By Brandon Gallagher Watson
Tree health practitioners are frequently asked to assess and diagnose tree problems. On most days this is not too hard. In any given area, most trees have three to five common issues and this makes the diagnostician’s job fairly easy. Properly identify the tree, run through your list of common ailments for that species, and then use your judgment and experience to key in on the right one. Now that you have your culprit, you can open up your tree health protocol playbook, and choose your management strategy from the known, predictable solutions. This method works great when the pest has readily available management protocol and you can properly set the client’s expectations. There are, however, many times where the management is not so straightforward and the job becomes more difficult. The client, however, is still looking to you to solve their problem, so what do you do for management of difficult to control pests?
Where do we start?
First, we need to understand what makes a pest difficult to manage. There are several reasons why we might call a pest “difficult.” One problem may be that we do not yet have a well-developed management protocol. This is often the case with newly introduced pests, but many times we at least know where to start. A newly introduced exotic pest can wreak havoc on native trees that have no natural defense; however, if we have experience with a similar or related species we can use that as a starting point. For example, if a new leaf-feeding beetle pops up on the East Coast, we have proven management tools of other leaf feeding beetles, such as Japanese beetles, so we could try a similar management approach to develop an effective protocol. If it proves successful, we now have a plan in place. When emerald ash borer (EAB) was first discovered in the United States arborists and researchers were able to implement treatment plans that been used for many years to manage bronze birch borer (BBB) as these two insects are closely related.
Just because it worked before…
The case of EAB actually brings up another factor that makes a pest difficult to manage; and that is a poor response to current management tools. Although we had a starting point for EAB by using a protocol similar to BBB, it quickly became apparent that EAB was going to be a different challenge that would require a different management strategy. Although BBB was effectively controlled using soil-applied imidacloprid, EAB treatments seemed to have mixed results, especially on larger trees. Questions were asked regarding why EAB was responding differently. Was there something about the uptake in ash versus birch trees? Were the EAB larvae more resistant to the product? Was it application timing? Was it that the current label rates were insufficient? Researchers looked at all of these questions and discovered that by applying imidacloprid at higher rates on larger trees they were able to get the same effective control of EAB that were seen on smaller trees. Now, by using an imidacloprid product labeled to protect larger trees, arborists have a soil-applied management strategy that can provide predictable results.
Do we have the right tools in the toolbox?
Sometimes the management challenge is a lack of available management tools altogether. Not every ash tree can or should be protected from EAB by soil applications, so a tree injection treatment should also be in the playbook. Now, protecting a valuable ash tree from EAB is not a one-time treatment, it is a long-term commitment that will require a treatment to be reapplied at frequent intervals. As any tree injection requires a wounding of the tree, it is not practical that a tree could be injected on an annual basis. Tree injection formulations of imidacloprid were already available and had shown good data for protecting ash trees, but imidacloprid is only effective for one year. If application by tree injection was to be a part of the EAB toolbox, there would need to be treatments that were effective for longer time periods to minimize the impact of wounding each time the treatment was applied. Researchers reviewed many treatment options and discovered emmamectin benzoate could do just that; it was effective at controlling EAB, and, most importantly, was shown to give acceptable control for more than one season. This now allows arborists to use tree injection treatments to effectively and responsibly protect trees when soil applications are not feasible.
It’s not “what” it’s “how”
The previous example of EAB treatments touches one more factor contributing to being labeled “difficult to control” and that is available application methods. There are plenty of examples of tree pests where we know a certain product works, but there is a limited range of options to either get it into the tree or get it to where the pest is doing damage. Many vascular diseases can only be treated by tree injection and many foliar diseases can only be treated by spray treatments. Although they have proven management protocols, they can be significant challenges from an operations standpoint. Apple scab is not thought of a “difficult pest” but it sure is if you have high winds for a week straight and can’t get to it. New tools do become available from time to time that allow another application method option, as in the case of treatments for caterpillars and spider mites. Both these pests have had effective treatments for decades, but always needed to be applied by spraying. This is not a problem in many cases, but take a tall tree, put it in a residential area on a windy day, and suddenly spider mites become a tremendous management headache. Arborists now have options to control pests like these though soil applications, which can greatly reduce the number of operational conflicts.
Difficult, but not impossible
Arborists have more options to protect trees and satisfy customers than ever before. The important thing to remember is “difficult to control” shouldn’t mean “impossible.” If you keep your tree health toolbox and protocol playbook open to new ideas, new methods, and new research, there shouldn’t be too many cases where you truly don’t have an option to meet your management objectives.
Brandon Gallagher Watson is communication director at Rainbow Treecare Scientific Advancements, and is an ISA Certified Arborist (#MN-4086A).