Control efforts being tested as ash borer continues to spread
METRO DETROIT — It has been more than a decade since the emerald ash borer was first detected in North America, and in that time, it has devastated ash trees as it spreads.
The invasive beetle is native to China, where it typically infests sick or dying ash trees, feeding on leaves and laying eggs within bark, which is then attacked by larvae as they hatch. When it was accidentally brought to the U.S., it found a slew of ash trees with little natural resistance and has since spread, killing millions of trees in the process.
A new study released in the fall by entomologists Deborah McCullough, of Michigan State University, and Daniel Herms, of Ohio State University, has found insecticide treatments to be the most effective way to protect trees during the infestation’s invasion wave and to keep plants alive afterward.
“In the past three or four years, new products have become available that are much more effective than what we had to work with when ash borer was first discovered,” McCullough said. “In one case, there’s a product that’s injected into the base of the tree, called ‘Treeage,’ and you have to hire someone to do it, and it’s not free, but one treatment will protect a tree for three years, and that’s 100 percent control.”
Furthermore, she said that city foresters in multiple states, such as Michigan, Illinois and Wisconsin, estimated costs and found that treating the trees was more cost-effective than total removal of all dead or in-danger ash trees.
This was not just because of the cost of removal, but because of the environmental benefits the mature trees provide, such as stormwater capture, shade, reduced water use, and increased property value, McCullough said.
The study found that not all trees needed to be treated for effective prevention. Simulations suggest that if 20 percent of ash trees were treated annually beginning four years after the ash borer’s introduction, 90 percent of all trees would still be alive after 10 years due to the reduced population growth of the ash borer — effectively creating a kind of herd immunity.
“If you treat a portion of the trees, the beetles during the leaf-feeding period probably won’t feed on leaves of a single tree,” McCullough said. “So by having treated trees, maybe you can’t afford to treat them all, but you treat 30-40 percent, there’s an increased chance the beetle will encounter toxic leaf, and all the trees that haven’t been treated benefit.”
Other potentially good news involves the blue ash tree. While it does not seem to be as cold-resistant as other species, such as white ash, it seems to be more resistant to ash borer. McCullough said studies so far indicate it is as resistant to the borer as the trees in China that co-evolved with the beetle.
“Even in a few woodlots near Ann Arbor where the big invasion wave went through in ’05-’06, virtually all the white ash trees have been killed, but 60-70 percent of blue ash are not only alive, but look great,” she said. “You can see some evidence of ash borer, but its just natural resistance. The beetles don’t like them as well as green ash or white ash.”
McCullough said researchers are still trying to pin down exactly why the blue ash, as well as its Chinese counterparts, is more resistant — believing it may be chemically based — and are hoping to either splice the resistant genes into other species or cross-breed it for either more cold-resistant blue ash, or more borer-resistant ash species.
Additionally, the study reports that biological control methods — using woodpeckers and both native parasitic microscopic wasps and similar species that are the ash borer’s natural predator in China — are currently in the experimental stage, but their usefulness long-term is not yet known.
The report said there are three identified predator species of stingless, tiny wasps called ‘parasitoids’ that have been introduced into Michigan sites since 2007 and are now established in 14 states. In areas that already have large ash borer populations, McCullough said, it may not be able to make an appreciable dent.
“Ideally, you want the population to build up and slow down the population growth of ash borer, so you don’t see this widespread really high mortality rate (of ash), and that hasn’t happened yet,” McCullough said. “It may be some years off; it may be a sort of thing where the invasion wave goes through and parasitoids can control the population that’s left, rather than being overwhelmed by them.”
The study is still ongoing on how effective these parasitoids will be in controlling ash borer where blue ash is more prevalent, or when the wave has passed through, she said.
The native parasitic wasps attacking ash borer is a new development, she said.
“They seem to be learning that they can go to the ash trees because there’s a whole bunch of those insects we like to parasitize on the ash trees,” she said. “Up until 2007 or so, we weren’t seeing evidence they were being parasitized, and now at least one (species of parasitoid) is abundant.”
The usefulness of these methods is limited for cash-strapped local communities that have been dealing with the borer for years. Jeff Schmidt, supervisor with the Roseville Department of Public Works, said the majority of ash trees in the city have been devastated by the invasive insect.
“They’re either dead, have been removed or are declining rapidly,” Schmidt said. “So we are probably the same as any other surrounding city at this point with that.”
The city has not had the resources or the money to attempt any of the control methods on its remaining trees, Schmidt said, though he knew that some residents in the city have opted for the insecticide treatments privately.
“A lot of that has substantial costs to it, and with the limited resources we have these days and the limited money, as well, it’s not something we can do,” Schmidt said.
Schmidt said removal can be an expensive option, depending on if buildings, swimming pools, electric poles and fences are nearby.
Unfortunately, McCullough said while the situation has improved substantially for ash trees in urban areas like those around metro Detroit, in rural and forested areas, there are still tens of millions of trees susceptible and dying to the beetle. Those areas are impractical to deal with tree removal and insecticide treatments, she said.
“I don’t see that rate slowing,” she said. “That part is pretty depressing.”
For more information on the emerald ash borer, McCullough suggested visiting the website www.emeraldashborer.info.