherwise harm humans. The US Department of Agriculture and the Forest Service have done extensive testing to make sure the wasps won’t parasitize other insect species or negatively impact the environment.

The EAB is believed to have been introduced to North America as early as the mid-1990’s. The insect likely arrived in the Great Lakes port region via ash shipping crates from China. Teams of researchers were sent to Northern China to research the EAB in its native habitat, and to study possible bio-control strategies. The two parasitoid wasp species being released here appeared to be the best choices.

By late May, Osthus has tied 100+ ash segments (called bolts) to trees along the river corridor from just north of the Franklin Bridge to Minnehaha Falls. These bolts will contain the larval form of the first parasitoid wasp, and will be restocked every two weeks throughout the summer.

Next to the ash bolt, Osthus will attach another device the size and shape of a pill bottle. These will contain the second bio-agent, a type of wasp that searches bark crevices to find EAB eggs. Each adult female will insert one of her eggs into an EAB egg. The wasps develop inside the EAB egg, emerge as adults and kill the EAB offspring in the process. It’s not a pretty scene but with parasitism rates as high as 60% (by Minnesota Department of Agriculture estimates), it’s considered effective.

The goal of biocontrol is to use natural enemies to bring the EAB population into balance and reduce damage to ash trees. In their native range of Northern China, the EAB presents no problems because it has naturally occurring predators. Since the EAB was introduced to North America, its predators have to be introduced too.

The EAB invasion has been devastating for Minnesota, which has the highest volume of ash trees of any state in the US. The environmental and economic costs of dealing with this loss are difficult to calculate.

According to Osthus, “The Twin Cities metropolitan area has been aggressive in its management plan. Slowing the spread of the EAB also means slowing the rate at which the city has to pay for removal of dead or dying trees. That makes it easier on the city’s budget, as well as on its taxpayers.”

What can be done to slow the EAB invasion at the citizen level? Don’t transport firewood unless certified by the Minnesota Department of Agriculture, which means it’s been kiln dried. Be vigilant in caring for ash trees on personal property. Diseased trees dry out very quickly because their food and water transport are cut off, and need to be removed.