Invasive insects threaten 1.4 million trees in the United States

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© Ryan Deberardinis

Researchers from McGill University have estimated that over the next 30 years, 1.4 million street trees will be killed by invasive insect species

At a projected cost of $900 million to replace, the first national space forecast of street tree mortality rates shows how invasive species will change our world.

Published in the British Ecological Society’s Journal of Applied Ecology, researchers have published data on 30,000 urban areas across the United States, to show hotspots at risk of major agricultural loss.

90% of the 1.4 million trees predicted to die in the study were thought to have been caused by the so-called emerald ash borer, an insect that is expected to kill virtually all ash trees in more than 6,000 urban areas.

Hunt down invasive insects to protect trees

To enable the prediction of invasive insects on street trees in the United States over the next 30 years, the researchers combined a series of four models. These included models of street tree populations in 30,000 communities, the predicted spread of 57 invasive insect species, the degree of mortality of these insects for different tree species, and the cost of removal and replacement of dead trees.

Due to data availability, the study focused specifically on street trees, which represent only a small fraction of all urban trees.

The study also only predicted the economic costs for municipalities faced with the destruction of street trees and no longer the ecological impacts. Dr. Frank Koch, research ecologist and team leader at Eastern Threat Center NC, said: “The ecological impacts of the loss of urban trees or an invasive species moving from urban forests to natural forests would both be considerable. However, these impacts were beyond the scope of our study.

The impact of invasive insects and hotspots

The researchers predicted that the impact of invasive insects will not be evenly distributed across the country, with less than a quarter of U.S. communities expected to experience 95% of total street tree mortality.

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

New York, Chicago and Milwaukee are some of the hotspots the study draws attention to, these areas have very high numbers of ash trees and are on or near the recent path of the emerald ash borer. Along with this, larger human populations are also expected to increase the influx of invasive insects into an area.

Insects such as the United States. Asian wood-boring insects like the citrus longhorn beetle have been labeled the biggest threat, with new establishments of these species potentially costing $4.9 billion over the next 30 years.

The potential to protect urban trees still exists

Trees are an important part of our urban environments and provide a multitude of benefits, including improving air quality, cooling streets, capturing carbon, providing habitats for wildlife and l improving the mental and physical health of citizens. We need to take note of excessive urban activity such as trade and travel that exposes urban trees to more invasive species.

This research can be used by future tree managers to learn which tree species in which areas will be most at risk from invasive insects. This information can be used to prioritize management efforts such as quarantining wood products.

Dr Emma Hudgins of McGill University and lead author of the research said: ‘These results can hopefully provide a cautionary tale against planting a single species of tree across entire cities like this was made with ash trees in North America. Increasing the diversity of urban trees provides resilience against pest infestations. Although we know it more intuitively for crop monocultures, many cities continue to plant what are essentially monoculture urban forests.

“This article shows that unless we plant a variety of tree species in our cities, urban trees are at serious risk from invasive pests. The take-home message for city planners is to plant multiple species in cities rather than just focusing on a few familiar species; It will keep the trees wonderful, and it will keep them in our cities.

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