In the summer of 2002, America came face to face with the emerald ash borer (EAB) for the first time. The larval form of this insect has been absolutely devastating to all 17 North American species of the Fraxinus genus. The natively Asian invasive species, Agrilus planipennis, was discovered in southeastern Michigan and has since spread to 30 of the 48 continental states. Most recently, in 2016, EAB was found in Delaware, Alabama and Oklahoma.

Because it’s the larvae of EAB that kills a tree from the inside out, not the adult beetle, it’s difficult to detect an infestation until the damage has already been done. EAB larvae feed on the tree’s water and nutrient transporters, dooming the tree before an infestation is even visible. The adult beetle’s tendency to fly long distances, along with accidental movement from humans, has created an infestation that’s quickly spreading throughout North America.

Economics

EAB has not only become an ecological disaster, but also an economic one. It is known to have killed over 100 million trees in the U.S. and is approaching an estimated $10 billion in damages. This is a serious threat to the very lucrative ash timber industry.

The United States Department of Agriculture estimates America’s ash trees to be worth approximately $282 billion in timber. Even if no more trees were to become infested, the damage would continue for years to come, as EAB takes one to five years to kill a tree. With EAB multiplying and spreading, the damage is projected to grow.

Researchers have found that trees that go untreated have a 99 percent mortality rate.

In many states, ash trees are a very common street tree. This means that a large majority of these costs land on the backs of homeowners and municipalities. With the average removal cost of a street tree being $5 per diameter inch, it’s very expensive to remove and replace such a large quantity of trees in a short period of time. States around the country have stopped planting ash trees until further notice. Dead ash trees will be replaced with a variety of other species. Urban foresters emphasize diversity in street tree species for maximum protection against invasive species and large-scale losses.

A large section of trees where EAB damage is widespread.

Life cycle monitoring

The majority of the forestry community agrees that EAB is here to stay. No matter what, the pest will most likely never be completely exterminated — but it’s still crucial to stabilize the population and slow the spreading of the pest.

Growing degree days (GDD) are a commonly used and reliable way to monitor pests and can be useful in planning population management efforts. GDD account for temperature, rather than the simple calendar date. Because EAB — like most insects — depends on higher temperatures to develop more quickly, GDD can be used to track its life cycle. To track GDD per day for EAB, a base temperature of 50 is subtracted from the average of the daily high and low temperatures.

An average temperature below 50 degrees will simply be zero GDD.

EAB tends to emerge, creating a D-shaped exit hole, as an adult between 450 and 550 GDD. This is useful to set a target time for population management efforts. EAB does its damage before it’s mature, so if using pesticides, it’s important to treat the tree using injection before the 450 GDD mark.

Drone detection

Because EAB is so difficult to protect against, new methods of detection and population control must be developed. With the mortality rate of infested trees so high, early detection is a vital step.

In Boulder, Colorado, drones are being used in early detection efforts. The drones, while expensive, have a variety of uses. They can assist in mapping infected areas as well as finding infested areas more quickly than a human on foot. An early sign of EAB infestation is the dieback and thinning of the tree’s upper canopy. The drones are able to keep track of and detect this issue very early in an infestation and in a timely manner.

EAB-infested ash trees along a street in Novi, Michigan.

GIS professionals can use the information from the drones to create heat maps and help forestry professionals decide where to focus their efforts.

Biocontrol

While it’s always a good idea to track and detect an invasive species, it’s useless without also finding ways to kill it. Pesticides are considered a short-term, but immediate, solution for individual trees. Alternatively, biological pest control can be used for long-term population control.

EAB has very few known predators, especially in the U.S. Fortunately, the Russian wasp Spathius galinae has been approved for release by the USDA. It works alongside three other similar wasps that have already been released. These small wasps don’t sting humans. The larvae of the parasitoid wasps feed on EAB eggs and larvae.

The idea is that each year, the population of these wasps will grow with EAB population and will keep them in check. Because this is such a slow, natural process, along with the quantity and long life cycle of ash trees in the U.S., it will be years before researchers can decide if these wasps are able to adequately manage the EAB population. This type of biocontrol can only be used in forested areas and is ineffective for urban or suburban settings.

Urban forestry control

In urban communities, the use of pesticides and traps has long been the main way to combat EAB infestation. Pesticides are injected into an infested tree through the phloem. In some instances, if the beetle has been present for too long, the phloem may have already been destroyed, making it impossible for the pesticides to travel through the tree properly. Multifunnel and prism traps are also used to detect EAB in preparation for the use of pesticides. Multifunnel traps are newer and have been found to be generally superior to prism traps. The green color of the multifunnel traps attracts more beetles and collects EAB easily and more efficiently. These traps capture the beetles without the hassle of glue and can be reused.

Even with the help of traps and pesticides, EAB continues to be devastating to residential communities. It’s very likely that the use of ash as a residential street tree is coming to an end.

One way to create an organized and useful tree inventory for the purposes of managing EAB is by using Purdue University’s EAB Cost Calculator, which can be downloaded for free online. This free, web-based calculator allows users to input tree categories based on size and class, which then projects growth in trees over time, the cost of treating trees, removing trees and replacing trees over a 25-year period. The cost calculator is driven by an EAB invasion wave model that assumes it takes eight years for EAB to kill all the ash trees in a given city after it has been detected in a given county. Essentially, you can stage your response to an EAB invasion based on the percentage of ash trees that are dying.

Outlook

Even with various population management efforts being implemented around the country, the outlook for the future of ash trees in the U.S. looks bleak. Pam Zipse, outreach coordinator of Rutgers Urban Forestry Program of NJAES, offered her projection for EAB in New Jersey: “Individual trees can be protected through ongoing chemical treatment, but we will not be able to get rid of EAB,” she says. “Researchers continue to study resistant individuals, but EAB is spreading at a fast pace. In New Jersey, it has been confirmed in 50 municipalities spanning 12 counties. I think that in New Jersey, the next couple years are going to be quite devastating.”

The rapid spread of EAB is not exclusive to New Jersey. This ecological and economic disaster has made its way across the country and is continuing to expand. Although the pest will most likely never be completely killed off, researchers will continue to develop methods to keep the EAB population in check.

Past experiences with hemlock wooly adelgid and Asian ambrosia beetle, among others, have taught us that once an invasive species establishes itself, it’s here to stay. The best we can do is manage it and control the population as effectively as possible.

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