Emerald ash borer

Range

The native range of the emerald ash borer is temperate northeastern Asia, which includes Russia, Mongolia, northern China, Japan, and Korea. It is invasive in North America where it has a core population in Michigan and surrounding states and provinces. Populations are more scattered outside the core area, and the edges of its known distribution range north to the upper peninsula of Michigan, south to northern Louisiana, west to Colorado, and east to Massachusetts

In northern Europe, a population was found in Moscow, Russia in 2003. This population has spread west to Sweden as of 2016 at 40 km (25 mi) per year.

Identification

Adults are a bright metallic green. Elytra are typically a darker green, but can also have copper hues. EAB is the only North American species of Agrilus with a bright red upper abdomen when viewed with the wings and elytra spread. The species also has a small spine found at the tip of the abdomen and serrate antennae that begin at the fourth antennal segment. The French priest and naturalist Armand David collected a specimen of the emerald ash borer during one of the trips he took through imperial China in the 1860s and 1870s. He found the beetle in Beijing and sent it back to France, where a brief description by the entomologist Léon Fairmaire was published in the Revue d'Entomologie in 1888.

Host plants

In its native range, emerald ash borer is only a nuisance pest on native trees, as population densities typically do not reach levels lethal to healthy trees. In China, it will infest native F. chinensis, F. mandshurica, and F. rhynchophylla; in Japan it will also infest F. japonica and F. lanuginosa.

EAB primarily infest and can cause significant damage to ash species including green ash, black ash, white ash, and blue ash in North America. In Europe, Fraxinus excelsior is the main ash species colonized by EAB. Ash susceptibility can vary due to the attractiveness of chemical volatiles to adults, or the ability of larvae to detoxify phenolic compounds. EAB has also been found infesting white fringe tree in North America, which is a non-ash host, but it is unclear whether the trees were healthy when first infested, or were already in decline due to drought.

Adults prefer to lay eggs on open grown or stressed ash, but readily lay eggs on healthy trees amongst other tree species. Young trees with bark between 1.5 millimeters (0.059 in) to 5 millimeters (0.20 in) are preferred. Both males and females use leaf volatiles and sesquiterpenes in the bark to locate hosts. Damage occurs in infested trees due to larval feeding. The serpentine feeding galleries of the larvae disrupt the flow of nutrients and water, effectively girdling (killing) the tree. On susceptible species or in the absence of organisms that suppress emerald ash borer populations, the tree will eventually no longer be able to transport sufficient water and nutrients to the leaves to survive.

Life cycle

The emerald ash borer life cycle can occur over one or two years depending on the time of year of oviposition, the health of the tree, and temperature.

Adult beetles are typically bright metallic green and about 8.5 millimeters (0.33 in) long and 1.6 millimeters (0.063 in) wide. After 400–500 accumulated growing degree days (GDD) at base 10 °C (50 °F), adults begin to emerge from trees, and peak emergence occurs around 1000 GDD. After emergence, adults feed for one week on ash leaves in the canopy before mating, but cause little defoliation in the process. Males hover around trees, locate females by visual cues, and drop directly onto the female to mate; mating can last 50 minutes, and females may mate with multiple males over their lifespan. A typical female can live around six weeks and lay approximately 40–70 eggs, but females that live longer can lay up to 200 eggs.

Eggs are deposited between bark crevices, flakes, or cracks and hatch about two weeks later. Eggs are approximately 0.6 to 1.0 millimeter (0.02 to 0.04 in) in diameter, and are initially white, but later turn reddish-brown if fertile. After hatching, larvae chew through the bark to the inner phloem, cambium, and outer xylem where they feed and develop. Emerald ash borer has four larval instars. By feeding, larvae create long serpentine galleries. Fully mature fourth-instar larvae are 26 to 32 millimeters (1.0 to 1.3 in) long. In fall, mature fourth-instars excavate chambers about 1.25 centimeters (0.49 in) into the sapwood or outer bark where they fold into a J-shape. These J-shaped larvae shorten into prepupae and develop into pupae and adults the following spring. To exit the tree, adults chew holes from their chamber through the bark, which leaves a characteristic D-shaped exit hole. Immature larvae can overwinter in their larval gallery, but can require an additional summer of feeding before overwintering again and emerging as adults the following spring. This two-year life cycle is more common in cool climates, such as European Russia.

Ecology

Outside its native range, emerald ash borer is an invasive species that is highly destructive to ash trees in its introduced range. Prior to EAB being found in North America, very little was known about EAB in its native range aside from a short description of life-history traits and taxonomic descriptions, which resulted in focused research on its biology in North America. Since its accidental introduction into the United States and Canada in the 1990s and its subsequent detection in 2002 in Canton, Michigan, it has since spread other parts of the North America. It is suspected that it was introduced from overseas in shipping materials such as packing crates.

Invasiveness and spread

Without factors that would normally suppress EAB populations in its native range (e.g., resistant trees, predators, and parasitoid wasps), EAB populations can quickly rise to damaging levels. After initial infestation, all ash trees are expected to die in an area within 10 years without control measures. Every North American ash species shows susceptibility to EAB, as North American species planted in China also show high mortality due to EAB infestation, but some Chinese ash species show resistance.

Green ash and black ash trees are preferred by EAB. White ash is also killed rapidly, but usually only after all green and black ash trees are eliminated. Blue ash displays some resistance to the emerald ash borer by forming callus tissue around EAB galleries, but is eventually killed. Many of the specialized predators and parasitoids that suppressed EAB populations in Asia were not present in North America. Predators and parasitoids native to North America do not sufficiently suppress EAB, so populations continue to grow. EAB populations can spread between 2.5 to 80 km (1.6 to 49.7 mi) a year. It primarily spreads through flight or by transportation of ash bark containing products such as firewood or nursery stock, which allows EAB to reach new areas and create satellite populations outside of the main infestation.

Other factors can limit EAB's spread. Winter temperatures of approximately −38 °C (−36 °F) limit range expansion., and overwintering EAB survive down to average temperatures of −30 °C (−22 °F) due to antifreeze chemicals in the body and insulation provided by tree bark. Larvae can also high heat up to 53 °C (127 °F).

North American predators and parasitoids can occasionally cause high EAB mortality, but generally offer only limited control. Mortality due to native woodpeckers is variable. Parasitism by parasitoids such as Atanycolus cappaerti can be high, but overall such control is generally low.

Environmental and economic impacts

EAB threatens the entire North American Fraxinus genus. It has killed at least tens of millions of ash trees so far and threatens to kill most of the 8.7 billion ash trees throughout North America. Emerald ash borer kills young trees several years before reaching their seeding age of 10 years. In both North American and Europe, the loss of ash from an ecosystem can result in increased numbers of invasive plants, changes in soil nutrients, and effects on species that feed on ash.

Damage and efforts to control the spread of EAB have affected businesses that sell ash trees or wood products, property owners, and local or state governments. Quarantines can limit the transport of ash trees and products, but economic impacts are especially high for urban and residential areas due to treatment or removal costs and decreased land value from dying trees. Costs for managing these trees can fall upon homeowners or local municipalities. For municipalities, removing large numbers of dead or infested trees at once is costly, so slowing down the rate at which trees die through removing known infested trees and treating trees with insecticides can allow local governments more time to plan, remove, and replace trees that would eventually die. This strategy saves money as it would cost $10.7 billion in urban areas of 25 states between 2009–2019, while removing and replacing all ash trees in these same areas at once would cost $25 billion. Some urban areas such as Minneapolis, Minnesota, have large amounts of ash with slightly more than 20% of their urban forest as ash.

Monitoring and management

In areas where EAB has not yet been detected, surveys are used to monitor for new infestations. Visual surveys are used to find ash trees displaying symptoms of EAB damage and traps with colors attractive to EAB, such as purple or green, are hung in trees as part of a monitoring program. These traps can also have volatile pheromones applied to them that attract primarily males.

Sometimes trees are also girdled to act as a trap tree by attracting egg-laying female EAB in the spring and debarking the trees in the fall to search for larvae. If detected, an area is often placed under a quarantine to prevent infested wood material from causing new infestations. Further control measures are then taken within the area to slow population growth by reducing EAB numbers, preventing them from reaching reproductive maturity and dispersing, and reducing the abundance of ash trees.

Government agencies in both the USA and Canada have utilized a native species of wasp, Cerceris fumipennis, as a means of detecting areas to which EAB has spread. The females of these wasps hunt beetles in the same family as EAB and will hunt EAB if it is present. The wasps stun the beetles and carry them back to their burrows in the ground where they are stored until the wasps’ eggs hatch and the wasp larvae feed on the beetles. Volunteers catch the wasps as they return to their burrows carrying the beetles to determine whether any of the catch consists of EAB. If it does, the agencies running the program may institute quarantine measures. This methodology is known as biological surveillance, as opposed to biological control, because it does not appear that the wasps have a significant negative impact on EAB populations.

Quarantine and tree removal

Once an infestation is detected, quarantines are typically imposed by state or national government agencies disallowing transport of ash firewood or live plants outside of these areas without permits indicating the material has been inspected or treated (i.e., heat treatment or wood chipping) to ensure no live EAB are present in the bark and phloem. In urban areas, trees are often removed once an infestation is found to reduce EAB population densities and the likelihood of further spread. Urban ash are typically replaced with non-ash species such as maple, oak, or linden to limit food sources for EAB. In rural areas, trees can be harvested for lumber or firewood to reduce ash stand density, but quarantines may apply for this material, especially in areas where the material could be infested.

Insecticides

Insecticides with active ingredients such as azadirachtin, imidacloprid, emamectin benzoate, and dinotefuran are currently used since they are systemic (i.e., incorporated into the tree) and remain effective for one to three years depending on the product. Insecticides are typically only considered a viable option in urban areas with high value trees near an infestation. Ash trees are primarily treated by direct injection into the tree or soil drench. Some insecticides cannot be applied by homeowners and must be applied by licensed applicators. Initially, tree injections will not compromise tree health, but over many years drilling and chemical wounds will compromise the tree's health. Damage from EAB can continue to increase over time even with insecticide applications. Insecticide treatments are not feasible for large forested areas outside of urban areas.

Biological control

The native range of EAB in Asia was surveyed for parasitoid species that parasitize EAB and do not attack other insect species in the hope they would suppress EAB populations when released in North America. Three species imported from China were approved for release by the USDA in 2007 and in Canada in 2013: Spathius agrili, Tetrastichus planipennisi, and Oobius agrili, while Spathius galinae was approved for release in 2015. Excluding Spathius galinae, which has only recently been released, the other three species have been documented parasitizing EAB larvae one year after release, indicating that they survived the winter, but establishment varied among species and locations. Tetrastichus planipennisi and Oobius agrili established and have had increasing populations in Michigan since 2008; Spathius agrili has had lower establishment success in North America, which could be due to a lack of available EAB larvae at the time of adult emergence in spring, limited cold tolerance, and better suitability to regions of North America below the 40th parallel.

The USDA is also assessing the application of Beauveria bassiana, an insect fungal pathogen, for controlling EAB in conjunction with parasitoid wasps.