Invasive insects could kill 1.4 million trees by 2050 in cities across the United States, which could cost more than $900 million to replace, according to a new study.

The study, published in the British Ecological Society's Journal of Applied Ecology, used data from around 30,000 urban areas across the country to create the forecast and recommend that cities take steps to plant a variety of trees rather than trees from a single species.

The study was conducted by researchers from McGill University and North Carolina State University with the U.S. Forest Service's Southern Research Station.

"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," Emma Hudgins, the study's lead author, said in a press release.

"Increasing urban tree diversity provides resilience against pest infestations. While we know this more intuitively for monocultures of crops, many cities continue to plant what are essentially monoculture urban forests."

The emerald ash borer, a green jewel beetle native to northeast Asia, is projected to kill nearly all ash trees in more than 6,000 urban areas. The beetle, which lays eggs and feeds under the bark of the ash trees, is expected to cause 90% of the 1.4 million urban tree deaths projected in the study.

The U.S. Department of Agriculture and state agencies have long struggled to fight the invasive species, which has already killed millions of ash trees across North America. Last year, the Montana Department of Agriculture issued an emergency quarantine order in an attempt to quell their spread into the state.

The new study found that the death of urban trees will be concentrated into "hot spot" cities that include New York, Chicago and Milwaukee, which have high numbers of ash trees and have been introduced to the emerald ash borer or are in its path.

Researchers also examined the potential impacts of other insect species that have not arrived in the United States. The study found that wood-boring insects from Asia, such as the citrus long-horned beetle, could cost the United States as much as $4.9 billion by 2050 if they are introduced in the country.

"This paper shows that unless we plant a variety of tree species in our cities, urban trees are seriously at risk from invasive pests," said Jane Memmott, an ecologist and entomologist at the University of Bristol, who was not a part of the study.

"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."

Hudgins noted that the study specifically researched street trees in the United States because of the availability and accuracy of the data used in the modeling, but that the findings also apply to neighboring countries like Canada.

"We can see a similar situation in Canada, since emerald ash borer arrived here by spreading across the border with the United States, and cities like Montréal are in the process of losing all of their ash trees," she said.

"Colder cities like Winnipeg appear to be seeing delayed impacts of emerald ash borer due to its need to complete a longer life cycle at low temperatures."

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