The emerald ash borer, an invasive beetle from Asia, is blitzing ash trees in urban and forested ecosystems across North America. To date, this metallic green beetle has attacked and killed hundreds of millions of ash trees in 25 states, and its spread continues. To control this beetle, the U.S. Department of Agriculture (USDA) is using biological control, a long-term sustainable management tool that involves the introduction of specialized natural enemies from a pest’s native region.
The emerald ash borer likely arrived in the U.S. during the 1990s after navigating the oceans hidden away inside wood packing materials from Asia. Upon the arrival of these beetles, they quickly went on the offensive, attacking an abundant ash resource. Their larvae kill trees by feeding on the phloem, and disrupting its flow of nutrients. On average, land managers spend tens of millions of dollars per year to combat the beetles, which infest hundreds of billions of dollars in potential ash timber.
In its native home, the beetle is subject to population control by a group of natural enemies including several specialized parasitic wasps or parasitoids. “The wasps are on a seek and destroy mission,” explained Leah Bauer, a research entomologist with the Forest Service’s Northern Research Station. Because these tiny stingless wasps co-evolved with the beetles in Asia, they can find them even when densities are low. The wasps target and attack beetle eggs or larvae, depending on the species.
After thorough research, the USDA authorized use of four species of the tiny wasps as biocontrol agents in states besieged by the bright green beetles. They target the beetles almost exclusively and pose no significant threat to livestock, crops, pollinators, or people. The first three biocontrol agent deployments began in 2007, and releases of a fourth species began in 2015.
When resource managers first release the biocontrol agents in forests with high beetle densities, the infested ash trees do not recover. However, the trees provide abundant beetle larvae and eggs as food for wasps, increasing the probability of establishment in the area. Bauer, working closely with other USDA and university collaborators, finds that the wasps also attack beetles in young, and regenerating ash saplings, protecting the next generation of ash trees.