Over the next 30 years, invasive insects will kill 1.4 million urban trees in the United States, a new study finds. Most of those deaths will be caused by the emerald ash borer, which researchers expect will kill nearly all ash trees in more than 6,000 communities.
The study was part of lead author Emma Hudgins’ Ph.D. thesis at McGill University in Montreal. Hudgins is now a postdoctoral fellow at Carleton University in Ottawa, Ontario.
“Over the course of my Ph.D., I had built models to predict how invasive insects would spread and establish across the United States, and I wanted to translate these spread predictions to predictions of ecological and economic impact,” Hudgins tells Treehugger. “I was also fortunate to have access to a new database of urban tree distributions and pest severity estimates from two recent studies, which made this project possible.”
Researchers specifically concentrated on the effects of the insects on city trees, because they point out that 82% of the U.S. population lives in urban settings and those numbers continue to grow.
“More generally, we were interested in looking at future impacts of urban trees because of their myriad benefits to city-dwellers, combined with the fact that urban areas are important bridgeheads for invasions,” Hudgins says.
“Beyond looking at current threats, we also wanted to create a list of high-impact characteristics in order to predict future insect invaders that would have the greatest impact that have not yet established in the U.S.”
Forecasting Insect Spread and Tree Deaths
For the study, Hudgins built models to estimate the impacts of invasive insects on urban trees in the United States from 2020 to 2050. The models encompassed about 30,000 communities across the country.
“These included a simulation model that forecasted insect spread and population growth over time, a distributional model of where susceptible trees were located, a probabilistic model of how deadly each insect was on each tree species, and a basic model of the cost of removing and replacing trees of different sizes,” she says.
“We put these all together to get forecasts of the precise locations, species, and cause of death of each future dead tree, as well as the associated cost to remove and replace it.”
They were able to classify “hot spot” cities with the most expected dead street trees over the next three decades.
“These particular locations are all at the top of our list because they have very high numbers of ash trees planted on their streets, and they are in the recent or near-future path of the emerald ash borer—a highly deadly wood-boring insect pest,” Hudgins says. “One reason why these cities are in the path of this species is their high human populations, which are predicted to be a factor increasing the influx of invaders of all kinds to cities in our underlying spread model.”
These are large cities, including New York and Chicago.
“There are many villages, towns, and small cities across the United States where ash trees represent a large share of all their street trees,” Hudgins says. “While they didn’t stand out as mortality hot spots, many of them will be invaded by the emerald ash borer and will have to bear the costs of removing and replacing lots of dead ash trees, often with very limited budgets and resources to do so.”
The findings estimated that about 1.4 million street trees will be killed in the next 30 years, costing approximately $30 million each year ($900 million total) to care for and replace them.
Emerald Ash Borer's Impact
The tree deaths aren’t spread equally in all communities, according to the research models. They estimated that just about one-quarter (23%) of urban communities will be home to 95% of insect-induced tree mortality.
And 90% of all tree deaths from invasive insects will be due to the emerald ash borer (Agrilus planipennis), which is predicted to kill nearly all the ash trees in more than 6,000 areas.
The emerald ash borer (EAB) arrived accidentally in the U.S. in 2002 via cargo imported from Asia. When infested by the insect, trees lose most of their canopy within two years and usually die within three or four years.
Hudgins points out that in the U.S., the invasive species that have had the most detrimental effects on urban trees are those that are native to Asia.
“This is likely due to higher volumes of trade imports from Asian countries that cause greater rates of introduction for these species. Wood-boring species like emerald ash borer have the most invasive style of feeding on trees, so they are the most deadly,” she says.
“Wood-boring species feed on the parts of trees responsible for supplying nutrients, and they can effectively cut off a tree’s ‘circulation’ in a process known as girdling. This is much more deadly than feeding on the leaves of a tree, which trees can tolerate even in really high amounts without much mortality.”
Another thing that increases the risk of an invasive insect is whether it feeds on common tree species like oaks, maples, or ash trees. Because there are so many of these trees, those insects can have a much larger impact.
Not only is the emerald ash borer a threat to trees, but it’s also a threat to some cultural practices in parts of North America.
“Black ash basket-making is a longstanding tradition and livelihood for Indigenous peoples in the Great Lakes region and the eastern United States and Canada, including for the Anishinaabe peoples, and peoples within the Haudenosaunee (Iroquois) and Wabanaki confederacies,” Hudgins says. “However, many weavers have been forced to use different tree species, or abandon the practice altogether, due to a lack of viable ash trees.”
Conservation and Planning
The researchers say their findings can be used to help with conservation efforts, by pointing out which trees in which communities are most at risk.
Hudgins points out that emerald ash borers had already killed hundreds of thousands of trees before the start of their 30-year study window. The city of Montreal alone estimated they cut 40,000 trees by 2020.
Researchers estimate that the invasive insect has killed about 100 million trees in North America so far.
“These results can not only allow cities to plan for major future impacts of pests, but they can hopefully motivate a change in urban tree planting strategies. Cities could decrease their risk of large-scale tree mortality by planting a diverse set of native trees, rather than large swaths of a single species,” Hudgins says.
“Putting a price tag on the urban impacts of these pests will hopefully also motivate greater compliance in campaigns to limit the movement of firewood, which can transport many insect pests.”