Sustainability Environment

Invasive insects could kill 1.4M urban trees over the next 30 years, study says

Emerald ash borer. Istock

Story at a glance

  • Estimates suggest 95 percent of the tree mortalities caused by invasive insects will occur in roughly 25 percent of 30,000 urban areas.
  • Hot spots are expected in New York, Chicago and Milwaukee.
  • The street tree mortalities could cost communities approximately $900 million.

A new study suggests invasive insects could kill 1.4 million street trees in urban areas across the U.S. over the next 30 years, costing communities $900 million.  

Researchers from McGill University, the USDA Forest Service Southern Research Station and North Carolina State University, using data from approximately 30,000 localities, found 90 percent of the trees will succumb to the emerald ash borer. Emerald ash borers are expected to kill nearly all ash trees in around 6,000 communities. 

Estimates suggest 95 percent of the tree mortalities caused by invasive insects will occur in roughly 25 percent of urban areas — where conditions allow species to spread — with hotspots expected in New York, Chicago and Milwaukee.  

The researchers combined a series of models that forecasted street tree populations in 30,000 communities, the predicted spread of 57 invasive insect species, how lethal the various insects are to different tree species, and the financial impact of tree removal and replacement.  

“These results can hopefully provide a cautionary tale against planting a single species of tree throughout entire cities, as has been done with ash trees in North America. Increasing urban tree diversity provides resilience against pest infestations,” the study’s lead author Emma Hudgins said in a news release.  

“While we know this more intuitively for monocultures of crops, many cities continue to plant what are essentially monoculture urban forests,” Hudgins added.  


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Researchers also projected the potential toll of invasive insects not yet discovered in the U.S. They estimated Asian wood boring insects could cost cities $4.9 billion over a 30-year period.  

“Urban trees do a variety of wonderful things – they keep cities cool, they take the sting out of heavy downpours, they are good for biodiversity and they even make people happier,” Jane Memmott of the University of Bristol, who was not involved in the study, said in the release.  

“This paper shows that unless we plant a variety of tree species in our cities, urban trees are seriously at risk from invasive pests. The take home message to urban planners, is to plant multiple species in cities rather than focus on just a few familiar species; It’ll keep trees wonderful, and it will keep them in our cities.” 


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