Emerald Ash Borer: Insecticide Options for Protecting Ash Trees and Their Effectiveness

By R. Chris Williamson, Ph.D., and Fredric Miller, Ph.D.

Since June 2002, the emerald ash borer (EAB) has been discovered in nine states in the U.S. as well as in Ontario, Canada. Consequently, many questions have been asked by homeowners and industry professionals regarding the capability and need of insecticides for protecting ash trees from EAB.

There has been much confusion surrounding the question of whether insecticides are an effective management option for EAB. Research and experience has shown that insecticides can protect ash trees from being killed by EAB. However, success of insecticides is not guaranteed! In some university trials, insecticide treatments were effective, but in other trials the same treatments failed. Some studies conducted over multiple years revealed that EAB infestations continued to increase despite ongoing treatment programs.

Insecticides are not effective in eradicating EAB infestations, which is why they have not been used as an eradication tool by the Cooperative EAB program in affected states. Research suggests that the best control can be achieved when insecticide treatments are started in the earliest stages of infestation, before visible symptoms are present or possibly the year before trees are infested. It is important to understand that most insecticide treatments must be repeated each year. Consequently, it may be more cost-effective to remove and replace the tree.

There are several insecticide options available for those people who want to treat their trees. It is important to understand that controlling wood-boring insects with insecticides has always been a difficult proposition. This is especially true with EAB because our native North American ash trees have no known natural resistance to this pest. Insecticide research programs are showing promise, but research on chemical control of EAB is still in relatively early stages. Scientists from universities, government agencies and private businesses are conducting intensive studies to understand the circumstances under which insecticide treatments will be most effective.

Insecticide options for controlling EAB
Insecticides used for control of EAB fall into four categories: 1) systemic insecticides that are applied as soil injections or drenches; 2) systemic insecticides applied as trunk injections or trunk implants; 3) trunk-applied systemic insecticides and 4) protective cover sprays that are applied to the trunk, main branches and (depending on the label) foliage. Insecticide formulations and application methods that have been evaluated for control of EAB are listed in Table 1. Some products can be purchased and applied by homeowners while others can only be applied by professional applicators. Strategies for their effective use are described below. It is important to note that pesticide labels and registrations may change. It is the pesticide applicator’s legal responsibility to read, clearly understand and follow all current label directions for the specific pesticide product being used.

Soil-applied systemic insecticides
Systemic insecticides applied to the soil are taken up by the roots and translocated (moved) throughout the tree vascular tissues that are not compromised. Probably the most widely tested systemic insecticide for control of EAB is imidacloprid. It is available for use by homeowners and professional applicators. The homeowner formulation of imidacloprid is Bayer Advanced Tree & Shrub Insect Control, Bonide Annual Tree & Shrub Insect Control, Ferti-lome Tree & Shrub Systemic Insect Drench and Gordon’s Tree and Shrub Insect Killer. Professional use formulations of soil-applied imidacloprid include Merit (75WP, 75WSP and 2F) and Xytect (2F and 75 WSP). Additional formulations of imidacloprid with different brand names are also becoming available.

All imidacloprid formulations can be applied as a drench by mixing with water and pouring directly on the soil at the base of the trunk. The application rates for both the homeowner and professional formulations of imidacloprid are quite similar. Soil drenches offer the advantage of requiring no specialized equipment to apply (other than a bucket or watering can). However, surface layers of organic matter, such as mulch or leaf litter, can bind the insecticide and reduce uptake. Prior to applying soil drenches, it is important to remove or pull back any mulch or dead leaves so the insecticide solution can be poured directly on mineral soil.

Imidacloprid formulations can also be applied as soil injections, which require special equipment, but offer the advantage of placing the insecticide directly into the root zone. Soil injections should be made only deep enough (2-3 inches) to place the insecticide under the turf or mulch layer. Soil injections can be made either at the base of the trunk or on a grid pattern extending to the edge of the tree canopy. Recent research has revealed that soil injections made immediately adjacent to the trunk (within 6-18 inches) are more effective than those made on a grid pattern under the tree canopy. Density of fine root hairs is very high at the base of the trunk and declines quickly as you move away from the tree. This pattern of root distribution can be clearly observed on trees that have been recently uprooted in a storm or when taking soil cores under the tree canopy.

Optimal timing for imidacloprid soil injections and drenches is mid-April to mid-May, depending on your region. Allow four to six weeks for uptake and distribution of the insecticide within the tree. In southern Ohio, for example, you would apply the product by mid-April while in southern Michigan, you should apply the product by early to mid-May. When treating larger trees (e.g. greater than 12 inches DBH), treat on the earlier side of the recommended timing. Large trees will require more time for uptake and transportation of the insecticide than will small trees. Recent tests show that imidacloprid soil drenches can also be successful when applied in the fall.

EAB larvae damage the vascular system (a.k.a. tree plumbing) as they feed, which interferes with the movement within the tree of systemic insecticides. Soil drenches or injections are aimed primarily at preventative treatment applications. However, in some cases, this approach may provide corrective control of low populations of EAB infested ash trees. Studies are ongoing to determine how much injury a tree can sustain before systemic insecticide treatments are no longer effective. Research results suggest that ash trees showing greater than 40 percent dieback are not likely to be salvaged, and any damage can reduce the effectiveness of systemic treatments.

Trunk-applied systemic insecticides
Several systemic insecticide products can be injected directly into the trunk of the tree, including formulations of imidacloprid and emamectin benzoate (see Table 1). An advantage of trunk injections is that they can be used on sites where soil treatments may not be practical or effective, including trees growing on excessively wet, compacted or restricted soil environments. However, trunk injections do wound the trunk, which may cause long-term damage, especially if treatments are applied annually.

Products applied as trunk injections are typically absorbed and transported within the tree more quickly than soil applications. Allow three to four weeks for most trunk-injected products to move through the tree. Optimal timing of trunk injections occurs after trees have leafed out in spring but before EAB eggs have hatched, or generally between mid-May and mid-June. Uptake of trunk-injected insecticides will be most efficient when trees are actively transpiring. Best results are usually obtained by injecting trees in the morning when soil moisture is good (but not saturated). Uptake will be slowed by hot afternoon temperatures and dry soil conditions.

Noninvasive, systemic basal trunk sprays
Dinotefuran belongs to the same chemical class as imidacloprid (neonicotinoids), but it is much more soluble. This product is labeled for EAB control in some states for application as a noninvasive, systemic bark spray by professionals. Although dinotefuran is labeled for use as a soil treatment against other insect pests, the current label permits it to be used for EAB only if it is applied as a trunk spray.

The formulated insecticide is sprayed on the lower six feet of the trunk using a common garden sprayer and low pressure. Research has shown that the insecticide penetrates the bark and moves systemically throughout the rest of the tree. The dinotefuran can be mixed with surfactants , which may facilitate its movement into the tree, particularly on large trees with thick bark. However, in field trials, adding a surfactant did not consistently increase the amount of insecticide recovered from the leaves of treated trees.

The basal trunk spray offers the advantage of being quick and easy to apply and requires no special equipment other than a garden sprayer. This application technique does not wound the tree and, when applied correctly, the insecticide does not enter the soil.

Protective cover sprays
The objectives of protective bark cover sprays are to kill newly hatched EAB larvae on the bark before they enter the tree and, depending on the label, adults as they feed on the foliage prior to laying eggs. Products that have been evaluated as cover sprays for control of EAB include Onyx (bifenthrin), Tempo (cyfluthrin), Sevin SL (carbaryl), Orthene (acephate) and BontaniGard (contains spores of the insect-killing fungus Beauveria bassiana). Some of these insecticides have been more effective than others (see discussion below).

Protective cover sprays are designed to prevent EAB infestations and must be timed precisely to be effective. Because protective residues must be present on the tree bark before egg hatch to prevent infestation, applications must be timed to coincide with adult emergence and oviposition (egg laying), which is difficult to monitor because there are no effective pheromone traps for EAB adults. However, first emergence of EAB adults generally occurs around 450-500 degree days (base 50 F), which corresponds closely with full bloom of black locust (Robinia psuedoacacia), which can serve as a useful phenological indicator for accurately timing applications. Best results with cover sprays have been obtained when two sequential applications are made, with the first as black locust reaches full floral bloom, and the second four weeks later. It is recommended that homeowners hire professional applicators to apply protective bark cover sprays as homeowners typically do not have the appropriate application equipment, especially on larger trees, i.e. greater than 15 feet tall.

When should EAB treatments begin?
It is quite difficult to determine exactly when to initiate insecticide treatments. Research suggests that the best control of EAB will be obtained when treatments are initiated in the earliest stages of EAB infestation before visible symptoms are present, or perhaps even the year before trees are infested. Treatment programs that begin too early represent an unnecessary expense. We suggest that those who want to protect their ash trees initiate EAB insecticide treatments if they are located within an EAB quarantine, or outside a quarantine but within the immediate vicinity (i.e., 10-12 miles) of a known EAB infestation. Locations of EAB infestations, current quarantine maps, and other important information regarding EAB can be found at the following Web sites:

www.aphis.usda.gov/planthealth/plantpestinfo/emeraldashb/downloads/multista
teeab.pdf

www.entomology.wisc.edu/emeraldashborer

www.emeraldashborer.wi.gov

How effective are insecticides for control of EAB?
Extensive testing of insecticides for control of EAB has been conducted by researchers at Michigan State University (MSU) and The Ohio State University (OSU). Results of some of the MSU trials are available at: www.emeraldashborer.info.

Soil-applied systemic insecticides
Efficacy of imidacloprid soil injections for controlling EAB has been inconsistent; some trials provide excellent control, and others yield poor results. Differences in application protocols and conditions of the trials have varied considerably, making it difficult to reach firm conclusions about sources of variation in efficacy. For example, an MSU study found that low-volume soil injections of imidacloprid applied to small trees averaging 4 inches DBH using the Kioritz applicator (hand-held device for making low-volume injections) provided good control at one site. However, control was poor at another site where the same application protocols were used to treat larger trees (13 inch DBH). Imidacloprid levels may have been too low in the larger trees to provide adequate control. Higher pest pressure at the second site also may have contributed to poor control in the large trees.

In the same trials, high pressure soil injections of imidacloprid (applied in two concentric rings, with one at the base of the tree and the other halfway to the drip line of the canopy) provided excellent control at one site. At another site, however, soil injections applied using the same rate, timing and application method were completely ineffective, even though tree size and infestation pressure were very similar. It should be noted that recent studies have shown that imidacloprid soil injections made at the base of the trunk result in more effective uptake than applications made on grid or circular patterns under the canopy.

Imidacloprid soil drenches have also generated mixed results. In some studies conducted by MSU and OSU researchers, imidacloprid soil drenches have provided excellent control of EAB. However, in other studies, control has been inconsistent. Experience and research indicate that imidacloprid soil drenches are most effective on smaller trees and control of EAB on trees with a DBH that exceeds 15 inches is less consistent. This inconsistency may be due to the fact that application rates for systemic insecticides are based on amount of product per inch of trunk diameter or circumference. As the DBH of a tree increases, its leaf area and total biomass increase exponentially. Consequently, for a particular application rate, the amount of insecticide applied as a function of tree size is proportionally decreased as trunk diameter increases. Hence, systemic insecticides may generally provide better control of some pests on small trees than on large trees if the application rate does not increase as DBH of the tree increases.

In an OSU study conducted in Toledo, imidacloprid soil drenches provided excellent control of EAB on 15-22 inch DBH trees when applied as double application, but control was less consistent when applied as a single application. Therefore, when treating trees greater than 15 inch DBH with imidacloprid soil drenches or soil injections, two applications are probably necessary. Applications can be made either in fall and again in spring, or twice in spring, about four weeks apart (for example in late April and again in late May). This is not an option for homeowner formulations of imidacloprid, which are limited by the label to one application per year. In all cases, applicators must comply with the limits specified on the label regarding the maximum amount of imidacloprid that can be applied per acre during a given year.

Trunk-applied systemic insecticides
Emamectin benzoate
In several intensive studies conducted by MSU and OSU researchers, a single injection of emamectin benzoate in mid-May or early June provided excellent control of EAB for at least two years, even under high pest pressure. For example, a highly-replicated study was conducted on trees ranging in size from 5-20 inches DBH at three sites in Michigan with moderate to high levels of EAB. Untreated trees had an average of 68 to 132 EAB larvae per square meter (m2) of bark surface, which represents high pest pressure. In contrast, trees treated with emamectin benzoate had, on average, only 0.2 larvae per m2, a reduction of more than 99 percent. When additional trees were felled and debarked two years after the emamectin benzoate injection, there were still virtually no larvae on the treated trees, while adjacent, untreated trees at the same sites had hundreds of larvae.

In two OSU studies conducted in Toledo with street trees ranging in size from 15-25 inches DBH, emamectin benzoate also provided excellent control over two years. There was no sign of canopy decline in treated trees, while the canopies of adjacent untreated trees severely declined.

One study suggests that a single injection of emamectin benzoate may even control EAB for three years. Studies to further evaluate the long-term effectiveness of emamectin benzoate are underway. To date, this is the only product that controls EAB for more than one year with a single application. In addition, in side-by-side comparisons with other systemic products (neonicotinoids), emamectin benzoate was more effective than other products.

Imidacloprid
Trunk injections with imidacloprid products have provided varying degrees of EAB control in trials conducted at different sites in Ohio and Michigan. In an MSU study, larval density in trees treated with Imicide injections were reduced by 60 to 96 percent, compared to untreated controls. There was no apparent relationship between efficacy and trunk diameter or infestation pressure. In another MSU trial, imidacloprid trunk injections made on May 24 were more effective than those made on July 19, and IMA-jet injections provided higher levels of control than did Imicide, perhaps because of the greater amount of active ingredient injected when following the IMA-jet label. In an OSU study in Toledo, IMA-jet provided excellent control of EAB on 15-25 inch trees under high pest pressure when trees were injected annually. However, trees that were injected every other year were not consistently protected.

In a discouraging study conducted in Michigan, ash trees continued to decline from one year to the next despite being treated in both years with either imidacloprid (Imicide, Pointer) or Bidrin (Inject-A-Cide B) trunk injections. Imicide, Pointer and Inject-A-Cide B trunk injections all suppressed EAB infestation levels in both years, with Imicide generally providing best control under high pest pressure in both small (6-inch DBH) and larger (16-inch DBH) caliper trees. However, larval density increased in treated and untreated trees from one year to the next. Furthermore, canopy dieback increased by at least 67 percent in all treated trees (although this was substantially less than the amount of dieback observed in untreated trees). Although untreated trees were more severely impacted, these results indicate that even consecutive years of treatment with these trunk injection treatments may only slow or delay ash decline when pest pressure is severe. In another MSU study, ACECAP trunk implants were not effective under high pest pressure.

Noninvasive basal trunk sprays with dinotefuran
Results of studies to date indicate that the effectiveness of this treatment is similar to that of many imidacloprid products applied as trunk or soil injections. MSU and OSU studies have evaluated residues in leaves from trees treated with the basal trunk spray. Results show that the dinotefuran effectively moved into the trees and was translocated to the canopy at rates similar to those of other trunk-injected insecticides, and faster than other soil-applied neonicotinoid products.

As with imidacloprid treatments, control of EAB with dinotefuran has been variable in research trials. In an MSU study conducted in 2007 and 2008, dinotefuran and imidacloprid applied as trunk sprays reduced EAB larval density by approximately 30-60 percent compared to the heavily infested untreated trees. Neither treatment provided control for two years; thus both would have to be applied annually, which is consistent with other studies. In general, control is better and more consistent in smaller trees than in large trees, but more research is needed with larger trees. Long-term effectiveness of dinotefuran for control of EAB control is not yet known, although studies to address this are underway.

Protective Cover Sprays
MSU studies have shown that applications of Tempo and Sevin SL provided good control of EAB, especially when the insecticides were applied in late May and again in early July. Acephate sprays were less effective. Onyx cover sprays also gave good control the first year under relatively light EAB pressure. However, in the second year, under heavier pest pressure, they were not effective. BotaniGard (Beauvaria bassiana) was also ineffective under high pest pressure. Astro (permethrin) was not evaluated against EAB in these tests, but has been effective for controlling other species of wood-borers and bark beetles.

In another MSU study, spraying Tempo just on the foliage and upper branches or spraying the entire tree was more effective than simply spraying just the trunk and large branches. This suggests that cover sprays may be especially effective for controlling EAB adults as they feed on leaves in the canopy. They also found a single, well-timed spray provided good control of EAB, although two sprays may provide extra assurance, given the long period of adult EAB activity.

It should be noted that spraying large trees is likely to result in a considerable amount of insecticide drift, even when conditions are ideal. Drift and potential effects of insecticides on non-target organisms should be considered when selecting options for EAB control.

Key Points and Summary Recommendations:
Insecticides can effectively protect ash trees from EAB.

Unnecessary insecticide applications waste money. If EAB has not been detected within 10-15 miles, your trees are at low risk. Be aware of the status of EAB in your location. Current maps of known EAB populations can be found at www.emeraldashborer.info. Remember, however, that once a county is quarantined, maps for that county are no longer updated.

Emamectin benzoate is the only product tested to date that controls EAB for more than one year with a single application. It also provided a higher level of control than other products in side-by-side field studies.

Research and experience suggest that EAB control with insecticides becomes less consistent on larger trees. Research has not been conducted on trees larger than 25-inch DBH. When treating very large trees under high pest pressure, it may be necessary to consider combining two treatment strategies.

Imidacloprid soil drenches and soil injections are most effective when made at the base of the trunk. Applications made in the spring or the fall have been shown to be equally effective.

Imidacloprid soil injections should be no more than 2-4 inches deep, to avoid placing the insecticide beneath feeder roots.

When treating trees greater than 15-inch DBH with imidacloprid soil treatments, best results will be obtained with two applications per year. However, imidacloprid formulations for homeowners (Table 1) can be applied only once per year. Treatment programs must comply with label restrictions on the amount of imidacloprid that can be applied per acre in a given year.

To facilitate uptake, systemic trunk and soil insecticides should be applied when the soil is moist but not saturated or excessively dry.

When using imidacloprid or dinotefuran for EAB control, use the highest labeled rate. Insects that feed under the bark are difficult to control; resist the temptation to cut corners to save money.

Trees that are already infested and showing signs of canopy decline when treatments are initiated may continue to decline in the first year after treatment, and then begin to show improvement in the second year due to time lag associated with vascular healing. Trees exhibiting more than 50 percent canopy decline are unlikely to recover even if treated.

Summary
Insecticides are valuable tools that have shown potential for protecting trees from EAB, including soil-applied systemic insecticides, trunk-injected systemic insecticides and protective cover sprays applied to the trunk, branches and (depending on the label) foliage. It is important to understand that success in not assured, and that trees will have to be treated each year. In many cases, it may be more cost-effective to remove and replace the tree.

Insecticide applications have effectively protected ash trees from EAB. However, in some university research trials, trees have continued to decline from EAB attack despite being treated over consecutive years. In other studies, EAB treatments have failed completely! The bottom line is that research on chemical (insecticide) control of EAB remains in the early stages, and we still do not have enough experience to know under what circumstances insecticides treatments will be effective over the long term.

R. Chris Williamson, Ph.D., is an associate professor in the Department of Entomology at the University of Wisconsin-Madison. Fredric Miller, Ph.D., is a professor in the Department of Horticulture at Joliet Junior College in Joliet, Illinois, and a research associate-in entomology at The Morton Arboretum in Lisle, Ill. This article was based on their presentation on the subject at TCI EXPO 2008 in Milwaukee.

References
McCullough, D.G., D.R. Smitley, and T.M. Poland. 2004. “Evaluation of insecticides to control emerald ash borer adults and larvae. Summary of research conducted in 2003.” www.emeraldashborer.info/treatment.cfm. Accessed 7 May 2007.

McCullough, D.G., D. Cappaert, and T.M. Poland. 2005. “Evaluation of
insecticides for control of emerald ash borer: a summary of 2004 trials.”
www.emeraldashborer.info/treatment.cfm. Accessed 7 May 2007.

Smitley, D., T. Davis, E.J. Rebek, and K. Newhouse. 2005a. “Troy EAB test results.” www.emeraldashborer.info/Research.cfm . Accessed 7 May 2007.

Smitley, D. T., Davis, E.J. Rebek, and K. Newhouse. 2005b. “Imidacloprid soil drench test at Bay Pointe Country Club, 2004.” www.emeraldashborer.info/Research.cfm. Accessed 7 May 2007.

Smitley, D., D.A. Herms, and E.J. Rebek. 2006. “Timing of imidacloprid soil drenches for emerald ash borer control.” In: V. Mastro, R. Reardon, and G. Parra, eds. “Proceedings for Emerald Ash Borer Research and Technology” Development Meeting, p. 26-30. USDA Forest Service Forest Health Technology Enterprise Team FHTET-2005-16, 72pp.

Cited News from: http://www.treecareindustry.org/articles/magazine/TCI0609_p8.htm