The Ash Assassin

Picture this…there is a tree that grows fast and strong, lives a long time, and has a beautiful, spready canopy. The durability and disease resistance of this tree makes it ideal for urban settings. In native, forest environments, this tree adds great biodiversity and stability to the surrounding area and soil. This tree is so good, it even benefits the economy via the lumber markets and the nursery stocks.

Can you think of a more perfect tree? Well…This tree would be the Ash tree. 

The Ash tree is from the Flaxinus family and is known for its tight, diamond shaped bark and vast canopy of compounded leaves. Certain species are found native to wetlands (Black Ash), while others prefer moist, rich soils (White Ash). When the benefits of the Ash tree were discovered, the species was the recommended tree for urban forestry expansions across North America. This meant that Ash trees now line roadways and urban communities, it populates vast forests, spreading across multiple townships and borders. At the time of the popularity of the Ash, a disease targeting the species seemed unlikely, since it was so immune to other common defects and diseases. But, in 2002, the life of an Ash tree would be forever changed…enter the Emerald Ash Borer. 

The Emerald Ash Borer (EAB), or the Agrilus planipennis, is a small metallic green boring beetle native to Asia. It is believed that the foreign beetle was introduced to North America as a result of wood packing materials. To date, the EAB has killed millions of Ash trees across Southern Ontario. Once the EAB infests an area, it is expected that all the Ash trees will be dead within 10 years. This is evident throughout the GTA and southern Ontario, with some areas left with nothing but barren, crispy dead Ash trees making it look like winter in the dead of summer. But how can one, small bug create so much damage to a supposedly resilient tree? Well, let's discuss the bug so we can understand how and why it is the ultimate Ash tree killer. 

The EAB starts its life cycle by laying eggs in those deep diamond shaped crevices of the Ash bark. One female EAB will lay about 90 eggs at once. These eggs will hatch into larvae, which feed off the cambium, creating S shaped tunnels under the bark. In this early stage of life, the EAB is immature, but highly damaging. The cambium is known as the “living” portion of the tree, responsible for transporting water and nutrients from roots to leaves. The tunnels caused by the EAB larvae damage the living tissue, progressively weakening the tree. While many bugs are killed off during the harsh weather of winter, these larvae can survive under the bark over winter, continuing to cause more and more damage as the EAB matures. 

Once the larvae have matured and eaten their share of Ash tissue, they begin to pupate, aka they enter adolescence. At this point, it is April or May, and within a few weeks, the EAB are considered “adults”, emerging from the tree via D shaped exit holes by June. But, the Ash tree is not off the hook yet…the adult beetles continue to use the Ash tree as a food source, munching on leaves, further damaging the tree's ability to survive. Once the signs of changing seasons occur, the EAB is ready to restart the process, finding new Ash trees to lay their eggs and continue the vicious cycle.

Now, you may be wondering, WHY target the Ash if it is supposedly “durable” and “disease resistant”? Well, the answer is (once again) evolution. Ash trees naturally give off a special chemical cue that repels other bugs. But, being an invasive, foreign species, the EAB senses this biochemical and uses it as a homing beacon. Seems silly that the Ash is essentially asking for EAB infestations…But the trees did not ask for it. The beetle has a shorter life cycle than the tree, allowing it to fine tune its targeting abilities faster than the tree can change its natural biochemistry. The EAB has a life cycle of about one year, while an Ash tree has a life cycle of 80-100 years. EAB targeting Ash trees are more successful that those that target, let's say, a maple tree. The offspring that thrive off the Ash are then wired to also target Ash and inherit the ability to sense the chemicals coming from the Ash. Scientifically, an Ash tree targeted by the EAB is likely to succumb to the damage caused by the beetle, and will not pass on its genes to future generations. Those that do survive an infestation likely give off a slightly different chemical, and will have a better chance at producing more Ash trees. However, as we mentioned, the EAB evolve at a much faster rate, so an EAB may adapt to this new beacon, while these “resistant” Ash become the new target for future EAB generations. Did we mention yet that this is a VICIOUS cycle?

 So, we know the EAB is in Ontario and causing ridiculous amounts of damage to the Ash population, but how do we identify an infestation and what do we do about it? 

Signs of on EAB infestation are:

• S shaped “galleries” or tunnels under the bark of Ash Trees from EAB larvae;

  

• D shaped holes in the bark from adult EAB; 

  

• Lack of bark from girdling and loss of cambium;

  

• Branch mortality and wilted or yellow foliage due to loss of water/nutrient flow.

How to control EAB:

1. Phyto-sanitation, or, in non-science terms, the control of plant disease, suggests the best thing to do is remove infected Ash trees to limit the food source for the bug, hopefully reducing the EAB population in the process. Infected wood should not be moved from an affected area. This is because the larvae and the beetle can survive in the wood long enough to re-emerge in a new location, spreading to once unaffected areas. 

2. Another form of control is bio-surveillance. This is essentially using one species to control another. In this case, a special type of wasp is used to target EAB, killing only the beetle and saving the Ash tree from devastation. Like the EAB, the wasp targets Ash for laying their eggs, but does so on the surface of the bark rather than under the bark, not damaging the tree. Young wasps hatch and feed on the EAB larvae, reducing the population and liberating the Ash tree. This form of control is harder to pitch as a solution since the majority of people are more opposed to scary killer wasps than some dead trees. It should be the other way around, as millions of dead trees is MUCH worse than some specialized-combat wasps. 

3. Another tactic for combating the EAB is insecticides. Other borer bugs are controlled preventatively by killing the young beetles on contact once they emerge. However, much of the damage to the Ash is done during the larva stage of EAB infestation, and once they emerge, the beetle is protected by its hard exterior. Thus, traditional preventative insecticides are a useless defense against the EAB and a systemic insecticide must be used to effectively control the EAB. A systemic insecticide does not work on-contact, but by being taken into the tree, making the tree itself toxic to the EAB. EAB-specific insecticides and other systemic  insecticides are injected into the tree or soil. This allows the tree to spread the compounds through the cambium and into the leaves. When the EAB go in for a snack, they ingest the insecticide, and are either deterred or killed. 

   
   

   What does this all mean?

Well, for starters, homeowners with Ash on their property need to be aware of potential risks of EAB infestation. An infested Ash is a dying and weak Ash. This means that the once strong and durable tree is much more likely to have brittle, broken limbs or structural damage, endangering immediate surroundings. Biodiversity is at risk, as the Ash tree is known to be home to numerous birds, beneficial insects, squirrels and other forest critters. The loss of an entire species will always impact the environment, especially when it was once such a critical tree species. Lastly, the economy is also at risk. Ash trees account for $22 million of the lumber market. Infested or damaged Ash lumber is no longer acceptable, limiting an already limited natural source even further. Additionally, the cost to remove all Ash from infected areas and replace them with new species is an astronomical feat financially and physically. Just imagine the labor to remove and dispose of the millions of dead Ash trees throughout Ontario, let alone the rest of Canada and the northern States also affected. And nothing comes free these days…

The Ash tree is still a strong tree, the EAB is just stronger. Since 2002, the EAB has become a household name, with affected regions despising its existence, and unaffected areas fearing its arrival. Here in Haliburton ON, the first individual infestations have been identified in recent years. This means that over the next 10 years, we are in for large quantities of Ash dying off. Be ahead of the curve by keeping a close eye on the Ash trees and know how to identify the signs of EAB. While small in size, this little bug has a big impact on the Ash population and the world around it!