1.4 Million Urban Trees May Fall to Invasive Insects by 2050

It could cost 30 million a year to replace infected trees in areas hardest hit by the non-native species

An image of a ash tree with emerald ash borer damage next to a healthy tree
Emerald ash borers are already predicted to kill all ash trees in more than 6,000 urban areas. Ryan Armburst, Kansas Forest Service

In a new study, researchers found that invasive insects in the United States could kill approximately 1.4 million urban trees by 2050, which would cost over $900 million to replace, reports Vishwam Sankaran for the Independent. 

Hot spots predicted to have the most urban tree mortality were Milwaukee, Wisconsin; Chicago, Illinois; and New York, New York. The study, published in the Journal of Applied Ecology, is the first nationwide forecast of street tree mortality from invasive insects, like the emerald ash borer.

The emerald ash borer is an invasive metallic green beetle native to Asia. It was first detected in 2002 in southeastern Michigan, and experts suspect it was brought into the United States on wood packing material carried on cargo ships or airplanes traveling from Asia. Since then, the bug has been wreaking havoc on ash trees. While adult beetles only nibble on foliage, the real damage is inflicted by developing ash borer larvae. The larvae tunnel under a tree's bark and disrupt its ability to transport water and nutrients. In the last two decades since the insect was first detected, hundreds of millions of trees have been killed in North America.  

Data collected from 30,000 communities across the country was used to estimate tree mortality in the next 30 years, a statement explains. The researchers combined this data with a model that predicted the spread of 57 different invasive insect species. The team also considered how deadly the invading insects are to which tree species and how much it would cost to remove and replace infected trees.

Their results show emerald ash borers alone could cause 90 percent of the estimated 1.4 million tree deaths, reports Jon Jackson for Newsweek. Ash borers are already predicted to "kill virtually all ash trees" in more than 6,000 urban areas, per Newsweek.

An image of a long metallic green colored insect resting on a leaf. The insect is the Emerald ash borer.
Emerald ash borer larvae tunnel under the tree's bark and disrupt the tree's ability to transport water and nutrients. Dr. James E. Zablotny/USDA

However, the insect's invasion won't spread evenly across the country. Less than 25 percent of 30,000 urban areas in the U.S. are expected to experience 95 percent of all street-tree mortality, reports Adam Barnes for the Hill. The areas that will experience the most tree loss include cities in the Midwest and East Coast. These locations are expected to have the most tree loss because large numbers of ash trees occupy the streets and parks in these areas, per the Independent. The urban areas are also located within ash borers' recent and predicted reach.

Other insects the team predicted would pose a threat to street trees included the Asian wood-boring insect. Though not yet detected in the U.S., this invasive insect could post an estimated $4.9 billion in damages over the next three decades, per a statement.

One way to save urban areas from becoming treeless is to diversify the types of trees planted, says study author Emma Hudgins, a biologist at McGill University, in a statement.

"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," Hudgins says. "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."