Biodiversity Management & ForestryISSN: 2327-4417

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Research Article, J Biodivers Manage Forestry Vol: 4 Issue: 1

Use of Chemical Protection and Host Tree Reduction to Control an Emerald Ash Borer Infestation in West Virginia

Phillip A Lewis1* and Richard M Turcotte2
1USDA APHIS, Pest Survey, Detection and Exclusion Laboratory, 1398 W. Truck Rd, Buzzards Bay, MA 02542, USA
2USDA Forest Service, Forest Health Protection, Morgantown, WV 26505, USA
Corresponding author : Phillip A Lewis
USDA APHIS, Pest Survey, Detection and Exclusion Laboratory, 1398 W. Truck Rd, Buzzards Bay, MA 02542, USA
Tel: 508-563-0914, Fax: 508-563-0903
E-mail: [email protected]
Received: November 12, 2014 Accepted: December 19, 2014 Published: December 24, 2014
Citation: Lewis PA, Turcotte RM (2015) Use of Chemical Protection and Host Tree Reduction to Control an Emerald Ash Borer Infestation in West Virginia. J Biodivers Manage Forestry 4:1. doi:2327-4417.1000136

Abstract

Use of Chemical Protection and Host Tree Reduction to Control an Emerald Ash Borer Infestation in West Virginia

The destruction of the ash (Fraxinus) resource in North America by the emerald ash borer has progressed rapidly since it was first identified in 2002 in and around Detroit, Michigan. A 2004 survey estimated that 15 million ash trees had been severely impacted or killed by this pest insect and by late 2005 infested ash trees were detected in Indiana and Ohio. The emerald ash borer infestation has continued to expand, currently encompassing 24 states and various locations in southern Ontario and Québec, Canada.
 

Keywords: Agrilus planipennis; Emerald ash borer; Management; Emamectin benzoate; Tree injection

Keywords

Agrilus planipennis; Emerald ash borer; Management; Emamectin benzoate; Tree injection

Introduction

The destruction of the ash (Fraxinus spp.) resource in North America by the emerald ash borer (EAB) (Agrilus planipennis Fairmaire) (Coleoptera: Buprestidae) has progressed rapidly since it was first identified in 2002 in the Detroit metro area and in Windsor, Ontario [1]. A survey in late 2004 estimated that 15 million ash trees had been severely impacted or killed by this pest insect and by the end of 2005 areas with infested trees had been identified in numerous counties throughout Michigan as well as in several counties in Indiana and Ohio [2]. The emerald ash borer infestation has continued to spread and currently encompasses 24 states and various locations in southern Ontario and Québec [3]. Considering the rapid spread and the general lack of resistance to this insect by North American Fraxinus species, the entire ash resource in North America is now considered at risk [4].
Chemical control of EAB has had mixed success and is hampered by the difficulty in exposing the larval stage to toxic levels of pesticide as it feeds in a serpentine fashion in the phloem. In addition, there is often rapid colonization of ash by this insect in the upper tree canopy that is coupled with a delay in visible symptoms such that trees are often dead or in severe decline within a year after an infestation becomes evident [2]. Current recommendations suggest that homeowners, businesses and municipalities begin treatment of their ash trees when EAB has been identified nearby or within their county and not to wait for symptoms such as canopy dieback, D-shaped exit holes, and bark cracking in the trees to become evident [5].
One of the most promising of the chemical control options is direct trunk injection of a systemic 4% emamectin benzoate solution (TREE-age®, Arborjet, Inc., Woburn, MA). A single treatment may be effective over multiple years and achieve up to 99% or better control of the larval population. It has been used as a rescue treatment for severely declining ash trees that are of particular value [5,6]. The formulation is applied as a pressurized trunk injection, and formulation viscosity, application rate, tree size, health and growth conditions all impact uptake times and hence application costs.
The abrupt presence of large quantities of dead and dying mature ash trees has become a monetary as well as a logistical challenge for municipalities and land managers coping with the aftermath of this destructive beetle. Ash is commonly planted or present in public areas, parks and along streets and the potential liability associated with falling limbs in public areas is great. In order to have some semblance of control over the process, we are presenting a case study using an ash-phloem reduction (select tree removal) and ash tree protection strategy (systemic pesticide application) that can be employed by both urban and rural arborists and managers. Such an approach would allow ash to be removed or retained as desired during the initial phase of an EAB infestation and provide a means to maintain a healthy ash resource for as long as possible.
The initial detection of EAB in West Virginia was determined to be an isolated population located within a recreational area, perhaps having arrived with campers transporting infested firewood [7,8]. This emerald ash borer infestation possessed a number of attractive attributes that provided a unique opportunity to investigate the potential of controlling and managing this insect within a forested area. We present this as a management case study that investigated the persistence of the systemic pesticide following application and tree health parameters as well as adult and larval EAB densities within and outside of the management area.

Methods

Emerald ash borer was discovered in late 2007 at ACE Adventure Resort near Oak Hill in Fayette Co., West Virginia (37.9743° N, 81.0966° W). The find was a result of a joint state and federal effort to identify infestations using girdled trees to attract egg-laying female EAB. After the infestation was discovered in one of these “trap” trees, ash borer larvae were also detected in several trees in the immediate and surrounding area. A survey conducted within an 800 meter radius of the original trap tree identified a total of 309 ash trees (Figure 1). Of these only 25 trees displayed obvious signs of EAB infestation, including canopy decline, bark cracks, D-shaped exit holes and woodpecker foraging. A combination control strategy of ash tree host removal and chemical protection of remaining ash was initiated in 2008.
Figure 1: Aerial photograph detailing the West Virginia emerald ash borer management area (White dots indicate ash tree locations; the circle measures 800 meters in radius and is centered at the original trap tree).
Ash phloem was reduced by the removal and chipper disposal of ash trees that had a diameter at breast height (1.4 m; dbh) of about 7.6 cm or less (n=121). Twenty-five infested ash trees, ranging in size between 18 and 30 cm dbh, were felled and the bark surfaces sprayed with a solution of an imidacloprid insecticide (Merit, Bayer Environmental Science, Research Triangle Park, NC) to suppress any adults that were to emerge from those trees in 2008. The remaining ash trees (n=163) were chemically protected with emamectin benzoate, applied once by trunk injection between April-May, 2008. Treated trees were grouped by size class: sapling (5-13 cm dbh), pole (15-20 cm dbh) and saw timber (>23 cm dbh) and trunk injected, as appropriate to size, with progressively higher dosages of product (0.1, 0.2 or 0.4 g active ingredient per 2.5 cm dbh). Since total tree mass is an exponential function of tree diameter, we were seeking an optimal pesticide level in the phloem and leaf tissues of the larger trees (Table 1) .
Table 1: Size and treatment information of the West Virginia ash trees.
Trunk injections with emamectin benzoate were delivered using a hydraulic injection device (VIPER Hydraulic Device, Arborjet, Inc.) set at between 13.8 – 24.0 bar and custom fitted with a tapered 6.4 mm stainless steel injection tip and flexible hose. The higher injection pressures were necessary as trees had extremely slow uptake times, possibly due to the extended drought at the site and the cold spring temperatures which made the formulation quite viscous during the application. A single injection hole was placed at the base of the tree for every five centimeters in tree diameter and holes were drilled using a 5.6 mm brad-point bit.
A total of 31 of the treated trees (19%) were randomly chosen from each size class to track pesticide residue levels over time and to rate several health parameters of the trees over a four year period (Table 1). Between 2008 and 2011, sample trees were assessed twice each summer in early July and late August for D-shaped exit holes, woodpecker feeding and for general tree health through canopy crown ratings. Assessments were made using binoculars to examine the main stem of the ash trees for emergence holes and woodpecker damage by standing in one place and slowly sweeping the view from a point 10 m above the ground down to the base of the tree. The vantage point for each tree was fixed in order to achieve consistency between observations over time [7].
Foliage samples were collected to quantify pesticide residue using either a pole pruner or a Big Shot® line launcher (Sherrill Tree, Greensboro, NC) during the tree health assessments. Leaves were taken from four to eight mid-canopy branches and the four cardinal directions and kept frozen until analysis [6]. Leaves from each sample were separated from the stems and petioles and placed within paper grocery bags at room temperature for several days until dry and brittle. Dried leaves were compressed and broken using a gloved hand and then placed into a 1.9L stainless steel vessel atop a commercial blender (Waring® Model 51BL31, Waring Inc., USA). The blender was run on high speed for approximately 30 seconds to homogenize the sample and break up the leaf tissue into a fine powder. Vessels were thoroughly cleaned with detergent and an alcohol rinse after each use to avoid cross-contamination between samples.
Ground leaf samples of 0.5 g were weighed into a 50 mL plastic centrifuge tube and extracted in 10 mL of pure methanol for 3 hours on a table-top shaker. Sample tubes were spun down in a high-speed centrifuge at 6,000 rpm for 10 minutes and the supernatant was used to quantify emamectin benzoate using a commercially available Enzyme Linked Immuno-sorbent Assay (ELISA) kit (SmartAssay kit #3100176052, Horiba, Ltd., Kyoto, Japan). The assay kit is marketed for the determination of residues in aqueous samples, so a slight modification was made through a 20-fold dilution of the methanol extractions in order to more accurately quantify chemical residue.
Insect traps (n=40) were set up annually within the 800 m radius management area and monitored for EAB adults between 2008 and 2011. All traps were EAB program panel traps, green in color and baited with Phoebe oil (2008) or Phoebe oil + Manuka oil lures (2009-2011) [9]. Traps were placed within ash trees by June and were lowered and visually checked for adult EAB twice per season in July and August; lures were changed during the check in July.
Post-treatment EAB population assessment
In October of 2011, four trees from each of the three size classes were selected, felled and debarked to characterize the EAB population within the treated trees. Four control trees were selected from just outside of the management area as positive controls by which to draw comparisons. Trees were felled and a visual check for exit holes was made of the entire trunk, which was then debarked up to 10 m, with alternate meter sections debarked beyond that. Visual checks were made on all branches >5 cm diameter for bark cracking and exit holes. For the control trees the infestation was so heavy that only a visual check of the main trunk and on branches >5 cm diameter was performed in order to quantify the numerous EAB exit holes and woodpecker attacks.

Statistical Analysis

A randomized complete block design was used to evaluate the data generated by this study. Data were tested for normality using PROC UNIVARIATE. The data were modeled using a gamma distribution, with injection time and tree size as fixed effects in the model [10]. To account for dependence between years we used the autoregressive order 1 heterogeneous covariance structures. Using this distribution and log link allowed all the assumptions of residual analysis to be met. We tested for difference among pesticide residual by application rate, year and year by application rate interaction along with injection time, tree size and injection time by tree size interaction with the gamma distribution with log link in PROC GLIMMIX [11]. Degrees of freedom were adjusted using the Kenward–Rogers method, in a repeated measure framework where year was the repeated measures variable, followed by the Tukey-Kramer least significant means test [12] when significant differences occurred. All analyses were conducted at the P<0.05 level of significance using SAS version 9.1.3.

Results

The average time needed for the trees to uptake the TREE-age® formulation of emamectin benzoate significantly increased with tree size class, from 7.2, 13.6, to 28.3 minutes per tree as both tree size and product dosage increased (F5,139 =38.94, P<0.001). When tree dbh is divided by injection time, the fastest uptake times were seen in trees 33 cm dbh and greater (n=36; 0.74 minutes/cm dbh) as compared to the 23-25 cm trees which had the slowest uptake times (n=15; 1.2 minutes/cm dbh) (T-test; P<0.005). The greater transport capability of the larger diameter trees was perhaps a factor that allowed for better absorption of the tree injections. Approximate treatment time in minutes for trees treated with emamectin benzoate in this study were typically a value that was double or triple the dbh measurement of the tree.
The accuracy of our ELISA method was confirmed through the comparison of split samples (n=62) analyzed using an LC/MS analytical method by the pesticide manufacturer (Syngenta Crop Protection, Inc., Greensboro, NC). Average foliar residuals by month and year did not differ significantly between the two sample periods of a particular year and so were combined and averaged for analysis (Table 2). Emamectin residue differed significantly by treatment size class (F2, 34.9 = 19.18, P<0.001), year (F3, 47.5 = 171.98, P<0.001) and size class by year (F6, 55.8 = 5.44, P<0.001). Statistical differences of residue amounts between the size class groupings were only observed within the 2008 data. Mean (±SD) pesticide residuals were significantly different only between the smallest and largest size classes; residual pesticide values ranged from 2.98 (2.66), 5.45 (5.63) and 10.15 (11.89) μg/g, respectively. Chemical residue detected in ash foliage dropped off rapidly. Values in 2009 were 4-5% of the amount seen in the previous year and continued to decline in 2010 to between 2-3% of the 2008 values. In 2011 there was only a single sample from August that had a positive value (0.32 μg/g), the rest of the samples were below the detection limit of the pesticide assay (Table 2).
Table 2: Average ± SD of pesticide residuals (μg/g) by sample date and year for composite leaf samples collected in 2008 to 2011 from Fraxinus sp. trees trunk injected with emamectin benzoate.
The tree health assessments that were conducted soon after treatments were completed in August of 2008 found that 29.0% of the trees showed evidence of canopy die back, and this group of trees had an overall dieback rating of 11.6%. There were between 1 and 7 EAB exit holes per tree and 11 total woodpecker attacks, confirmation that a light infestation was present within these ash trees. The final assessment that was made in 2011 found no evidence of woodpecker damage, no visible EAB exit holes and stable, healthy tree canopies in this group of trees.
Traps set up in the management area did not catch any EAB adults in 2008 and 2010. One trap, which happened to be placed on the edge of the management area, caught 29 beetles in 2009. The final year of trapping in 2011 indicates that the insect pressure was beginning to build up throughout the management area, with 12 of 40 traps catching a total of 27 beetles. In tandem with the above trapping, several hundred traps were monitoring the EAB infestation within a broader 5 mile radius circle outside of the management area between 2008 and 2010. There were 2 positive traps in 2008 and 27 and 36 positive traps the following two years, which helped to identify heavily infested areas, one of which was just north of the management area.
EAB Population in the Management Area
For the treated trees, a total of 129 m2 of log area was processed. No exit holes were found and there was evidence of 7 woodpecker attacks (<0.1/m2). All of the bark scraping of those logs revealed only 2 galleries (one a year old) from one of the trees and no live EAB larvae were found. In contrast, for the 36 m2 of log area processed for the control trees there were 392 EAB exit holes identified (11/m2) and there were 2,232 instances of woodpecker damage (62/m2).

Discussion

The chemical treatment of ash trees is an option that many towns and municipalities are attempting as an alternative to the devastation that EAB leaves in its wake. Cost estimates of ash treatment scenarios modeled in an urban setting demonstrate that tree treatments can have a long-term cost advantage over a removal and replacement approach [13]. An estimate of the costs to conduct the treatments as outlined in this case study using current rates of commercial labor and chemical cost range from $120-$160 per tree, so a substantial investment and commitment by landowners and town managers would be needed with this approach.
Although untreated ash trees outside of the management area were not being monitored, we did observe the rapid mortality of a single untreated 46 cm dbh ash tree just inside the management boundary that had escaped our notice during the initial tree survey. This tree exhibited crown-dieback symptoms in 2009 and was completely dead the following year as a result of intensive attack from EAB. In contrast, several of the treated trees located within a few hundred meters of this tree remained healthy and continued to have good canopy cover until the last assessment in 2011.
The data presented here demonstrate that it is possible to maintain the health of a group of ash trees up to 4 years post-treatment in the midst of building EAB pest pressure, using a single application of emamectin benzoate. Even though chemical residue in the trees declined rapidly within a year following treatment, these trees were protected and healthy canopies were present within the treated trees between the spring of 2008 until the tree harvest and infestation assessment in 2011. Our attempt of phloem reduction (removal of smaller diameter ash) and phloem protection (chemical treatment) of this ash resource on an area-wide basis, appeared to impede the ability of EAB to reinfest the management area. The finding of no live larvae amongst the 12 trees that were cut down and intensively sampled 4 years after treatment is indicative of a gradual reinfestation from infested trees on the periphery into our low-density ash management area. This is also borne out by the insect trapping data that did not detect the widespread presence of adult EAB until the final year. While this study observed extended protection of ash trees from EAB-induced mortality with a single chemical treatment, management stands containing a high ash density and a more severe EAB infestation may require more frequent chemical treatments to maintain tree health.
The expansion of the EAB infestation in West Virginia has been quite rapid from the initial find at our study site in 2007. It is currently confirmed in 30 counties, 19 of which were added within the 2 year span of 2011 and 2012 (Table 3) and the entire state is now under a federal quarantine.
Table 3: History of the EAB infestation in West Virginia.
Management of the ash resource is a necessary component in dealing with EAB infestations and integrated approaches using currently available tools can provide land managers with options to remove and retain trees as desired. Thinning, timber stand improvement and crop tree techniques used in concert with EAB exclusion via area-wide chemical treatment can be used to control the mortality trajectory of EAB, manage canopy disturbance and tree fall if efforts are made at the early stages of an EAB infestation.

Acknowledgments

This project would not have been possible without the valuable assistance of the ACE Adventure Center, Al Sawyer, Dave Cowan, Erin O’Brien, D-Jay Laffoon, Sarah Bello, Rachel Braud, Rachel Messineo and Jason Watkins (APHIS); Danielle Martin, Will Harris, Sam Forbeck, Terry Burhans Jr., Nathan Sites, Chelsea Gibson, and Dan Snider (FS). The emamectin benzoate formulation used in this study was graciously donated by Syngenta Crop Protection, Inc. The authors are grateful to several anonymous reviewers who provided valuable critique to improve this manuscript.

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