Scientists hope this parasitic wasp from Asia can help stop the emerald ash borer’s spread in North America.
DAVID CAPPAERT
Later this month, the U.S. Department of Agriculture (USDA) will formally admit defeat along one front of its battle against a devastating invasive insect. Starting 14 January, the agency will no longer regulate the movement of living ash trees or borer-infested wood within the United States. This quarantine has, for more than 10 years, formed the cornerstone of the federal government’s strategy for curbing the spread of the emerald ash borer, an iridescent green beetle that threatens to wipe out North America’s ash trees, an ecological linchpin of many forests. Instead, USDA plans to ramp up an effort to control the borer by releasing tiny wasps that parasitize and kill the beetles.
The shift is controversial. Some scientists and environmental advocates agree that, after spending some $350 million over the past 2 decades to fight the ash borer, the government should redirect scarce resources to more promising strategies. But others argue the surrender is premature, and some states are vowing to maintain local controls on ash tree and wood movement. “I worry that this decision hastens the rate at which [ash] trees are threatened,” says Leigh Greenwood, a forest health specialist at the Nature Conservancy. “This is one layer of protection we’re taking away.”
The emerald ash borer first gained notoriety in 2002, when ash trees in the Detroit area started mysteriously dying. After researchers identified the insect, which was accidentally imported from Asia, Michigan and USDA’s Animal and Plant Health Inspection Service (APHIS) imposed a quarantine that prohibited export of ash trees and wood from inside the infested zone. Biologists also began to set traps to monitor the spread of the beetle.
But stopping the borer’s expansion has proved difficult; adults can fly up to 10 kilometers and often go undetected in new areas for years. The borer has attacked and killed tens of millions of trees in at least 35 states, mostly in the eastern and central United States; it has also infested southern Canada. USDA’s quarantine zone has expanded along with the beetle. But in 2017, the International Union for Conservation of Nature declared the borer had caused six North American ash species to become endangered or critically endangered. And in 2018, APHIS proposed ending its quarantine. “The regulations haven’t stopped it from spreading,” Robyn Rose, who managed the agency’s borer program, said that year.
USDA’s proposal drew some 150 comments. Some scientists and groups supported the move, in part to free up scarce resources. But others argued that USDA’s withdrawal could hamper efforts to prevent the borer from spreading to several as-yet-uninfested western states and Mexico, which hold the only known populations of numerous ash species. Oregon officials, for example, fear losing Oregon ash, an important riverside tree. Other experts worry city budgets could be strained by the costs of removing thousands of dead ash trees if the beetle attacks urban forests. Several Native American tribes have argued the move would endanger ash trees on their lands, which are vital to cultural practices such as basketmaking.
Despite those concerns, USDA last month announced it would end the quarantine in favor of what it deems “more effective and less intrusive methods” to fight the borer. The agency will instead emphasize biological control, or introducing natural enemies of the borer. In particular, scientists have identified four species of parasitic wasps native to Asia that lay eggs in ash borer larvae or eggs.
So far, researchers have released the parasitoids on an experimental basis in 340 counties in 30 states. Three of the wasps have established self-sustaining populations. At some sites, researchers report the wasps have killed 20% to 85% of borer larvae feeding on ash saplings, and are helping young trees survive to reproductive age. “All the information seems to be pointing in the same direction—the parasitoids are having an impact,” says Herbert Bolton, who now manages the ash borer program at APHIS.
For biocontrol to protect ash trees in the long term, however, wasps would have to establish everywhere ash trees grow—a still-distant goal. And wasps appear less effective at protecting mature ash trees, perhaps because they can’t penetrate the thick bark to find borers. To keep older trees standing, APHIS is investigating strategies that include chemical pesticides, but such efforts would be practical only in select ash stands. USDA has also backed longer term efforts to breed borer-resistant ash trees that could be used to replant forests.
Minnesota won’t follow USDA’s lead, saying it will enforce its own internal quarantine. Other states plan to start to certify firewood—which can carry the borer—as pest-free, a task the federal government had been doing. Such piecemeal efforts won’t be enough, fears Reginald Defoe, resource manager for the Fond du Lac Band of Lake Superior Chippewa in northern Minnesota. After USDA released its proposal, he wrote that, without a uniform national policy, “the movement of [the borer] is anticipated to be much faster.”
*Clarification, 7 January, 4:15 p.m.: This story has been revised to clarify USDA’s authority to regulate the movements of wood and trees.