The Emerald Ash Borer (EAB) is a species of jewel beetle native to eastern Asia. In 2002, the beetle was detected for the first time in North America. First in Michigan, then Ontario, although tree ring analysis suggests that it has likely been present in those regions since the early 1990s. Since then, the number of EABs have increased year after year as the bugs spread across Ontario, Quebec and more than half the continental U.S.
An infection of EABs can kill an otherwise healthy ash in 2-5 years. But how can an 8.5 mm long insect kill a tree anyways? One way would be by eating all of its leaves. Without foliage, a tree has no way to photosynthesize, and therefore no way to make energy. Adult EABs do munch on leaves—a loss of tree canopy is a warning sign of EAB infestation—but not usually to the degree that would kill an ash. Instead, it's the EAB larva that cause the majority of the damage.
EAB eggs are laid on ash branches, and larvae, once hatched, chomp their way under the bark. The little grubs will chew out 6 mm wide S-shaped tunnels called galleries to live in that can be up to 30 cm long. These galleries disrupt a tree's internal water transport system, taking away its ability to send necessary nutrients up to its branches and leaves. As a result of nutrient deficiency, EAB-infected ash trees often show signs of chlorosis, or a lack of green colour in their uppermost leaves. Dying ash trees will sometimes send out epicormic shoots—little sprouts from the roots or lower trunk and branches—in an attempt to survive.
Most EABs spend winter inside ashes in their larval form. They're able to withstand temperatures down to -30 ˚C, and are partially insulated by the tree bark. Eventually, come spring, the fully matured beetles will emerge from the ash trees, leaving small capital D-shaped exit holes about 4 mm wide.
The loss of one type of tree might not seem like such a cause for alarm, but the widespread death of ash trees is having many repercussions. In 2015, Montreal was home to roughly 200,000 ash trees. Mont Royal, the iconic park in the centre of the island was, until recently, home to over 10,000 of those trees. But, as a result of the EAB infestation the City of Montreal was forced to cut down about one-third of those ashes. The other two-thirds they chose to treat with preventative insecticides. To make up for the over 3000 lost trees, the city will plant 40,000 saplings. Of these, about 50% are expected to thrive. In 2016 Montreal committed $18 million to fighting the EAB and replacing the ashes it kills. In the U.S., affected states spend an average of $29.5 million per year to manage EAB populations.
The loss of ash trees can impede ecosystems, bring down home values or disrupt food webs. During bad weather, sick or dying ashes can pose a safety risk if they fall or drop branches. And with the loss of these trees comes an increased risk of landslides and flooding, both of which tree roots help to prevent.
Pointe-Claire, a municipality within Montreal, has banned the planting of any new ash trees since 2014, enforced with fines up to $2000. Any property owners with dead ash trees must cut them down—with any ash featuring more than 30% dead branches considered dead—or face fines. Additionally, the cutting must be done between October and March when there is thought to be a lessened chance of spreading the insects.
While the EAB is wreaking havoc on North American forests, in Asia it poses no threat. One reason is that the forests of its homeland are much patchier, limiting the spread of the beetles. Another is the innate level of defense that Asian ash species have against EABs, as a result of evolving alongside them. A big part of why the EAB can spread like wildfire in Canada and the U.S. is its total lack of natural predators. While American woodpeckers have developed a big appetite for EABs—to the point that woodpeckers on an ash tree is considered almost diagnostic for EAB infection—nothing else on this continent has.
Back home in Asia, EAB eggs are predated upon by several types of parasitic wasps. Oobius agrili rely on the beetles' eggs, not only for food, but for reproduction. Female wasps inject their own eggs into the eggs of the EAB. The wasp larva will hatch first, and kill the EAB larva in the process of growing. In some parts of China, Oobius agrili parasitize up to 60% of EAB eggs laid. They are far from the only game in town either. Spathius agrili is responsible for parasitizing up to 90% of EAB larva east of Beijing, and there are several other species, such as Spathius galinae, that are specially acclimated to cold or warm conditions.
Unfortunately, none of these wasps are found naturally in North America. But that doesn't mean that they can't survive here. The USDA Biological Control Production Facility in Brighton, MI is responsible for raising more than 5 million EAB parasitoids (insects that ask as parasites to the EAB) that have already been released across 27 states as well as Ontario and Quebec. The hope is that introducing a natural predator of the EAB will help to bring down the number of beetles and keep them in check moving forward.
Another similar method for EAB control also uses an organism that naturally infects EABs, but a fungus! Researchers have utilized strategically placed containers holding spores from the fungus Beauveria bassiana to infect EAB populations and cull their numbers. Research shows that the fungus is effectively passed from beetle to beetle during mating.
Our last weapon in the fight against the EABs are pesticides. There are quite a few brand names for EAB pesticides, although they all use the same 10 or so active ingredients. There are a variety of ways to apply them to ash trees, depending on the product. Some are applied to the soil, typically either by drenching the ground around the tree with a water and pesticide solution, or by injecting the solution a few centimeters below ground near the base of the tree. Either way, the compounds will be slowly taken up by the tree and spread throughout it.
Some compounds can also be applied to trees as a spray. These tend to be less preferred, since the chemicals can easily drift around while spraying, affecting unintended species. Interestingly, one of the approved sprays is permethrin, which is also effective at repelling mosquitos!
A relatively new technology, tree injectors, allow the direct injection of anti-EAB pesticides into a tree's inner tissue. Trunk injected pesticides work faster, much like IV drugs work faster than pills in humans. In studies, this method has been the most successful, however it is more expensive. Some municipal governments are reimbursing tree owners for part of the cost of treating their ashes. Montreal West, for example, will reimburse tree-owners up to $250.
Unfortunately, On June 30th the Emerald Ash Borer was detected for the first time on the West Coast of North America, in Oregon.
Much like COVID infections, EAB infections are widespread in their host organism, despite many efforts, over several years, both governmental and personal, to stop the spread. Just like COVID, even if we ignore the EAB, it's still there. Both conditions require remarkably similar solutions: isolate when infected to the best of your ability, treat pre-emptively to help avoid infection, and remember that even the best preventative treatment needs to be updated every so often.
