By David Brynn, VFF Conservation Forester
OVERVIEW: Vermont Family Forest’s Ash Grove Seed Project is our attempt to respond effectively to the Emerald Ash Borer (EAB) invasion that is upon us. Our goal is help maintain Vermont’s three native ash tree species over time. Taking into account recent research on ash tree survival in the wake of EAB, along with the practicalities of organic treatment of ash trees to protect against EAB infestation, we have explored four options landowners can undertake on behalf of ash trees and suggest two, all of which I describe below.
OPTION 1 – No treatment: Recent research suggests that white ash trees may have more resistance to EAB than was originally reported. Leaving most, if not all, white ash trees to grow (rather than cutting them ahead of EAB infestation) may help protect genetic diversity, since resistant trees will survive and produce seeds. Green ash is exhibiting less resilience and much higher mortality than white ash and will most likely require chemical intervention to retain viable seed sources. The mortality rate of black ash is less clear, but as with green ash, some intervention on their behalf will likely be required to perpetuate the species.
OPTION 2 – Non-organic plugs: Vermont landowners can purchase and install non-organic, systemic insecticide plugs to combat EAB in Vermont. Acecap offers 0.25-inch diameter plugs for trees measuring 3 inches in diameter at breast height (DBH) or less. These can be installed in 1-inch deep holes located every two inches around the base of the tree. Larger 0.375-inch diameter plugs are available for trees that are over 3 inches DBH. A 10-inch tree would require 15 of the 0.375-inch size plug. They cost about $2 per plug. The treatment would run $30 per tree plus labor and travel. However, they are not organic and they introduce synthetic toxins into the environment. VFF does not recommend this approach.
OPTION 3 – High pressure injection of organic TreeAzin: We have done extensive research into organic options for EAB treatment, searching for an effective and affordable option. One of the only commercial organic options available is a product called TreeAzin, a pesticide derived from neem oil. Certified pesticide applicators can purchase and inject the product. As of January 3, 2020, the price for TreeAzin (5% azadirachtin) was $420 per liter plus $25 in shipping. One liter treats roughly 80 inches of tree diameter at breast height, depending upon size (larger trees require more). Eight 10-inch DBH ash trees could be treated with one liter of the product. The TreeAzin chemical alone (not including time and travel) costs $52.50 per tree.
Several certified pesticide applicators stated that TreeAzin is generally prohibitively expensive for forest-based applications. However, green and black ash seed groves might warrant the cost. Several certified pesticide applicators said that if VFF would organize the project so that their responsibility would be only to conduct the actual injections, the estimated cost would be about $6 to $10 per DBH-inch or $60 – $100 for a 10-inch tree plus travel. If VFF is not involved, the cost would be $10 to $15 per DBH inch or $100 – 150 for a 10-inch DBH tree plus travel. So, if you identify three ash trees to inoculate with TreeAzin, you are committing to paying at least $300/year for treatment.
Two regional licensed pesticide applicators/certified arborists we’ve talked with who work with TreeAzin are Richard Wood (of Trees Vermont in Colchester, 802-343-6905) and Adam and Marilyn Whitney (of Whitney Tree Service in Waitsfield, 802-496-9975).
TreeAzin injections would need to happen annually or at least every two years. It is hard to imagine landowners could sustain these costs over time. The good news, however, is that TreeAzin’s active ingredient, azadirachtin, comes from neem oil, which has long been used in organic agriculture. Organic, cold-pressed neem oil is available and can be effectively and safely applied by landowners (see Option 4).
OPTION 4: Stem and foliar spraying, soil drenching, and systemic gravity injection of organic, cold-pressed, neem oil: Neem oil has been used for centuries as a plant fertilizer and invigorant, as well as an insect repellent. As such, neem oil may both boost the health of ash trees and decrease their palatability to EAB. Emerald Ash Borers feed on ash tree leaves as adults and on ash tree phloem as larvae. Some organic orchardists effectively use organically produced, cold-pressed neem oil as a deterrent for leaf chewing by adult insects. When leaves are coated with a 0.5% concentration of neem oil, adult EABs that feed on them experience a regurgitation reaction and they no longer feed on that surface. Mating and oviposition are confused and interrupted as well. When insects in the larval stage feed on phloem which has been soaked with a 1.0% concentration of neem oil, their molting cycle is disrupted. Though EABs are not killed in either scenario, their potential to cause damage to ash trees could be significantly reduced.
While orchard trees are quite suitable for stem and foliar spraying, because they are short and readily accessible, forest-grown trees present significant challenges due to their height and locations. This is one reason why systemic injection has such appeal. The chemical is injected into the xylem and drawn up the stem to the leaves and then returned to the roots through the phloem.
Pure neem oil has an azadirachtin concentration of only about 0.15% to 0.20% compared to the 5.0% azadirachtin in TreeAzin. In other words, it would take 25 times more neem oil by volume to deliver the same amount of azadirachtin contained in TreeAzin. However—and this is important—organic orchardist Michael Phillips has made the case that organic, cold-pressed neem oil is a far superior product to TreeAzin.
Writing about the use of neem oil in orcharding, Phillips says, “The fatty compounds in neem (derived from palmitic, stearic, linoleic, and oleic acids primarily) contribute considerably to overall plant health, which in turn help the fruit tree to be that much more able to resist disease….Azadirachtin by itself has impact but better results can be obtained by using all the liminoid compounds in the whole plant medicine. A kind of synergistic effect is obtained whereby one component strengthens and increases the working of another.” The obvious question arises: Can neem oil be effective as an EAB repellent in a forest setting?
Neem oil, as a repellent, can be sprayed on the leaves and bark in a solution of warm water that is 0.5% and 1.0% neem oil respectively plus 0.3% to 0.6% dish soap. This would seem to be especially effective for use on smaller ash trees up to 15 feet tall. Soil drenches could also be used as part of this treatment option. What about the cost? A 5-gallon bucket of cold-pressed organic neem oil currently costs $250 plus shipping. A 0.5% solution of neem oil for foliar application would require about one-half cup (i.e. 3.2 ounces) of neem oil mixed with dish detergent and 5 gallons of warm water. The cost of the neem oil would be $2. The cost of 5 gallons of the 1.0% bark and soil drench solution would be $4.
Ideally some of the cold-pressed, organic neem oil would be injected into the tree. This systemic approach would send azadirachtin to the leaves and the phloem. The 0.5% and 1.0% concentrations are a very dilute mix that is readily translocated in the tree when spring growth is active. Coupled with foliar treatment on smaller trees, soil drench, and bark soaking this would seem to be an effective way to send EABs the signal that they are not welcome in this tree. Proper timing is essential.
High pressure injection is fussy and can cause damage to the tree tissues. It is speedy and it gets professional applicators on their way quickly. However, the ash trees may appreciate being part of the work crew, via a slower process. As they break bud and grow leaves, ash trees suck moisture from the forest soil when there is plenty of moisture available to them. It is essential that the participating forest soils have plenty of rich organic matter to hold moisture and nutrients. This is a key part of the wholistic organic EAB treatment.
Could injection of neem oil work as an EAB repellent? In addition to rich forest soil that is ready to send moisture to the ash trees on demand, a reservoir of the 1.0% concentration of organic, cold-pressed neem oil plus dish detergent might help to repel EAB. Gravity injection equipment suitable for tree treatment were once commercially available, but I have not been able to find them in my recent equipment search. Alternatively, a 5-gallon pail with lid seems like an ideal vessel. A hole could be drilled at the bottom of the bucket and a ¾ inch fitting is inserted and secured with gaskets and lock nuts. A release valve will be required to contain the neem oil dilution in the bucket until the tubing and spouts are firmly in place. A maple sugaring fitting has 6 ports that will accept the 5/16-inch tubing. A maple health spout is attached to the end of each of the six maple tubes. Ideally the bucket will be secured gently to the ash tree with sturdy straps at about DBH. A shelf might be better but that will add cost and hassle.
A 10-inch DBH ash tree would be drilled with six 0.25 inch holes about 5 inches apart. The dilute mix could then be slowly injected by gravity into the tree through maple health spouts inserted at the base of the tree. The goal would be to inject up to one quart of the organic, cold-pressed neem oil into the tree over the course of the early days of the growing season. A soil drench and stem soaking could be added to re-enforce the injection treatment.
The cost for the landowner doing the work would be about $12.50 for the neem oil plus $15 for the bucket and tubing. This would bring the foliar spray, soil drench, stem soaking, and neem oil injection to $33.50 per tree or about $3.40 per DBH inch. A 5-tree Ash Seed Grove could be treated in the first year for about $170 including the organic, cold-pressed neem oil, the dish detergent, and the gravity injection apparatus. Subsequent years the costs would be significantly less. The use of cold-pressed, organic neem oil as an EAB repellent appears to be a safe and relatively inexpensive option for landowners willing to do the work. There are no guarantees, however, that it will be effective in combatting EAB over time.
- Our sense at this point is that the best option is to leave as many of your ash trees to grow as possible—NOT cut in anticipation of EAB except when a standing dead ash tree poses a hazard. Look at your forest conservation plan. If it calls for harvesting ash, consider rethinking that, and waiting to see how your forest-grown white ash trees do over the next 10 years. Unless they are large and clear, they are relatively low in stumpage value. If they survive, they may pass genetic traits of EAB resistance on through their seeds. In addition to leaving all white ash trees to grow, you could opt to treat some particularly stellar ones to help ensure their survival. Since green and black ash appear to have a higher EAB mortality than white ash, consider focusing treatment on these species.
- If you’re interested in inoculating with TreeAzin, despite its costs, now is not too soon to identify trees. Know that inoculation will cost at least $300/treatment annually for a grove of three trees. If you want to proceed with identifying a 3-5 tree grove to innoculate, a VFF forester will spend 2 hours in your woods free of charge (for forestlands within Vermont’s Center-West Ecoregion only) to identify great trees to treat.
- We are not asking you to take action in any particular way, but to know and consider your options. We do not know enough about the use of cold-pressed organic neem oil as a repellent to recommend its use. Some limited use may be worth a try in order to save a few seed producers in the family forest. The jury is out on that one.
 White ash (Fraxinus americana) survival in the core of the emerald ash borer (Agrilus planipennis) invasion, by Molly A. Robinett and Deborah G. McCullough. Canadian Journal of Forest Research (2018).