Hitchhiking Bugs

More than 20 million ash trees have died in the Midwest. They were killed by a half-inch-long, metallic-green beetle that tunnels underneath bark. The emerald ash borer, a beetle native to Asia, threatens to completely remove ash trees from U.S. forests.

This is not the first time our forests have been devastated. During the early 1900s, chestnut blight ravaged our forests by killing almost every chestnut tree, and in recent decades, Dutch elm disease has caused the widespread death of elms.

The emerald ash borer is only one example of many non-native insects that have invaded North American forests. These invasive species are efficient hitchhikers on a variety of wood packing and other plant materials. Increasing international trade has provided more opportunities for non-native insects to travel to places where they’ve never existed before.

On Its Way

The emerald ash borer was first detected in the U.S. in 2002 when large numbers of ash trees began dying in southeastern Michigan. The beetle had arrived several years before, probably by hitchhiking within wood crating or other wood materials in a shipment from Asia.

By the time the infestation was discovered, it was already too late to contain it. The beetle had spread across Michigan’s Lower Peninsula and into Indiana, Ohio and Ontario.

More recently, populations have been found near Chicago and in Maryland. In southeastern Michigan, where nearly all ash trees have been killed, communities and homeowners are struggling to deal with the costs of removing the many hazardous standing dead trees.

These beetles continue to spread across the Midwest. Humans unknowingly transport emerald ash borers when they haul infested ash logs, firewood or nursery stock, because the insects spend much of their lives hidden beneath ash bark.

The beetle’s worm-like larvae feed just under the bark and create wandering S-shaped tunnels that disrupt the flow of water and nutrients within the tree. When large numbers of larvae are present, they completely girdle and kill trees within one to three years. All ash trees—from small to large—are susceptible, even healthy ones.

Missouri’s Ashes at Risk

Ashes, including white ash, green ash, blue ash and pumpkin ash, comprise about 3 percent of the trees in Missouri’s rural forests, and about 13 percent of the trees in our urban forests. In some places, they comprise as much as 30 percent of park and street trees. Cultivars of white and green ash are among the most commonly planted trees in urban landscapes.

Even before the emerald