emerald ash borer
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emerald ash borer, (Agrilus planipennis), species of beetle (order Coleoptera, family Buprestidae) that is native to East Asia and became a serious pest of ash trees (Fraxinus) in North America. The insect naturally occurs in China, eastern Russia, Mongolia, Taiwan, Japan, and Korea and was introduced into eastern and central North America in the 1990s. Being responsible for the deaths of hundreds of millions of ash trees in the United States and Canada, the emerald ash borer is considered one of the most destructive insect pests ever to be introduced to North America.
Life cycle
Emerald ash borers can have a one- or two-year life cycle. Adult beetles emerge in late May, June, and July to feed on ash trees and mate. The adult beetle is bright metallic green with a flattened back and measures about 1.3 cm (0.5 inch) long. It has purple abdominal segments beneath its wing covers. Females begin laying eggs on the bark of ash trees as soon as two weeks after emerging. After one or two weeks, the larvae hatch and tunnel into the tree to feed on the phloem and outer sapwood layers. As they grow, they produce distinctive S-shaped tunnel networks, known as galleries, that girdle and ultimately kill the tree by disrupting its vascular system. The larvae pass through four stages of growth and typically overwinter in chambers in the outer bark of the tree. They pupate in the spring and emerge from a characteristic D-shaped exit hole. Adults can fly distances up to 800 metres (about a half mile) to infest new trees.
In their native Asia, emerald ash borers tend to attack only stressed or weakened trees. However, North American tree species have no evolutionary defenses against the novel insect, and the larvae can kill both stressed and healthy trees. Small trees can die as soon as one to two years after infestation, whereas larger infested trees can survive for three to four years before succumbing. Heavy infestations can kill a tree more quickly. A thinning leaf canopy is one of the first symptoms of infestation; one-third to half of the branches may die on a large tree within the first year. Infested ash trees may also display splitting bark and increased woodpecker activity.
History of introduction and impact
There are 18 species of ash trees native to North America, and they are among the most abundant North American trees. They are important members of deciduous forests, wetlands, and riparian habitats across the continent and are frequently the codominant trees there, with maples (Acer) and beeches (Fagus). A number of insects, including at least 20 species of moths and butterflies, are heavily dependent or specialists on ash trees in North America, making the trees an important part of the food web. Prior to the introduction of the emerald ash borer, it was estimated that there were between seven and nine billion ash trees in the United States alone. In the eastern United States, timber from ash trees has been valued at $25 billion per year.
Although the beetle was first detected in millions of stricken trees near Detroit, Michigan, U.S., and Windsor, Ontario, Canada, in 2002, tree ring studies indicate that the emerald ash borer arrived in the United States in the 1990s, probably embedded in shipping materials from eastern Asia. Over the next decades the invasive species spread throughout five eastern Canadian provinces and some 35 U.S. states east of the Rocky Mountains. Given that adult beetle dispersal is somewhat limited, the long-distance spread to new areas primarily occurred through the transport of infested firewood, ash logs, or ash trees from nurseries into areas that did not have an infestation. Indeed, the initial infestations in Colorado, Indiana, Missouri, and West Virginia all were traced to the movement of firewood. The transport of trees and ash logs with bark is now regulated, though the continued transport of infested firewood remains an ongoing problem. In 2022 the pest was reported on the West Coast for the first time, having been found in an ash grove in Oregon.
The pest has almost completely extirpated native ash trees in some heavily infested areas, dramatically altering those ecosystems. Fewer than 1 in 1,000 trees survive attack by the insect. A plethora of dead trees can make forests more vulnerable to intense wildfire due to heavy fuel loads. The loss of trees also opens the forest canopy at a large scale, often facilitating the increase of invasive plant species that further alter and degrade the ecosystem. Studies have shown that few ash seedlings appear to reach reproductive age, raising fears that the emerald ash borer may permanently alter North American landscapes. The insect also kills ornamental ash trees used as shade and street trees, impacting the tree cover of urban and suburban areas. Some states have estimated costs in the tens of billions of dollars to treat, remove, and replace infested or dead ash trees on developed land.
Control methods
In 2003 the U.S. Department of Agriculture implemented a domestic regulatory program through its Animal and Plant Health Inspection Service in an attempt to control the spread of the emerald ash borer and possibly eradicate it. In 2021 eradication was deemed no longer feasible, and the program was terminated, though federal regulations continue to restrict nursery stock from Canada and the importation of ash wood. In both the U.S. and Canada, state and provincial regulations have remained in place to curtail the human-facilitated spread of the insect.
Given the speed at which the pest can kill a tree and the persistent risk of reinfestation, insecticides are costly to use effectively and must be applied annually. Appropriate insecticides can be used preventively or at the early stages of infestation but are generally not practical for forested landscapes. Most of the products are systemic insecticides that are absorbed by the tree; the application of some is done by trunk injection or trunk implant or by drenching the surrounding soil.
The most promising management strategy is biological control, in which natural enemies are deployed to contain the pest. Researchers found a number of stingless parasitoid wasps from Asia that attack emerald ash borers at various stages of their life cycle. Of the four species that were deemed safe to release in North America, three were successfully established in infested areas. One of these is a species of egg parasitoid (Oobius agrili); the female wasp lays its eggs within the eggs of the emerald ash borer for the larvae to feed on. The other species are larval parasitoids (Spathius agrili, S. galinae, and Tetrastichus planipennisi). The females of these wasps lay their eggs on or in emerald ash borer larvae as a food source for their emerging young. More than five million parasitoid wasps have been released in the United States since 2007, and up to 80 percent of emerald ash borer larvae have been parasitized by these wasps in some areas.