International travel and commerce have made the world a more porous place. One of the unintended consequences of international trade is the inadvertent translocation of non-native species of animals, plants and micro-organisms. The U.S. Forest Service estimates there are 4,300 non-native species in the U.S.
Approximately 15 percent of those are considered invasive. The criteria for a species to be considered an “invasive species” are as follows:
“They are non-native to the ecosystem that they are occupying,
“Their existence in that ecosystem causes or is likely to cause harm to the economy, environment, or human health.
“If left unchecked, invasive species can threaten native species, ecosystem services, recreation, and property values. Invasive species may be plants, animals, or microorganisms.”
Such is Agrilus planipennis Fairmaire – common name: Emerald Ash Borer (EAB) an insect in the beetle family with origins in northeast Asia. It arrived in the U.S. in the 1990’s. EAB most likely arrived Stateside in packing materials utilized in cargo containers from that area. EAB was first detected in the U.S. in the Detroit area in 2002, it has spread to 26 states and 2 Canadian Provinces.
The National Invasive Species Information Center, data analytics estimates put the economic cost associated with invasive species damage and control efforts at more than $100 billion a year in the United States.
THE BALANCE DISRUPTED
Regionally, native plants, animals and micro-organisms live in balance where there are not too many predators or too much prey. A consistent theme of nature, namely - the weak get eaten, helps maintain that balance. This “Law of the Jungle” holds true from the apex predators down to microscopic organisms.
Not surprising, Ash trees have natural enemies. There is a native species of Emerald Ash Borer. It fulfills a niche in the ecosystem by feeding on unhealthy, declining Ash trees.
When the non-native EAB comes into an area it kills Ash trees. Native North American Ash trees lack the defenses to repel the invasive predator. EAB does not discriminate, infesting and feeding on all species of North American Ash. Healthy and weak, large and small, all are subject to being attacked and killed by EAB.
Like many insects, EAB has four life stages - adult, egg, larvae and pupae. Adults feed on the leaves of their host plant. This feeding does little damage. Adult EAB mate and deposit eggs in the crevices and fissures of tree bark. When the EAB egg hatches as a larva, it immediate bores inward to the living, vital, nutrient rich tissue of the phloem and begins feeding. It is this feeding and disruption of nutrient flow that kills Ash trees.
Initially, the numbers of feeding larvae are small and minimally impact the vascular system of the host. Over a period of a couple of years, the numbers of feeding EAB larvae increase exponentially. It is at the larvae stage where the damage is done.
The larvae feed through the warm months of late spring, summer and early autumn. As temperatures cool, the feeding slows until the larvae cease feeding and enter the pupae stage (think cocoon).
The pupae undergo metamorphosis and emerge from the feeding chamber as an adult. The exit hole is approximately the size of a BB and has a distinctive D shape.
With Ash mortality at virtually 100 percent, Emerald Ash Borer will change the composition and complexion of our urban forests and woodlands. In its wake are hundreds of millions of dead Ash trees. In its future are the remaining Ash trees that make up the Fraxinus (spp) family of trees in North America.
Urban forests consisting of private and public trees as well as woodlands and riparian corridors will be altered dramatically.
Does this mean we resign ourselves to let nature run its course? Are there no options for municipalities and private property owners but to accept the loss and the cost? Fortunately, lessons have been learned. Knowledge has increased and our understanding of Best Practices in managing an EAB outbreak in an area have improved as the invasion has spread to new areas.
DETECTION vs PRESENCE
In areas of the country where EAB has completely decimated Ash populations, city managers and urban foresters frequently use phrases such as, ‘like trying to stop a freight train’ or ‘it hit like a tsunami’.
How can that be? Commonly EAB will be in an area for a couple of years before it is detected. By the time it is detected, the EAB populations are growing exponentially. From initial discovery, i.e. “Detection” until a tree’s demise – add 3 more years. Total 5 years. Some trees last longer but on average it only takes 5 - 6 years to kill an Ash tree. A tree may not be symptomatic in the first 2 years.
Ash trees were a popular choice for municipal street tree replacement in the 1970s when Dutch Elm Disease killed vast populations of Elm trees. Elm trees were popular choice as a street tree. Hopefully this time around we have learned to diversify and should not have too high a percentage of any one species.
EAB RESPONSE PLANNING
A municipal EAB Response Plan should be written and periodically updated to incorporate the most current knowledge and science. Prudent canopy conservation, public safety and fiscal responsibility should be the guiding principles for the management of EAB in urban areas.
Failing to plan is planning to fail. A community with Ash trees as a component of its urban forest should adopt an official EAB Response Plan or management plan. It should be noted, ‘taking them down when they die’ is not a plan. The math is against such an approach.
EAB killed Ash trees die in large numbers over a relatively short period of time. Resources (money, labor and equipment) likely will not be able to keep up. Without planning, a municipality will fall behind the curve in keeping pace with tree removals. Dead Ash wood decays very quickly, losing its tensile strength and subsequently the ability to remain standing.
Municipalities are not immune from liability in the event a municipal Ash tree fails and strikes “a target”. Targets are persons, property or activities that could be injured, damaged, or disrupted by a tree.
What portion of the liability would a municipality’s insurance company assume if a tree or tree part failed and that tree failure was is deemed forseeable and resulted in personal injury or death?
As arboricultural science advances so too must definitions become more refined. The following definition has been recommended by Natural Path Urban Forestry Consultants, a nationally recognized consulting firm. “High-Risk Trees are trees (or tree parts) that have a high potential for failing and a high potential to strike a foreseeable target resulting in significant consequences”.
OPTIONS BEYOND THE CHAINSAW
Much has been learned in the intervening 16 years since EAB was detected in the U.S. Highly effective systemic insecticides have been developed. These products are typically classified as a Restricted Use Pesticide (RUP). This means the purchasing and supervised application of them requires State certification and licensing.
For private property owners and others responsible for high value Ash trees, treatment may be a viable option. Before choosing to treat Ash trees, it is recommended the treatment candidates be inspected by a Certified Arborist. Each tree’s condition should be carefully reviewed, inspecting for mechanical flaws, decay, disease and vigor.
The U.S. Forest Service began seeking Biological Controls when it became evident that containment and eradication (the first option) of EAB control was not possible. Biological Control seeks to discover and utilize the native predators of an invasive species to help control or suppress invasive species.
Classical Biological Control of an invasive species involves traveling to the native habitat and range of the invasive species. Biocontrol efforts seek to identify the natural enemies of the invasive species. The goal is to find the natural predators and use them to attack the invasive species.
Once the invasive species’ predators have been identified, rigorous testing begins. Among concerns to be tested is the risk of collateral damage or predation of native, non-target beneficial species.
In 2007, permits were issued by APHIS - Animal and Plant Health Inspection Service, a division of USDA – Forest Service for release of 3 species of wasps as biological agents to parasitize EAB.
These EAB biocontrol agents consist of an egg parasitoid wasp and two larval parasitoids. In each case, the wasp uses its “scape” or ovipositor to deliver its eggs either on the eggs of EAB within the crevices and fissures of the bark or below the bark to the feeding larvae. In 2015, another EAB larval parasitoid from the Russian Far East was approved for release because one of the parasitoids did not establish in northern regions.
When the wasp eggs hatch their first meal is the host EAB larvae. The young wasps then emerge as adults, mate and repeat the process of where it ovi-posit its eggs unto the feeding EAB larva.
Another of the sanctioned parasitoid wasps deposits its eggs on the unhatched EAB eggs in the bark crevices. As the parasitoid wasp’s eggs hatch, their initial feeding is to consume the EAB’s egg.
Unfortunately, biological control by its very nature is a long term proposition with the ultimate quantifiable efficacy of the controls unknown. What is known is these wasps are “species specific” in their choice of host for depositing their eggs with no collateral predation to beneficial insects.
What may be surmised regarding biocontrol of EAB:
EAB populations are suppressed when its native predators are present in sufficient numbers.
Incorporating bio-controls may keep Ash trees viable longer. Using parasitoids to suppress EAB populations may buy time for communities. Extra time will allow them to spread the costs of tree removals over a longer period of time.
Riparian corridors (the trees along rivers and streams) are for the most part uninterrupted avenues where EAB may move into new territory. Bio-controls released in these riparian corridors where Ash trees make valuable ecological benefits may slow the spread of EAB.
THERE’S AN APP FOR THAT
Quantifying those tangible benefits from trees is now fairly simple and straight forward thanks to the U.S. Forest Service. They have developed a suite of software applications called i-Tree.
The following excerpt is taken from the i-Tree web page. “i-Tree is a state-of-the-art, peer-reviewed software suite from the USDA Forest Service that provides urban and rural forestry analysis and benefits assessment tools.”
i-Tree apps are tools to factor in and quantify in monetary terms, the unique benefits derived from trees. This suite of software tools covers from micro to macro parcels of land.
The point is, the case should not be closed and the decision final on removing all Ash trees cuz … you know … EAB. Crunching the numbers will help decision-makers make prudent decisions.
The costs and benefits of keeping and protecting high quality Ash trees ought to be modeled and quantified before making a decision to keep or cut down. The costs represented by injection of a systemic insecticide every two years versus the larger upfront expense of cutting down the tree(s) and purchasing a replacement tree(s). The bottom line: It will probably take 20 years to replace a 20 year old Ash.
There is a consensus among scientists and other tree research specialists that conservation makes economic sense. Recent cost-benefit analysis indicates that protecting healthy trees with one of the several systemic insecticides can be more cost-effective than tree removal and replacement.
To date, EAB has not been detected in Lincoln. Is it here? Dr. Leah Bauer, USFS Senior Entomologist - Northern Research Station, believes they are already present in the Capitol city. The difficulty once again is detection.
The city of Lincoln’s plan it to, “ … replace EAB-threatened public ash trees on a one-to-one basis …” Since all non-treated Ash tree are threatened, in essence, the plan calls for the removal of all of the city’s public Ash trees – some 14,000 of them.
In addition to public trees, it is estimated there are approximately 40,000 privately owned Ash trees in Lincoln. Residential and commercial property owners and managers with Ash trees will also need to decide how they will deal with the impending destruction as well.
Once again, waiting till they die is not much of a plan. Supply and demand for tree removal may dramatically drive up the price of tree removal and bring in less reputable companies to the area.
SUMMARY AND CONCLUSIONS
Emerald Ash Borer has been detected in Nebraska (Omaha 2016). The invasion is unstoppable but it can be managed. Recent research strongly supports treatment as an integral and cost-effective part of managing the impact of EAB.
For private property owners wishing to save their Ash trees, the first step is an inspection and evaluation of the trees in question. A viable candidate for treatment should be free of structural flaws, crown dieback, decay and the tree should be located in a suitable space.
At every phase of the invasion cycle it is critical for municipalities to have the support of the community. The involvement and understanding of the citizenry will be necessary when budgets bulge and what appears to be a healthy trees are taken down. The fight should be with EAB not the citizens.
As mentioned earlier, a written EAB Response Plan will be the roadmap through the crisis. The first item is an inventory. It is necessary to know what you have before you can manage it. Be proactive, count the trees, set the budget. The essential attributes of an inventory are the trees location, size and condition.
The city forester or municipal arborist should lead the charge. If your community does not have one and it is not in the budget to put one on staff full time then hire a consulting arborist. Check credentials. Bring in someone knowledgeable with background and training in EAB.
Utilizing Bio-controls ought to be considered when cities need to spread the expense of removals over longer periods of time due to budget and other resource constraints. It is preferable to let the parasitoid wasps help suppress the borer populations than leave EAB unchecked.
“Be Prepared” is the motto of the Boy Scouts. It should also be the mind-set of stakeholders in the battle with Emerald Ash Borer. Preparedness should include educating ourselves on the invasion wave and implementing a strategy that will mitigate liability and replenish our urban forests with greater diversity of tree species.
© 2018 Joseph Mangan All rights reserved
Joe Mangan is the owner of The Tree Consultants and a certified arborist.