In 2002, a small, emerald-green beetle was discovered near Detroit, Michigan. Within two decades, 40 million ash trees succumbed to the invasive emerald ash borer in Michigan alone, and the insect’s range expanded from southeast Michigan to 35 states and five Canadian territories. Hundreds of millions of ash trees have been destroyed, leading to five of North America’s six ash tree species to enter the IUCN Red List as Critically Endangered.

According to the U.S. Forest Service, the nation’s 8 billion ash trees are valued at $282 billion, with timber harvest value of $25 billion annually. Ash wood is used to make tool handles, baseball bats, cabinetry, furniture, baskets, wood packaging, paper, pulp, and railroad ties. A 2011 study estimated it would cost between $13.3 and $26 billion to remove and replace decimated ash trees in parks, private land, and along streets in the states of Illinois, Indiana, Michigan, and Wisconsin. This dollar number doesn’t factor in the invaluable social and environmental value of the hundreds of millions of ash trees lost since the early 2000s. For example, a 2013 study discovered that the loss of ash trees in a 15-state area correlated with 20,000 deaths from respiratory and cardiovascular disease.

The plight of the ash tree is a case study on the importance of North America’s forests—and the very real threats facing them in the reality of climate change, globalization, and urban sprawl. The ash tree, whose very existence still hangs in the balance, isn’t the only tree threatened by wood boring insect infestations. The Asian longhorned beetle, multiple fruit tree borers, and more are joining the growing ranks of wood-boring insects threatening America’s forests.

What is the value of trees?

The United States is home to 823 million acres of forests and woodlands, about eight percent of the world’s total forested area. These forests employ over one million Americans. The U.S. lumber industry is valued at $15.6 billion, and the fruit and nut industry rakes in an average $25 billion annually. 

But the economic benefits pale in comparison to the ecological services trees provide. Worldwide, forests cover nearly a third of the earth’s land surface, and serve a dizzying number of ecological functions such as storing untold amounts of carbon, purifying air and water, enhancing biodiversity, and providing livelihoods for millions of people. The total value of the world’s forests is estimated at up to $150 trillion—almost twice the value of global stock markets.

Value to urban settings

Urban trees—usually hardwoods—are responsible for $2 billion annually in energy savings. What’s more, they contribute greatly to reducing the heat island effect and will play a central role in cooling cities in the face of extreme heat. Shaded surfaces may be 20–45°F (11–25°C) cooler than unshaded areas. What’s more, trees increase happiness and property values in urban neighborhoods, and have been shown to cut energy bills by 25 percent. 

Trees are invaluable sources of human mental and physical health. A single tree can absorb 10 pounds of air pollutants every year, saving over 850 lives annually and preventing 670,000 cases of acute respiratory illness. 

Trees are closely linked to human happiness and development. Unfortunately, the nation’s urban trees are at-risk for invasive pests, and some of the most common and beneficial urban species—ash, American elm, and American chestnut—have virtually disappeared from America’s streets and parks. 

Value to the climate

Forests are the most important land-based mechanism for carbon dioxide sequestration. Currently, U.S. forests offset approximately 14 percent of the nation’s annual GHG emissions, and the longer they are allowed to grow, the more capacity for sequestration they embody. Studies suggest protecting existing trees is more effective at sequestration than planting new ones, with proper forest management capable of mitigating up to 20 percent of the climate crisis.

According to Project Drawdown, trees and the myriad ways they can be utilized for food, shelter, material, culture, and ecosystems services, can sequester 181.31-292.94 gigatons of carbon dioxide from the atmosphere by 2050, equating to $270.18-$440.92 billion in net profit. Unfortunately, America’s forests are in trouble, largely from wood-boring insects. 

Wood boring beetles: What threatens trees?

Wood boring beetles are a group of insects that eat and destroy wood during their larval and/or adult stages. While many wood boring beetles are native and benefit forests by culling dead or dying trees, others have become an existential threat to some tree species and even entire ecosystems.

The emerald ash borer is responsible for the deaths of at least 100 million ash trees, and the Asian longhorned beetle—less established than emerald ash borer but able to infect a greater number of tree species—has killed nearly 100,000 trees. Wood- and phloem-boring insects likely cause the largest economic impacts among insect infestations, costing local governments and private property owners a combined $1.7 billion annually. Emerald ash borer and the Asian longhorned beetle are the worst culprits among wood boring insects, but there are others that officials are keeping an eye on, some established, and some not:

• Sirex woodwasp (Sirex noctilio F.)
• Coconut Rhinoceros Beetle (Oryctes rhinoceros)
• Citrus Longhorned Beetle (Anoplophora chinensis)
• Brown Spruce Longhorned Beetle (Tetropium fuscum)
• European Spruce Bark Beetle (Ips typographus L.)
• Common Pine Shoot Beetle (Tomicus piniperda L.)
• Roundheaded appletree borer (Saperda candida)
• Flatheaded appletree borer (Chrysobothris femorata)
• Shothole Borer (Scolytus rugulosus)
• Peachtree borer (Synanthedon exitiosa)
• Lesser peachtree borer (Synanthedon pictipes)
• Dogwood Borer (Synanthedon scitula)
• Termites

Early detection of wood boring insects is essential to saving trees. A suite of tools enables citizens and foresters alike to document sightings of species like the Asian longhorn beetle before they become a widespread infestation. But when the charismatic adults of wood boring pests emerge from a tree, it’s often too late. Early action can only be enabled by early detection, and good data that quantifies the unseen is essential in nipping wood boring pests in the bud. If infestations could be detected prior to the emergence of adult insects, a tree has vastly increased odds of surviving an initial attack. 

Those odds could increase with recent developments in acoustic detection technology that hears deep inside the tissue of trees before larvae emerge.