Elm trees were once one of the hardwood trees that dominated the East Coast of North America.
This changed in the late 60’s early 70’s when the trees began to die, street by street. By the time horticulturalists found the problem, a fungus carried by the European elm bark beetle and native elm bark beetle, boulevards were stump lined and the problem was headed West.
Elm beetles over winter as larva under dead bark. This can be on a dead or dying tree or elm firewood.
Once the weather becomes warm, the insects complete their growth cycle and emerge as beetles where they actively feed on elm trees from mid-May until the weather becomes cool in the fall. Once again the burrow under the bark to wait for spring.
Scientists have found that elm beetles usually live and die without moving any great distance. Beetles will travel from tree to neighbouring tree and occasionally be blown further by the wind but they rarely if ever migrate long distances on their own. Lack of migration makes it easier to prevent new infections and control old ones
One method is to keep the existing trees healthy as insects usually attack stressed plants not healthy ones. Elms can be kept healthy by making sure that they get enough water during the growing season — April to the end of July. They should also be watered well in the late fall, insuring that they have plenty of moisture in their system going into dormancy.
All dead or dying branches should be removed between, Oct. 1 – April 1 when the beetles are not moving. Do not prune elm trees during the summer months as the cuts allow beetles to enter the tree. Those with a dead branch on their tree must now wait until October before they remove any material.
The exception is if the tree is infected with the Dutch elm fungus. If the tree is infected it must be cut down and burned immediately.
Transporting, storing or selling wood from elm trees is prohibited.
These human restraints have been put in place to keep the beetle from spreading and killing existing trees. It has worked well as Alberta has only had occasional outbreaks.
Park workers set and monitor insect traps in elm trees, allowing the insect populations to be monitored on a regular basis. When an infected tree is found, as one was in Wainwright in 1998, it is immediately removed and burned. Leaving it in place would result in beetles spreading to other trees in the vicinity. This infection was an isolated incident that happened when a homeowner brought home firewood from another province.
Symptoms of Dutch elm disease differ depending on which beetles are present.
The smaller European elm bark beetle feeds on small twigs at the top of the tree. The first sign that the disease is present is the yellowing, then wilting of small branches and twigs. When the branch is removed and examined, brown streaking will be present under the bark. This beetle has been found in elm trees in Alberta but at present time the beetles captured are not contaminated with fungus.
The native elm bark beetle is a larger insect that burrows into branches that are at lest two – four inches (six – 10 cm) in diameter. First symptoms are usually wilting and browning of an entire branch or major section of the tree. Brown streaking will be present under the bark, in the larger branches, down from where the initial infection occurred. Small branches at the top of the tree will be free of all discoloration.
The fungus associated with Dutch elm disease works similar to the pine beetle, it clogs all phloem and xylem tubes which move liquid and nutrients through out the tree. As the tubes are slowly blocked, affected parts of the tree wilt and die.
Elm trees, American and Siberian, grow into magnificent shade trees that can easily be recognized by their growth habit. branches join the main trunk at an angle forming a V.
When well cared for, elms will grow for hundreds of years forming large shade trees. There is a place for these trees in every large yard, park, boulevard, farm and acreage.
There are a number of these trees growing locally. Following provincial regulations will keep them healthy.
Linda Tomlinson is a horticulturalist and educator living in Rocky Mountain House. You can contact her at email@example.com