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A Tiny, Green Beetle Is Killing Thousands Of Colorado's Trees. Why It Can't Be Stopped

In the spring of 2016, Bonnie Prushnok noticed a dying tree in the neighborhood across the street from her condo. It had a strange appearance, she said, like it was gasping for life.

Its branches on top were bare, as if dormant for winter. But the lower branches looked full and healthy. A few months later, dozens of mature ash trees in her neighborhood, Reynolds Farm in Longmont, took on a similar characteristic.

"Branches would just break off," said Prushnok, vice president of the Reynolds Farm Homeowners Association. "That was the beginning."

The culprit, she found out, was a tiny, green beetle.


First discovered in Boulder in 2013, the emerald ash borer has since popped up in nine Front Range communities. Over the summer, foresters in Westminster and Broomfield found it in city limits. This month, just outside the town of Berthoud in Larimer County, local foresters discovered the bug burrowed inside an ash log.

Meanwhile, foresters in Denver guess it's only a matter of time until they find it in the state's capital, where there's an estimated 1.4 million ash trees.

The tree species makes up about 15% of the entire state's urban canopy — in other words, millions of trees along city streets, parks and in people's yards from Fort Collins to Colorado Springs.

The borer's rapid spread is basically a death sentence for any ash tree in its path. Once a tree is infested, it has less than four years to live.

So far, no one's been able to beat it.

"It makes me sad that we haven't gotten a cure," Prushnok said, walking past an infested tree near her home, its top branches bare and broken. "It's frustrating."


The emerald ash borer is native to Asia and arrived in North America during the 1990s, according to the Colorado State Forest Service. Across the country, the insect has killed millions of ash trees in at least 35 states.

Immediately after local foresters discovered the bug in Boulder in 2013, the Colorado Department of Agriculture looked for ways to halt — or at least slow — its spread.

So, they established an emergency quarantine. It included all of Boulder County.

It worked. Kind of.

The boundary banned the sale and movement of ash saplings and untreated logs, lumber, wood chips, stumps, roots and branches. Anyone found moving ash would be fined up to $1,000.

­Nevertheless, the beetle kept moving. ­

By the end of 2015, the entire city of Boulder was infested. In 2017, local foresters found it in Lafayette. The next year, residents spotted it in Lyons.

After a Broomfield homeowner discovered a dead borer lying on their sidewalk in September, state agriculture officials announced they would repeal the quarantine.

Laura Pottorff, plant health and certification section chief for the Colorado Department of Agriculture, told KUNC's Colorado Edition it would be lifted by the end of 2019.

"Once the insect had breached our quarantine area, it was clear that we couldn't stop it," Pottorff said. "It no longer makes sense or is economically feasible to place restrictions on the movement of wood."

Pottorff said the beetle beat the quarantine two ways: It simply flew from tree to tree. Humans also helped by moving infested ash.

But not all was lost, she said. The beetle would have traveled much faster without it.

"We're just glad we've given Front Range communities more time to better plan and prepare for its arrival," Pottorff said.


Emerald ash borer
Credit Will Parson / Chesapeake Bay Program
Chesapeake Bay Program
The emerald ash borer is native to Asia and arrived in the United States in the 1990’s. Local foresters first detected the beetle in Boulder in 2013.

Scientists have also tried to stop the beetle using the borer's natural enemy: wasps.

In 2014, John Kaltenbach had just started his job as a biological control specialist with the state. As local and state officials struggled to contain the new pest, he caught wind of a study the USDA was conducting.

The department — already battling the ash borer in other parts of the country — was breeding wasps from Asia in government labs. They planned to release thousands of them in communities across the country were the ash borer had taken hold.

Syracuse, New York and Naperville, Illinois were on the list. So, Kaltenbach thought, why not Boulder?

Five years later, the study is nearing its end. Official results haven't been published, but Kaltenbach said it's been somewhat successful.

In a bark sample taken from an infested ash tree in Boulder earlier this year, his team found that 28% of the ash borer larvae had been parasitized by one of the wasps.

"We also found that woodpeckers have discovered emerald ash borer," he said. "So, the combination of those two is killing a lot of them."

Despite the moderate success, the biocontrol won't be enough to spare Colorado's ash trees, he said.

"Probably our biggest problem is detection," he said. "We don't have a good trap and lure."

Typically, when foresters find the borer in a community, it's already been in town for a couple years, he said.

"And that makes management almost impossible because you just don't know where it is," he said.


Credit Matt Bloom / KUNC
Workers with the City and County of Denver's "Be A Smart Ash" program plant a new sapling in the city's Highland neighborhood.

Preparing for the worst

More than two dozen communities along the Front Range are preparing for or are already dealing with the ash borer's invasion.

In Berthoud, the city is offering financial aid to residents who choose to inoculate their ash trees with pesticides.

In Broomfield, local foresters have already ripped out 1,500 trees from public spaces. There's also free app that helps homeowners identify if they have an ash tree on their property.

And in Denver, parks and recreation staff are on the defense, said Coleman Loughery, a forestry inspector with the city and county.

"There are no confirmed infested trees yet," Loughery said. "We're keeping our eyes open."

Loughery, through the city's Be A Smart Ash program, is overseeing the planting of 3,000 new trees each year — all of them non-ash species. The idea is to identify blocks with a lot of ash and plant a variety of trees before the borer arrives.

"We know that one-sixth of the canopy (in Denver) is going to go away because of this," he said. "So, we're trying to get ahead of that and plant diverse species."

Loughery said little moments, like planting a new tree, are a silver lining in an otherwise tragic environmental story.

Homeowner James Ganswin, who qualified for the program and received a free Rocky Mountain Glow Maple, said he had no complaints.

"I mean, a free tree is a free tree," Ganswin said.

A new reality

The cost of managing an infested tree can range from a couple hundred dollars to several thousand, depending on the size.

For Bonnie Prushnok and the 57 other households in Reynolds Farm, the bill meant making a few adjustments.

"We're talking about almost $3,000 per tree just to take it down," she said. "Last year we had residents out shoveling snow to save our plow money to take them down."

Prushnok's advice to anyone who might be in this situation? Pick a few of the healthiest ash trees in your neighborhood and have them inoculated. It costs a couple hundred dollars every two to three years, even though the protection won't last forever.

I cover a wide range of issues within Colorado’s dynamic economy including energy, labor, housing, beer, marijuana, elections and other general assignment stories.
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