MADISON, WI (SPECTRUM NEWS) — An invasive bug is killing trees across Wisconsin.

The Emerald Ash Borer first came to the U.S. In 2002 to Detroit in wood packing material. It was first discovered in Wisconsin in 2008.

Since then it has swept across the southern part of the state, infesting any variety of Ash Trees it can find.

“It's not one of the most common trees in the state, but it's reasonably common, so there is a big resource,” said Michael Hillstrom, a forest health specialist with the Wisconsin Department of Natural Resources.

Hillstrom said the DNR doesn't have an exact number of how many Ash Trees are in Wisconsin, though he said he's seen figures that put it in the range of 800 million trees. In parts of the state he said it can range from 10 to 20 percent of trees.

The borer lays eggs in the Ash Trees and then the larvae feed in the food and water conducting tissue of it. The larvae effectively cut off the water and nutrient supply in the tree killing them. Something they've done more prevalently in the southern part of the state.

“In the northern half of the state it's still much more scattered, and that's where most of our ash is in the state, or where most of our trees are,” Hillstrom said. “Our bigger concern is with the northern part of the state and that's where we're still trying to get the message out statewide.”

Once the borer gets into the trees it will almost certainly kill them.

At the University of Wisconsin Arboretum they've had to simply watch as that happens to the Ash Tree collection in the horticulture garden.

“At this stage in time it's simply watching our beloved collection of ash slowly decline and be removed,” said David Stevens, the Arboretum curator.

The Arboretum tried to treat two before the bugs came, one of which was a variety discovered as a seedling by the Arboretum's original director.

“Some of his ashes were spread below this tree,” Stevens said.

Treatment is expensive and needs to be done periodically. The Arboretum decided to just try and treat two. The treatment didn't keep the borer out though.

Stephenson said if someone decides to treat an Ash Tree in their yard they should seek a professional to do it. He does caution the price is high and only gets higher per tree.

Hillstrom agrees, treating can be simply out of many people's price range.

Overall, experts don't have much hope to stop the spread of the borer at this point.

“Over time we do expect that about 99 percent of the ash trees are going to die of all of the different species,” Hillstrom said.

All of those dead trees can create a safety issue too. Hilstrom said in other states the dead trees have fallen on people and property like houses or cars.

“Safety is really the biggest thing, Ash Trees get brittle very quickly when they die, big branches can snap off all of the sudden, entire trees can snap off really quickly,” Hillstrom said.

Invasive species have killed trees in the state before. A similar exodus of Elms and Chesnut trees has happened in Wisconsin. Hillstrom said they can try to replant Ash Trees after the current ones die, and the state can learn something from the problem.

“The hope is we're learning to diversify our forests where appropriate so that we don't have all of one species,” Hillstrom said.

When the tree is first infested one of the warning signs is called crow die-back — when the branches at the top die first. The bug then works itself down according to Stevens. He says once it starts, there's no going back.

“At this point we are simply watching them fall by the wayside so it is a rather sad appearance and a sad thing to be a part of and it's certainly a sense of helplessness that there's really not much we can do at this stage in the game,” Stevens said.

The DNR says one reason the borer was able to spread so quickly was by people taking fire wood great distances. Otherwise the bug can only travel half a mile a year. Hillstrom said people should not take fire wood further than 10 miles from where they got it.