The fate of ash trees is in our hands
I would like to continue the discussion started by Mr. Robinson in his article entitled “Battle of the Bugs” published on January 22nd, 2016. I believe your readers would benefit from some additional information regarding emerald ash borer (EAB) management options that may be available to Centre Wellington. I will highlight the importance of ash trees, elaborate on the pesticide treatment strategy and present another potential option to deal with this serious issue. Currently, your county is in an early-to-mid stage of infestation and most trees are in good condition. However, eminent infestation is a very real possibility and survival rate among affected trees is very low.
Knowing that a deadly infestation could occur, why should we bother to save ash trees? For one thing, they are an important part of the urban landscape in Centre Wellington: over 2,500 ash trees were counted on public streets and parks, and surely thousands more reside on residential properties, rural areas and surrounding natural wood lots. They are popular in urban areas because they growth quickly and are salt tolerant. These trees offer many beneficial services to your community: they filter air pollutants, mitigate storm water runoff, remove carbon dioxide from the atmosphere, promote energy savings and increase property value. From an economic perspective, ash lumber is a valuable natural resource; the wood is very strong and is used to make tool handles, baseball bats and furniture. Furthermore, ash trees are a vital component of the web of life that makes up northern hardwood forests and wetlands. If this tree is wiped out, major changes in forest structure and nutrient cycling patterns will occur. Also, native organisms that depend entirely on ash trees are in serious peril. For example, at least 21 species of moths and butterflies feed exclusively on North American ash. What will happen to these native insects if ash trees disappear? Clearly, these trees have immense value for your community and ecosystems at large, therefore they should be protected.
In fact, I believe that we have a moral obligation to deal with this issue. The EAB was introduced to our forests because of human error. It is our responsibility to manage this pest and clean up the mess. Although there are significant challenges and costs associated with managing this invasive species, the consequences of inaction are far greater.
One of the management strategies briefly mentioned in the previous article was “treating” the ash. This method consists of using pesticides to protect healthy ash trees. The two most common pesticides used in Canada to prevent EAB attacks are TreeAzin and IMA-jet. These chemicals are systemic, meaning that they travel throughout all the plant tissues. Although it is possible to apply them to the soil surface and spray them on the bark, direct syringe injections into the base of trees greatly reduces dispersal of the pesticides in the environment. The cost of TreeAzin treatments for a medium size tree is approximately 100$ per tree per year. TreeAzin is produced from Neem tree seed extracts, therefore it can be considered as a “natural” pesticide. However, it is important to note that even natural substances can be very toxic and that any pesticide has inherent toxicity.
The active ingredients of TreeAzin and IMA-jet, azadirachtin and imidacloprid respectively, have non-target effects. This means that insects other than the EAB may be harmed by the use of these pesticides. Azadirachtin is an insect growth regulator, disrupting the molting process and preventing immature larvae to become adults. It has low toxicity towards mammals, which is important since this pesticide will be used in urban settings. This compound affects insects only if it has been ingested. Caterpillars are particularly sensitive to this pesticide. Imidacloprid also has low mammal toxicity. It does not harm caterpillars, but is toxic for most leaf-feeding beetles and sap-feeding insects. It is also harmful to beneficial insect predators such as lady beetles and lacewings, which ingest the pesticide through their prey.
An important consideration whenever applying pesticides is the potential for the pest to develop resistance. Resistance arises when a large proportion of a pest population is exposed to the pesticide. In this case, only high-value urban ash trees will be treated due to high costs. Treated trees would only represent a small proportion of the total population, so most EAB will not be exposed to the pesticide. Consequently, EAB will probably remain sensitive to the pesticide and therefore they are unlikely to develop resistance. Thus, these pesticides would remain effective over time. It is not feasible to use pesticide treatment on all ash trees in Centre Wellington, therefore broader control strategies must be considered.
Biological control strategies using parasitoids have been proposed to deal with this invasion, but we should be weary of their potentially harmful ecological impacts. Parasitoids are organisms that are only parasitic in their immature stages. Wasps are the parasitoid of choice when dealing with EAB. These wasps lay their eggs inside EAB eggs or larvae; the wasp larvae then feed upon and kill the host. In Canada, two non-native parasitic wasps have been approved and released in the wild: Tetrastichus planipennisi and Oobius agrili. These wasps from China are effective parasitoids because they co-evolved with EAB. Preliminary lab tests have demonstrated that the T. planipennisi only attacks EAB. On the other hand, O. agrili targets both native insect species and EAB. I strongly believe that we must exercise caution when introducing a non-native species to deal with a pest, because unintended consequences are possible and dangerous.
It has also been proposed that mass-breeding native parasitic wasps would be a more sustainable approach. Again, the issue with these wasps is that they do not solely prey upon EAB – other insects including various kinds of metallic wood-boring beetles could be at risk. Regardless of their origin, massive surges of native or foreign parasitic wasps could destabilize natural insect populations since they can attack many other non-target insects. There are clearly many risks and uncertainties that are associated with this approach, but some scientists believe that the benefits will outweigh the risks.
In my humble opinion, the take home message is that ash trees are extremely valuable and that we should implement management strategies to protect and preserve them despite high costs. Pesticide injection is a viable treatment option for high-value urban trees. However, it is not realistic to treat every ash tree with pesticides due to high associated costs. The only feasible widespread control mechanism currently proposed is the use of parasitoids. To evaluate the ecological risks and potential benefits of these parasitic wasps, controlled small-scale releases could be safely implemented in your community. This truly is a battle of the bugs!
Dominic Demers