An invasive beetle called the emerald ash borer (Agrilus planipennis) will kill up to 99 per cent of ash trees on the streets of many US cities by 2060, according to modelling by Emma Hudgins at McGill University in Canada and her colleagues.
Invasive pests and diseases, such as Dutch elm disease, have had a devastating impact on some types of tree in many places around the world. Hudgins has previously found that despite the different nature of these pests, it is relatively simple to model their spread because they are being carried around by people, for instance, in firewood. 鈥淗umans are doing the transport of these species,鈥� she says.
Hudgins鈥檚 team has now applied this finding to model how the spread of insect pests already present in the US will affect street trees in cities. The researchers discovered that the emerald ash borer is set to have the biggest impact.
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This Asian insect, first detected in the US in 2002, . It is poised to wipe out most ash trees in forests and will hit cities hard too. Some, including New York, Chicago, Cleveland and Philadelphia, could lose 99 per cent of their ash trees, the model suggests.
That means the loss of the many benefits of urban trees until replacements grow. Greener cities are good for people鈥檚 mental well-being, says Hudgins, and trees provide habitat for animals. They also , improving the health of those who can鈥檛 afford air conditioning and reducing the bills of those who can.
Clearing dead ash trees could cost each city up to $13 million, the team forecasts. 鈥淚f they fall on someone, that鈥檚 a huge liability,鈥� says Hudgins.
Cities replanting trees or planting new ones should choose a diverse mix of pest-resistant species rather than having streets lined with genetically identical trees, she says.
The model suggests the emerald ash borer will reach Seattle but not the rest of the West Coast, as there are few ash trees in forests there. But a long-distance jump can鈥檛 be ruled out, says Hudgins.
It is possible to save ash trees by treating them with insecticides, but this has to be repeated every couple of years, making it expensive and impractical on a large scale. Parasitic wasps are now being released in some places to combat the emerald ash borer, although it isn鈥檛 yet clear how effective this will be.
鈥淚t鈥檚 still very preliminary,鈥� says Hudgins.
The emerald ash borer has also been spreading across Russia and was detected in Ukraine in 2019, so it could eventually reach western Europe. There, a fungal disease called ash dieback is already killing most infected trees.
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