biodiversity · Invasive species · wildlife

Tiny insects, big problems

Here in Montreal, there have recently been a lot of tree losses due the emerald ash borer (EAB). When walking around my neighbourhood, my heart breaks when I see big, beautiful ash trees flagged for removal because of the wrath of the EAB. All this had me wondering: how did it all start and how do we stop it? Logically, if it is killing these trees at such an alarming rate, then it can’t possibly be natural, or at least not natural for this part of the world.

The EAB has been a problem in North America since it was first noticed, in Detroit and Windsor back in 2002, but of course, the insect probably arrived on the continent, several years earlier. The pest is native to Asia, living in and feeding off trees of similar genus or family to the ash tree. It is not considered a pest in Asia because Asian ash trees tend to be more resistant and the pest occurs at lower densities, however, its behaviours and impacts are much different in North America. The EAB most likely arrived in solid wood packing material that was being imported to North America. Finding the ash tree as a suitable host, it was able to establish and multiply, causing increasingly devastating losses to ash tree populations. This is particularly problematic for urban areas because ash trees are frequently planted in urban forests due to their shaded-providing and aesthetic properties.

The adult EABs are not the ones that cause significant harm the trees, it is actually their larvae. They live underneath the bark, feeding off the internal fibres of the tree, specifically, the phloem and the cambium of the tree, impeding water and nutrient supply to outer parts of the tree, causing it to die within 3 years.

It is difficult (unless you are a professional) to notice if an ash tree is infested before it is too late for treatment. Some noticeable signs are loss of foliage and new shoots appearing on the trunk of the tree, which is the tree’s natural response to loss of foliage. Closer to the end of its life, the bark of the tree will crack open or bulge near to where the larvae have been feeding.

Once a tree is infested, in can either be treated or cut down. Treatment is only possible for larger ash trees (15 cm or more in diameter) that have lost less than 30% of their foliage. In urban areas, smaller trees and trees that are too far gone, must be cut down as there is no other way to save it and they are at risk of falling down suddenly and harming someone.

Cutting down all infested trees, is not an effective solution to eliminating the EAB problem, as the EAB will just find another host tree. There is still a lot of research being done on this front, and it is suspected that biological controls and natural tree resistance can be possible contributors to the control or eradication of the EAB in North America. Biological controls have a long history of causing more problems than they solve, so it is possible that with population control via treatment of infested trees and regulations on transported materials, the problem will eventually be solved as ash trees gain natural resistance.

Here are some pictures I’ve taken of local ash trees that have been seriously infested:

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