The Science of Systemic Bark Sprays

Systemic applications were a revolution in plant science when they became available in the mid-20th Century. For trees, we most often think of systemic treatments as those applied to the roots. This is usually performed by applying a product within a foot of the trunk where the fibrous roots absorb it and move it up the vascular system to the canopy. Similarly, tree injection treatments place the treatment directly into the vascular system through the root flares and the treatment goes up from there. Within the last few years, a third option -- systemic bark spray applications – has been gaining popularity with arborists. How does this method work? Is it as effective as other application methods? And, perhaps more importantly, why have so many tree health care professionals been adopting this method for their client’s trees?

First, let’s step back and look at systemic treatments in general. Systemic treatments are any applications that utilize the vascular system of a plant for distributing the treatment to other parts of the plant. Systemic treatments are the go-to treatment option arborists most often employ for managing a number of different tree health issues. Although they are widely used in every area of horticulture and agriculture, systemic treatments are especially valuable to tree care as the height, locations in the landscape, and long windows of susceptibility make many other treatments difficult in practice.

Systemic treatments have many advantages over other modes of action. Chemicals that are known as “contact treatments” are quite effective, but require the pest to be present at the time of treatment. If, for example, you spray a contact insecticide for Japanese beetles, it will kill any beetle that the spray touches. It will not, however, stop any that were on the underside of the leaf if you did not hit directly, and may not control any that land on the tree a week after you sprayed. With a systemic application, the treatment is contained inside the tree, and, if timed correctly, the protection for the tree is already present when the first pest arrives. Additionally, once they have been absorbed, these treatments cannot be washed off by rain or broken down quickly by exposure to sunlight. This not only provides longer windows of efficacy, it also reduces the total volume of chemicals that are released into the environment by eliminating the need for multiple applications.

So taking advantage of the vascular system is a good thing, but how do we get the systemic treatment into the vascular system? As mentioned earlier, roots and drilling holes directly into the root flare or trunk are common, but spraying certain types of treatments directly on the bark of a tree is an option as well. Those new to this method are skeptical that this could work, often because of long-standing misbeliefs on the structure of a tree.

In dicot flowering plants and conifers, bark is basically anything on the outside of the vascular cambium. Without going into too much detail on tree anatomy, bark can basically be thought of as a hard, dead protective tissue whose primary function is defense from pests, fire, physical damage, and the sun. However, interspersed throughout that protective tissue are myriad little pores, known as lenticels. Every one of these tiny pores on the surface of the bark is linked directly to the flow inside the tree -- no different than the pores on your skin. And, taking that analogy further, just as there are many medical treatments applied directly to human skin that absorb though our pores, bark spray treatments work essentially the same way.

Bark spray treatments are not considered ‘new’ in tree health management. Spraying the bole of a tree has been the primary defense against bark beetles on conifers for a number of years.. These treatments persist on the treated bark surface repelling beetle attacks. Newer chemistries, such as dinotefuran (Transtect,) have the ability to move through the lenticels into the tree, and move systemically to the rest of the limbs and canopy. This allows an applicator to simply spray the trunk, from about eye height down to the base, and protect the entire tree.

So, do they work as effectively as applying a treatment to the soil or directly injected into the vascular system? Systemic bark spray applications began to be evaluated in the mid-2000s as a management option for emerald ash borer in several different studies. In head-to-head trials of systemic bark sprays of dinotefuran, soil applications of dinotefuran, and soil-applied treatments of imidacloprid, the bark application performed at a statistically significant equivalent as the more traditional soil applications. Additional studies have compared these methods for the treatment of scale insects and hemlock woolly adelgid, and found the results to be comparable to treatments by other application methods.

Chart courtesy of Dr. Dan Herms, OSA and Valent Corp.

Initial research of dinotefuran bark sprays incorporated adjuvants (alkylphenol ethoxylate, polysiloxane polyether copolymer, propylene glycol) to help open lenticels and assist in the movement of treatments into the vascular system. This surfactant is more frequently combined with phosphites (Reliant) for the management of phytophora diseases like Sudden Oak Death and Beech Bark Canker. The researchers discovered that, while bark adjuvants (e.g. Scrimmage) were essential for penetrating bark with phosphites, they did not necessarily increase the uptake ability of dinotefuran on all tree species. The molecular structure of dinotefuran allows it to be highly soluble in water, and thus transfers easily through lenticels into the vascular tissue. Recent trials, however, suggest the use of Scrimmage for bark spray treatments when applying to red maples. Regardless of tree species the addition of bark penetrants can improve the performance of the dinotefuran solution, and is worth considering.

There are three primary reasons why this application method is gaining popularity with arborists and other tree health care professionals.
The first is the speed of application. On average, it takes about 30 to 60 seconds to walk around the tree wetting the bark from the base up to about five feet high. Even compared to quick applications such as soil injections, this is a significant labor saver, allowing the same applicator to treat many more trees in the same timeframe as other treatment methods. Spray applications are well known for operational challenges such as rain and wind, and although bark sprays are still subject to these limitations, needing to wet only the lower portion of the tree makes it more flexible than treatments aimed at covering the foliage of a 90-foot tree.

Second, as these treatments are quickly absorbed into the vascular system, they are also quick to work. Treatments of dinotefuran sprayed on the bark on Monday can be protecting the entire canopy by the weekend. Soil application of closely related products, such as imidacloprid, can take 6 to 8 weeks to reach the effective levels bark treatments can do in days. For this reason, these treatments are quickly becoming the choice for insect management during the growing season, or for when rapid results are needed. However, it is worth repeating that all systemic treatments are reliant on transpirational pull, thus the trees need to be watered for the best uptake.

Third, this application method gets the same results as the soil applications, but uses half the amount of chemical. This not only saves money on product costs, but reduces the chemical in the environment. How can the same product get the same results against the same pest when half the amount is used? It turns out that systemic bark spray treatments are much more efficient at getting the treatment in the tree, thus need less product. Also, bark treatments do not have issues of the products binding to organic matter in the soil, which often limits the quantity of product available to transfer into the tree.

Systemic bark treatments can save trees, time and money. Like all tree health care treatment options, this is another tool in the toolbox. Each situation will dictate what product and application method makes the most sense for the time of year, the pest in question, the location of the tree, and, of course, the client’s management objections. Choosing your tools wisely is still the name of the game, but now you have another tool from which to choose.

Brandon Gallagher Watson is an ISA Certified Arborist and the Creative Director of Rainbow Tree Company. Rainbow consists of a full-service tree care company, a lawn care and pest control division, and Rainbow Ecoscience, a research & development branch dedicated to developing tools and protocols for science-based tree care.