Emerald ash borer
|Emerald ash borer|
The emerald ash borer (Agrilus planipennis) is a green jewel beetle native to Asia and Eastern Russia. Outside its native region, the emerald ash borer (also referred to as EAB) is an invasive species and is highly destructive to ash trees in its introduced range. The emerald ash borer was first discovered in America in June 2002 in Michigan. It was accidentally brought to North America in ash wood used in shipping materials.
The natural range of the emerald ash borer is eastern Russia, northern China, Japan, and Korea. It is invasive outside of its native range. Its first confirmed North American detection was in June 2002 in Canton, Michigan. It is suspected that it was introduced from overseas in shipping materials such as packing crates. Surrounding states and provinces have experienced emerald ash borer migration from Michigan, and the species has continued to spread.
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.
EAB primarily infest and can cause significant damage to ash species including green ash, black ash, white ash, and blue ash. In its native range, emerald ash borer is only a sporadic pest on native trees as population densities typically do not reach levels lethal to healthy trees.
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 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 nutrients to the leaves to survive. EAB has also been found infesting white fringetree, but it was not apparent whether the trees were healthy when first infested, or were already in decline due to drought.
Adult beetles are typically bright metallic green and about 8.5 millimeters (0.33 in) long and 1.6 millimeters (0.063 in) wide. Underneath the elytra, the upper side of the abdomen is coppery-red, which is a distinctive feature of the species. 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. 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 phloem and cambium 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 in 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 emerging as adults the following spring.
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 the 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 callous tissue around EAB galleries; however, they too are 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 20 km (12 mi) a year. However, it primarily spreads longer distances by transport of firewood and other wood products that contain ash bark, which allows EAB to reach new areas and create satellite populations outside of the main infestation.
Other factors can limit its spread. Climate research suggests that EAB growth may be stemmed in areas too cold for the beetle to survive. 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. 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. 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 colored traps attractive to EAB are hung in trees as part of a monitoring program. 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, the area is typically 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, therefore, 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 know that proper quarantine measure must be instituted. 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.
Tree removal and replacement
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, especially in areas where the material could be infested.
Insecticides are typically only recommended in urban areas or high value trees near an infestation. Insecticides with active ingredients such as imidacloprid, emamectin benzoate, and dinotefuran are currently recommended since they are systemic (i.e., incorporated into the tree) and remain effective for one to three years depending on the product. 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.
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.
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|Wikimedia Commons has media related to Emerald ash borer.|
|Wikispecies has information related to: Agrilus planipennis|
- New interview about EAB with prominent Research Entomologist
- United States Department of Agriculture Public Awareness Campaign
- Canadian Food Inspection Agency
- USDA Forest Service Northern Research Station link to research on EAB
- Ohio Department of Agriculture infestation PDF map
- Species Profile- Emerald Ash Borer (Agrilus planipennis), National Invasive Species Information Center, United States National Agricultural Library. Lists general information and resources for Emerald Ash Borer.
- The short film Emerald Ash Borer: The Green Menace is available for free download at the Internet Archive