Spread of the invasive emerald ash borer in urban forests
Ash tree in Longmont, Colorado in August 2021 with dieback as a result of infestation by the emerald ash borer.
The emerald ash borer is a non-native insect that infests and kills ash trees. Accidentally introduced from Asia, it was first found in Michigan in 2002 and has since spread rapidly in the eastern United States, killing the majority of ash trees where it has invaded. In 2013, the insect was discovered in Boulder, Colorado and later spread across the Colorado Front Range corridor. Urban foresters and natural resource specialists in Colorado are concerned about the mortality of ash trees that the insect is likely to cause because ash trees are valued as shade and ornamental trees in these communities and removing dead trees in urban landscapes can be costly. To address these concerns, scientists from the USDA Forest Service’s Rocky Mountain Research Station and Northern Research Station studied how fast the insect is spreading in Colorado compared to the eastern United States.
The scientists found that emerald ash borer populations are expanding much more slowly in Colorado than in the East for several reasons. In the East, there are continuous native populations of ash trees that offer pathways for insects to readily move, but in Colorado, ash trees are restricted to the urban environment where they have been planted, limiting movement pathways for insects. Additionally, due to concerns about ash tree mortality in the Colorado Front Range, there have been management efforts involving the use of insecticides that appear to have had positive effects in reducing regional insect populations. Finally, the Colorado Front Range is a much drier environment compared to the East which may be less favorable for this insect’s population development.
Ultimately, the good news is that the slower spread of emerald ash borers in Colorado may provide more time for developing plans and management options to protect trees than in the East where the insect has moved much more rapidly. This pattern of slower spread is likely to be the case for other communities in the West as well. This may be good news for Western communities who rely on these trees for shade and beauty in parks and neighborhoods, summer temperature moderation, and reduced energy costs. State and local forestry agencies are a great source of information on emerald ash borer management options.