To walk down a street in Midland, Mich., this summer is to witness a scene of mass carnage: row upon row of tree stumps with just a scattering of sawdust around them. This trail of destruction is the work not of tornadoes or of man but of a voracious beetle known as the emerald ash borer, first found in the U.S. in Detroit in 2002. The spreading infestation has killed some 60 million ash trees in 15 states stretching east to New York and south to Tennessee, and by the end of this year the death toll will likely surpass that of Dutch elm disease. "It is now the most destructive forest insect ever to invade North America," says Deb McCullough, an entomologist at Michigan State University. "We literally cannot keep up with it."
The iridescent bug is a recent arrival. Native to China, it probably migrated to the U.S. burrowed inside wooden shipping pallets. It has few predators in North America, which is one reason the speed of the outbreak is unprecedented. And most of its damage is done unseen. Mature beetles bore through the outer bark and into the phloem the vascular tissue that carries sugars from the leaves to the rest of the tree and lay eggs there in May and June. When the eggs hatch, the larvae feed on the nutrients in the phloem, in effect starving the tree.
There are an estimated 8 billion ash trees in the U.S., and none of the 16 species is resistant to the pest, says researcher Dan Herms of Ohio State University. That includes the northern white ash, which provides the wood of choice for the Louisville Slugger baseball bat. Treating infested trees with insecticide kills adult ash borers but not always the larvae, so while it can slow down an infestation, it can't stop it entirely.
In the 1960s, American cities and towns began planting ash trees, long favored for their stately silhouette and abundant foliage, to replace trees killed off by the Dutch elm scourge. But this approach, known as monoculture in which block after block is lined with the same kind of tree has made those areas especially vulnerable to the ash borer. The city of Fort Wayne, Ind., has already cut down 2,000 ash trees to stop the insect's spread and will likely have to remove an additional 1,200. Chicago can expect most of its 93,000 ash trees to be affected and is projected to spend as much as $46 million by 2020 to defend its foliage. All told, U.S. cities will spend more than $10 billion over the next decade to treat or remove infested trees, according to a recent study in Ecological Economics.
Other efforts to thwart the borers are also under way. Quarantines on transporting firewood that might be incubating the winged invaders are widely in place. And another Chinese immigrant is being enlisted in the war on Agrilus planipennis. In eight states this summer, the U.S. Department of Agriculture (USDA) is releasing more than 150,000 stingless wasps, sworn enemy of the ash borer, imported from the same forests in China where the borer occurs naturally and bred in a Michigan laboratory. "These things are like little hunter-seekers. Their whole mission is to go and find the emerald ash borer," says USDA scientist Jon Lelito. The wasps, which look like tiny flying ants, lay their eggs on the larvae or the eggs of the ash borer. When the wasp eggs hatch, the wasps feed on the ash borers.
If that solution sounds ominous past efforts to deploy nonnative predators against pests have spiraled out of control Lelito says the USDA's environmental-impact study predicts that as ash-borer populations decline, so will the number of wasps, until the two populations reach an equilibrium. "That's the advantage of biocontrol" over harsh pesticides, Lelito says. "There's nothing that lingers in the environment."
In the meantime, arborists are redesigning their treescapes. In Milwaukee, each block will have up to four different tree types, including lindens, oaks, maples and hackberries. "We're trying to protect ourselves so we won't have entire blocks with no trees," says the city's forestry-services manager, David Sivyer. For cities across the Midwest, that barren landscape is already a reality.