The black ash tree is considered the most important species in hydric sites in the upper Midwest, and yet it’s on to verge of total extirpation because of the emerald ash borer (EAB). The EAB is an invasive beetle from southeast Asia that lays its eggs in ash trees and eats the leaves when they reach maturity.
There are about 8.5 billion ash trees and hundreds of millions have already been destroyed. Part of the reason this beetle is so deadly is because it infects ash trees before they can reproduce. The larvae burrow underneath the bark and keep the water from moving upward from the roots, and the adult beetles eat the leaves keeping nutrients from flowing throughout the tree. Without this keystone species, the ecosystem would change dramatically.
There are ways to combat the beetle, but funding is in short supply.
The EAB was first discovered in southeastern Michigan in 2002, but evidence suggests that it’s been in the area since the early ’90s. From Michigan, it spread via the transportation of the larvae in firewood and lumber. How the beetle first came to North America is still uncertain, but experts suspect that it was the same way it was able to spread here, on harvested wood. The EAB has been reported in 35 states, including the entire Midwest. Wisconsin is one of the states currently under EAB quarantine, meaning that there are regulations limiting transport of firewood outside of the state, and county regulations limiting it within the state.
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Unfortunately, early efforts to stop the EAB from spreading were abandoned because prior to its arrival ash trees were not widely studied. It was the emergence of this beetle that helped us understand just how fundamental this species is to our ecosystem.
“I lived through it – those were some rough years,” says Deborah McCullough, entomologist and professor at Michigan State University, recalling the first reports of the EAB. “There is just nothing on the outside of the tree to tell you that it is there.”
Larvae in low populations on the trees do not cause significant damage, but by the time the signs of their arrival are visible, they have already been there for three or four generations. Natural predators like the woodpecker feed on the larvae, but this is not effective enough to control the EAB populations and keep them from killing off ash trees, according to McCullough.
There are traps, but they are not an effective way to tell if there are EABs around. A detection method which proves effective is a practice called girdling a tree. A deep strip is carved into the ash, almost like a belt, which essentially starves and kills the tree. Historically, the EAB always chooses a wounded ash tree to lay eggs in over a healthy one. This is because ash trees emit different chemicals into the environment when they are wounded, and the EAB can sense this. But girding is not an ideal or efficient method of detection, as it necessitates killing a healthy tree and then cutting it down and shaving the bark off to find larvae, and even if successful is only good for one season at most.
Pesticides like Tree-äge are an effective method of controlling the populations, according to a study done by McCullough, which found that Tree-äge provided, “nearly 100% protection from emerald ash borer,” for two years. The pesticide is injected into the trunk or poured into the soil around the tree.
“Once it takes a bite out a treated tree, that beetle is going to die,” says McCullough.
This method is effective in urban areas, but when you get into forests, it becomes a whole heck-of-a-lot harder. Each tree needs to be individually treated, and there is not a logistical way to do this. At least not without significant funding. Targeting areas that are immediately vulnerable and areas that are already infested, then treating a percentage of them to try to reduce the population are one way to reduce the cost of saving forested ash.
Anticipating the black ash being wiped out, Robert Slesak, forester and researcher for the US Forest Service, has studied potential replacement species. Swamp white oak is the current front runner.
“That is fine and dandy, but it’s not black ash,” says McCullough. “Black ash occupied all of that area for a reason.”
Slesak also studied the impact on the environment if black ash is wiped out. Since it is a swamp tree, it helps regulate the height of the water table below the soil.
“When you remove those vegetation water pumps, which is essentially what they are, you just get this increase in water levels,” says Slesak. He has reported seeing a rise in the water table anywhere from 40 to 50 centimeters. While this may not seem like a lot, the vitality of many species depends on the conditions of the swamps with the black ash, and a change of even a few centimeters can mean the difference between life and death. Slesak is worried about the black ash wetlands becoming an open marsh.
The black ash has a strong effect on the temperature of the ecosystem. The leaves provide shade and keep the ground level cool for other plants to grow and species to thrive such as fish and crustaceans. Without the canopy, the ground will become warmer and many other species which relied on that shade will die or leave.
“And the temperatures, you can tell,” says McCullough. “When you are paddling down the rivers it’s hotter, it is definitely hotter.”
According to a study in the American Journal of Preventative Medicine, there is a relationship between trees and human health. The study concluded, “Tree loss from the spread of the emerald ash borer is associated to the cardiovascular and lower-respiratory systems.” While not entirely black ash specific, the loss of trees in an area cause the air quality to plummet. The study also indicated that in areas with more trees, people are more likely to go out and enjoy the scenery which leads to an increase in exercise. The study argues that the loss of ash because of EAB causes a decline in health.
Currently in Milwaukee, the department of public works has identified over 15,000 areas where ash trees are at risk of being killed by EAB. They ask anyone who has ash trees on their property and can inject pesticides to do so, and they also request that anyone who notices an infected tree report it to prevent the beetle spreading and to avoid the dying tree falling and endangering lives or property.
“The beetle is not trying to be bad, it is just doing what it does,” McCullough says. “It is people that keep screwing up, and I know it is an accident the emerald ash borer got here but we have got to get better at keeping some of these tree feeding insects out of the country because it is hard to predict the effects of that.”