killed by emerald ash borers with bark stripped by woodpeckers feeding on emerald ash borers Outside its native range, emerald ash borer is an
invasive species that is highly destructive to ash trees in its
introduced range. Without factors that would normally suppress emerald ash borer populations in its native range (e.g., resistant trees, predators, and
parasitoid wasps), populations can quickly rise to damaging levels.
Green ash and
black ash trees are preferred by emerald ash borer.
White ash is also killed rapidly but usually only after all green and black ash trees are eliminated.
Blue ash exhibits a higher degree of resistance to emerald ash borer, believed to be caused by the leaves' high
tannin content, making the foliage unpalatable to the insect. While most Asian ashes have evolved this defense, it is absent from American species other than blue ash. Researchers have examined populations of so-called "lingering ash", trees that have survived ash borer attack with little or no damage, for the grafting or breeding of new, resistant stock. Many lingering ashes have unusual phenotypes that may increase resistance. Aside from their higher tannin content, Asian ashes employ natural defenses to repel, trap, and kill emerald ash borer larvae. Although studies of American ashes have suggested that they are capable of mustering similar defensive mechanisms, the trees do not appear to recognize when they are under attack. Many of the specialized predators and parasitoids that suppress emerald ash borers in Asia are not present in North America. Predators and parasitoids native to North America do not sufficiently suppress emerald ash borers, so populations continue to grow. Birds such as woodpeckers feed on emerald ash borer larvae, although the adult beetles have not been used by any American fauna as food. and overwintering emerald ash borers survive down to average temperatures of because of antifreeze chemicals in the body and insulation provided by tree bark.
Environmental and economic impacts Emerald ash borers threaten the entire North American genus
Fraxinus. It has killed tens of millions of ash trees so far and threatens to kill most of the 8.7 billion ash trees throughout North America. 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 eventually die. This strategy saves money, as it would cost $10.7 billion in urban areas of 25 states over 10 years, while removing and replacing all ash trees in these same areas at once would cost $25 billion (with another estimate putting the removal alone at $20–60 billion).
Monitoring In areas where the emerald ash borer has not yet been detected, surveys are used to monitor for new infestations. Visual surveys are used to find ash trees displaying EAB damage, and traps with colors attractive to them, such as purple or green, are hung in trees as part of a monitoring program.
Management In areas where the emerald ash borer is non-native and invasive, quarantines, infested tree removal, insecticides, and biological control are used to reduce damage to ash trees.
Quarantine and tree removal Once an infestation is detected, quarantines are typically imposed by state, or previously, 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 emerald ash borer are present in the bark and phloem. In urban areas, trees are often removed once an infestation is found to reduce emerald ash borer 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. 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.
Kentucky Extension specialists suggest selecting uncommon species to replace removed ashes in the landscape. Previous generations created
monocultures by planting ash trees in an overabundance, a factor in the extent of the devastation caused by the emerald ash borer. Favoring instead a diversity in species helps keep urban forests healthy.
University of Kentucky scientists suggest choosing
monotypic species such as the
pawpaw,
yellowwood,
Franklin tree,
Kentucky coffeetree,
Osage orange,
sourwood, and
bald cypress.
Insecticides Insecticides with active ingredients such as
azadirachtin,
imidacloprid,
emamectin benzoate, and
dinotefuran are currently used. Dinotefuran and imidacloprid 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. Damage from emerald ash borers can continue to increase over time even with insecticide applications. 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 emerald ash borer larvae one year after release, indicating that they survived the winter, but establishment varied among species and locations. == See also ==