Emerald ash borer, invasive tree-killing insect, discovered in Denver

A deadly new enemy to Denver’s 330,000 ash trees just moved to town.

The emerald ash borer, an invasive insect that’s killed millions of trees across the United States, was discovered in several trees in the Congress Park neighborhood on June 17, city Parks and Recreation officials said Thursday.

City officials confirmed the wood-boring beetle’s presence with Colorado State University entomology experts and Denver’s forestry office.

“We’re grateful that our Office of the City Forester has been preparing for this inevitable discovery, yet this is not a low-stakes battle,” Parks and Recreation Executive Director Jolon Clark said in a statement.

Emerald ash borer beetles were first discovered in Michigan in 2002 and have since killed millions of ash trees nationwide. They are widely considered the most destructive tree insect ever introduced to North America, according to the Colorado State Forest Service.

In Colorado, emerald ash borers were first found in Boulder in 2013 and later spread throughout the state, including into Broomfield, Fort Collins, Littleton, Carbondale and — last year — Lakewood.

The small-but-destructive green pests invade both healthy and stressed ash trees, killing them as soon as two years after the infestation starts. They pose a significant threat to Colorado’s urban forests, 15% of which are ash trees, state officials said.

Denver has been preparing for the arrival of the emerald ash borer since 2016 through public education, replacing small ash trees on city land and rigorous treatment of historic ash trees on city property, officials said.