Strategies for managing emerald ash borer introductions within Minnesota - 2009

The emerald ash borer threat in Minnesota
Forest inventory and analysis data shows that Minnesota's forests include approximately 937 million ash trees. In addition, an estimated 3 million ash trees are planted throughout urban and rural landscapes. In many Minnesota communities ash is the most common tree found along streets, in yards and in parks. In Minnesota's forests three species are locally abundant; green ash, white ash, and black ash. Though found on upland sites, ash trees are more likely to be encountered in riparian areas where ash makes up nearly 50% of the lowland hardwood forest type. Minnesota has more ash to lose than any other state with the possible exception of Maine.

The arrival of emerald ash borer (EAB) into Minnesota is inevitable; in fact, it could already be present but not yet detected. Recent findings of EAB in Wisconsin and Missouri were from sites where the beetle was apparently introduced several years ago and locally established populations were not found until dead trees were observed 3-5 years after initial introduction.

Emerald ash borer spreads naturally at a slow pace. Movement over long distances occurs through the movement of firewood, logs and nursery stock. Surveys have found that transporting firewood from home to the cabin or campsite is a common practice. Much of this wood comes from a dead or dying tree in a backyard setting. If the dead tree was an ash and EAB killed it, larvae and adults could be under the bark, ready to emerge. In addition to firewood, logs transported for wood products and nursery stock also have been documented in EAB movement.

This document is intended to address the EAB threat after it is found in Minnesota. Strategies prior to its discovery include assessment, prevention, and early detection, all of which are detailed in the existing Emerald Ash Borer Readiness Plan (PDF: 530 KB / 23 pages). Likewise, the Emerald Ash Borer Response Plan outlines the activities immediately after the discovery of an infestation, taking into account survey, regulatory, investigative, and mitigation procedures. All of the management strategies discussed here takes place after the infestation has been defined and are assumed to be coupled with ongoing survey, outreach and regulatory components.

What if no management is done?
Under a no-management scenario, emerald ash borer is permitted to multiply and spread within a specified area or region without human intervention. The consensus, based on the following research findings and survey work, is that most ash trees will be eliminated from the landscape.

• There is little or no evidence of inherent resistance or tolerance to EAB attack in any species of native ash in the eastern U.S.
• There is little or no evidence of any environmentally-based resistance due to soil types, soil moisture or nutrient levels, light, heat, or topographic position.
• There is little or no evidence of any associational resistance due to stand density, tree age mixtures, or tree species mixtures.

The spread rate under a no-management scenario is expected to be 18 miles per year. This is based on evidence from southeast Michigan, where the initial introduction of EAB apparently occurred in the early 1990's. In this area, initial spread rates were estimated at about four miles per year in the late 1990's. That spread rate eventually increased to approximately 18 miles per year by 2001 and 2002 (Siegert et al. 2008).

What if EAB management strategies are implemented?
Under this scenario, EAB will still likely spread across the range of ash in Minnesota and largely eliminate the vast majority of ash trees from our forests and urban landscapes. However, that process would likely be much slower, hopefully at or below the four miles per year spread rates initially reported for Michigan.

Researchers and managers continue to develop new and more effective tools for managing EAB. Delaying the spread and population expansion of the insect would allow more time to develop new and perhaps very effective management tools. In addition, Minnesota communities, homeowners and woodland managers would have time to implement practices that would buffer the effects of the sudden loss of large numbers of ash trees.

What strategies are available in Minnesota?
Because EAB populations are not yet present in the state the following strategies could be effectively utilized:

• Eradication
• Integrated management to slow ash mortality

If an EAB introduction can be found quickly, before populations have dispersed widely, the possibility exists for a successful eradication strategy. However, eradication success has been very limited, and a more likely management scenario would be one that suppresses EAB population growth to the point where tree mortality spreads slowly. A number of management tools or options now exist that can be used in an integrated management strategy to slow ash mortality providing communities and woodland managers time to react to this pest.

Which strategy will be selected?
Selecting an appropriate management strategy will be based upon the following.

• Age and size of infestation. Recent research has provided a reliable method for determining the age of an infestation. Infestations that have been present for several years have had the opportunity to become established and spread and are much more difficult to delimit (locating the borders of an infestation). Large infestations covering a range of habitats and landownership are typically more difficult to manage aggressively.
• Ash density and distribution within and adjacent to an infestation. This information, obtained from a combination of existing resource maps, databases and newly collected data will give decision-makers insight into the amount of resource at risk in the local area. Areas with higher densities of ash will provide greater challenges for implementing management.
• Confidence in delimitation data. Delimitation should employ the best techniques for detecting infested trees. A high level of confidence in the delimitation data allows managers a better chance to predict the outcome of a management action.
• Proximity to other infestations. Examining how each infestation fits in the bigger picture of a larger infested area will help guide management decisions strategically, using resources more effectively. The proximity to existing infestations affects the likelihood for success of certain management strategies.
• Risk of reintroduction. Areas where reintroduction is highly probable will require diligent attention to movement of infested materials into the area and monitoring of the area for new infestations. If the likelihood of reintroduction is great, the effort made to manage EAB in that location may be altered accordingly.
• Risk of artificial spread from a location. Knowledge of the level of risk of artificial spread and how it may occur will help determine the likelihood of successfully managing an infestation as well as determining management strategies focusing on minimizing artificial spread. If a site is deemed a high risk for further spread then managers may wish to focus additional efforts at reducing that risk.
• Presence of natural dispersal corridors. The presence of natural dispersal corridors could provide an additional challenge and need to focus on options for limiting spread through this conduit. Further, the presence of dispersal corridors out of an area may increase the likelihood of certain management activities in that area so that dispersal can be reduced. In contrast a site that is very isolated with limited dispersal corridors may be managed differently since spread may be less likely.

The following factors will also be evaluated when choosing a management strategy: potential environmental impact, land ownership, land use or classification, cost of implementing management, availability of resources to carry out management, and sociological impact.

Eradication Strategy:

Overall goal: Locate any existing or newly-introduced EAB populations as quickly as possible and eliminate those populations if feasible.

Rationale: It is possible to eradicate isolated, relatively small EAB population centers. If eradication is successful, a local reproducing population is eliminated and the risk of spread out of that area will no longer exist. An eradication treatment in Brimley State Park in Michigan's Upper Peninsula appears to have been successful. The infestation was found early and there were not many ash trees in the Brimley area.

Tools and Options

A. Complete host removal
This is the complete elimination of a localized population of EAB by removing all existing infested trees and removing all potential host trees that could harbor undetected beetles. This is generally accomplished by removal of all ash trees within a designated radius of known EAB infested trees, generally one-half mile, of known infested trees. Seemingly healthy ash trees are removed because they may be infested but without obvious symptoms. Felled trees are chipped and burned. Stumps have been removed and ground up, or treated with an herbicide to prevent sprouting. Subsequent monitoring of eradication sites has occurred for three years to determine the success of the eradication effort and the need for follow-up. Eradication was a favored option for "outlier sites" separated by some distance from "generally infested sites".

Considerations:

• Studies conducted in 2003 and 2004 provide the basis for this eradication strategy. Each of the studies included a known point source of infested firewood or nursery stock. Sampling showed that most larvae were found close to the source of the infestation. Further, infested trees were within less than a half-mile radius of the source (McCullough et al. 2004).
• Unfortunately, few eradication projects have been deemed successful. One of the reasons for this is that it is very difficult to adequately delimit an infestation and eradication attempts have fallen short of the outer limits of infestations. Survey methods are labor-intensive, expensive and unpopular.

B. Combination of host removal and various tools and options discussed under the "Integrated EAB Management Strategies" below
This option would have the same goal as complete host removal but it would be accomplished in a different manner. It would entail the complete elimination of a localized population of EAB by removing all existing infested trees and utilizing a variety of treatment tactics to concentrate and eliminate the remaining beetle population.

Considerations:

• All known infested ash trees are cut and destroyed.
• Remaining ash trees within the half-mile area around the known infested trees may or may not be cut and destroyed.
• A variety of tactics are considered for the remaining ash trees, these are described in the following section under "phloem reduction, "sinks", and "insecticide treated trees".
• EAB that remain in the area after removal of the known infested trees should be concentrated and killed, e.g. through the use of "sinks", insecticide treatment, and tree removal, with the end result being eradication of the local population.
• There are no management examples for this method for eradication of EAB, but similar tactics were successful in Asian long-horned beetle projects in the Chicago area.

Integrated EAB Management Strategy, also referred to as a Slow Ash Mortality Strategy (SLAM):

Overall goal: Develop an integrated, long-term strategy to slow the onset and progression of widespread ash mortality following an introduction of EAB.

Rationale: Research has shown that the rate at which ash tree mortality advances is related to EAB density (Siegert et al. 2008). As outlier populations build and coalesce, the area of dead, dying and declining ash trees increases dramatically. Therefore, treatment tools and options that concentrate and eliminate EAB and reduce its food supply should stunt population growth and slow the expansion of tree mortality.

Tools and Options

A. Suppression of EAB populations
Actions, both direct and indirect, can be taken to reduce existing EAB populations and to minimize future population expansion. These actions can be used alone or in combination. Direct actions against EAB populations are possible by destroying EAB life stages, either in infested trees or in trees treated with insecticides. Indirect actions revolve around the reduction in the amount of food available for EAB population expansion. EAB larvae feed and develop on ash phloem.

Direct reduction of EAB populations

1. Removal of trees known to be infested: Prompt removal of infested trees, prior to adult emergence should be a priority in SLAM project areas.

Considerations:

• Infested trees will need to be removed, or treated so that developing EAB progeny are not allowed to emerge. This can entail chipping, complete bark removal, or burning.

2. Sinks – systematic grids of girdled trees: Girdled ash trees are more attractive to adult EAB than healthy ash trees (McCullough et al. 2009). Female beetles preferentially oviposit on girdled trees unless there are other stressed trees nearby. If girdled trees are removed prior to adult emergence, a large component of future adults can be eliminated. Girdled trees deployed in a systematic survey grid can concurrently serve as sinks for the subsequent generation of EAB.

Considerations :

• Deploying sink trees in a grid pattern can provide valuable survey information.
• As EAB densities build in an area, the effectiveness of girdled trees to function as traps or sinks is diminished. While the EAB density at which this will occur is currently unknown, pilot projects and related projects already in progress will help to define this threshold.

3. Sinks - clusters of girdled trees: In addition to the systematic grid of girdled trees, clusters of girdled trees can be established within the project area.

Considerations :

• A cluster of three to six freshly girdled ash trees should generate a substantial plume of volatile compounds that are attractive to adult beetles. Trees should be girdled several weeks before EAB might be expected to emerge and fly in any give area. Visiting adult females should lay a substantial component of their eggs on these trees. Destroying these trees in fall or winter should eliminate a considerable proportion of the larvae produced locally.
• Placement of girdled tree clusters can be determined using maps and on-site visits to assess factors such as ash density, distribution and habitat heterogeneity.

4. Insecticide treatments: Insecticide options are available for managing EAB (McCullough et al. 2008). In some situations, insecticide treatments can serve multiple purposes. First, they can create toxic trees that kill dispersing EAB adults, and with some products, kill eggs and larvae. Second some products can protect high-value trees from invading EAB.

Considerations :

• Treating ash trees for EAB control can be expensive and results vary depending upon a number of variables including the health of the tree, prior infestation status of a tree, the insecticide product selected, the application method and the local EAB population pressure.
• Treated trees may survive thus preserving a local ash resource. However, it may require regular annual applications to protect ash trees over a long period of time.
• Treating sink trees (see 2 and 3 above) with insecticides prior to girdling could create "lethal trap trees" that could kill EAB adults attracted to the trees. Depending upon the insecticide selected, larval mortality may also be possible in treated trees.

Indirect reduction of EAB populations

5. Phloem reduction: Phloem reduction revolves around constraining EAB density and spread by reducing the amount of food available to EAB. Less food should result in lower beetle production and fewer offspring (McCullough and Siegert 2007). Phloem reduction can be accomplished in many ways and does not imply complete removal of the ash resource in an area. Studies have found that a large proportion of phloem in a stand is often located in a relatively small number of large diameter trees; removal of a few large trees can sometimes eliminate much of the available EAB food resource. Phloem reduction models that can be used to compare and contrast different management approaches are available for land managers.

5a. Cut and leave. In this scenario, selected ash trees are felled and left on site. Cut and leave projects are often done in areas where access is limited or where site disturbance is a concern (wet sites that would be impacted by heavy equipment), or when trees are not merchantable. Managers can select trees based on size or other factors with the goal of reducing phloem levels to some designated amount.

Considerations:

• Cutting infested trees is unlikely to prevent EAB larvae already present from completing development. However, cut logs and branches would not be used as further host material.
• If possible, cut-and-leave projects would work best in un-infested trees. Therefore, cut-and-leave projects may prove most useful on the outer edges of project areas.
• Managers do not need to be overly concerned about stump sprouts following cutting. The sprouts will have a very minimal phloem resource.

5b. Commercial timber sales can be used if feasible.

Considerations:

• Timber sales can generate funds for private or public entities.
• EAB-infested areas will be regulated (under quarantine), therefore log movement will need to be coordinated with regulatory officials.
• Providing assistance to landowners in sale layout, contract assistance, site restoration and other timber harvest related activities could increase the amount of phloem removal accomplished in many areas.

5c. Noncommercial tree removal may be beneficial to accomplish phloem removal in some areas. This can be very similar to cut-and-leave projects described above. However, in this scenario the cut trees would be removed for safety or aesthetic reasons. This might occur along roads, trails, in parks or in landscape situations.

Ash trees are often common along road or trail rights-of-way. In addition to removing phloem, right-of-way cutting may also eliminate EAB dispersal corridors.

Considerations:

• Transportation corridors, (i.e., road corridors) may require tree removal after cutting in order to avoid hazards related to logs in the right-of-way or conflicts with private property.

5d. Insecticide-treated trees can serve as a form of phloem removal. If a tree's phloem is toxic to EAB larvae then it is unavailable as a potential food source for EAB.

Considerations:

• Phloem reduction only occurs on trees treated with insecticide products that prevent larval development.
• Insecticides will need to be reapplied in order to keep phloem unavailable to EAB.

5e. Herbicide treated trees can serve as a form of phloem removal. Herbicides can be used to kill ash trees thus removing them as a potential food source for EAB.

Considerations:

• Less damage to soil; faster and more efficient than felling trees
• Can be used where water or topography limit access for tree felling.

References

McCullough DG, Poland T, Cappaert D. 2004. Dispersal of emerald ash borer: a case study at Tipton, Michigan, pp. 6-7. In Mastro V, Reardon R (compilers). Emerald ash borer research and technology development meeting. Port Huron, MI 30 Sept.-1 Oct. 2003. USDA Forest Service, FHTET-2004-02.

McCullough DG, Poland TM, and Cappaert D. 2009. Emerald ash borer (Agrilus planipennis) attraction to trap trees stressed by girdling, herbicide, or wounding. Canadian Journal of Forest Research (submitted).

McCullough DG, Cappaert DL, Poland TM, Anulewicz AC, Lewis P, and Molongoski J. 2008. Evaluation of non-invasive trunk sprays and trunk-injected emamectin benzoate, pp. 40-42. In Mastro V, Lance D, Reardon R, Parra G (compilers) Emerald ash borer research and technology development meeting, 23-24 Oct 2007, Pittsburgh, PA. USDA Forest Service FHTET-2008-07.

McCullough DG, Siegert NW. 2007. Estimating potential emerald ash borer (Coleoptera: Buprestidae) populations using ash inventory data. Journal of Economic Entomology. 100(5): 1577-1586.

Siegert NW, McCullough DG, Liebhold AM, Telewski FW. 2008. Dendrochronological reconstruction of the establishment and spread of emerald ash borer, pp. 4-5. In Mastro V, Lance D, Reardon R, Parra G (compilers) Emerald ash borer research and technology development meeting, 23-24 Oct 2007, Pittsburgh, PA. USDA Forest Service FHTET-2008-07.

Proceedings of EAB Research and Technology Development Meetings are available at:

• Emerald Ash Borer Research Reports
• Emerald Ash Borer Research Publications