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|Emerald ash borer|
|Emerald ash borer|
The emerald ash borer (Agrilus planipennis) is a green 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 emerald ash borer infestation 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 is believed to have been brought to America unintentionally in ash wood which was used to stabilize crates during shipping.
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 by overseas shipping materials. It has since been found in several other parts of the North America. Ohio, Minnesota, and Ontario have experienced emerald ash borer migration from Michigan, and the species has continued to spread across the continent.
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 Leon Fairmaire published in the Revue d'Entomologie in 1888.
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. 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 (0.024 to 0.039 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.
Damage can occur in infested trees due to larval feeding. The serpentine feeding galleries of the larvae disrupt the flow of nutrients and water and can girdle the tree. 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. However, 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 survive.
Outside its native range, emerald ash borer is an invasive species, that is highly destructive to ash trees in its introduced range. 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 been found in several other parts of the North America. Ohio, Minnesota, and Ontario have experienced emerald ash borer migration from Michigan, and has continued to spread across the continent.
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. All North American ash species show susceptibility to EAB as North American species planted in China also show high mortality due to EAB infestation, but some Chinese species show resistance. Green ash and the black ash trees are preferred by EAB. White ash is also killed rapidly, but usually only after 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, usually they are also eventually killed. Upon the arrival of EAB in North America, many of the specialized predators and parasitoids that suppressed its 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 and kill nearly 100% of ash trees in an area approximately 10 years after introduction. EAB populations can spread 20 km a year. However, it primarily spreads long-distance by transport of firewood and other wood products that contain ash bark, which allows EAB to spread to new areas to create satellite populations outside of the main infestation and quickly increase its range.
Other factors can limit 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 cause high EAB mortality occasionally, but generally offer limited control. Mortality due to woodpeckers is variable, and parasitism by parasitoids such as Atanycolus cappaerti can be high, but overall parasitism is generally low.
EAB threatens the entire North American Fraxinus genus. It has killed at 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. Field studies of the first Michigan forests first infested with EAB showed that the borer had killed off > 99% of all living Fraxinus. Forest floor samples of these same plots resulted in ground soils void of seeds which could be capable of germinating continued generations. 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. Quaratines 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.
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.
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 are currently approved by the USDA for release: Spathius agrili, Tetrastichus planipennisi, and Oobius agrili. All three species have been documented parasitizing EAB larvae one year after release indicating they survived the winter.
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