What role do insects and diseases play in forests?

Insects and diseases are a normal part of balanced forest ecosystems, and under these conditions of equilibrium, they are said to be endemic. They accelerate mortality of weakened trees and other plants, and they also break down dead plant material, thus helping to recycle organic matter. However, when a forest ecosystem is in disequilibrium, such as during an insect outbreak, insects and disease organisms will attack perfectly healthy trees as well as weakened ones. Degradation and recycling processes are accelerated in this type of situation.

Insects and diseases also are an important link in the food chain since they provide food for various bird and mammal species. They play a role in soil formation and in incorporating humus resulting from the decomposition of leaves. Despite their key role in the ecosystem, insects and diseases sometimes pose a threat to human activities because they destroy forest resources.

Exotic insects and diseases always represent a source of disequilibrium for a forest. Indigenous trees and plants have not developed mechanisms of resistance to these pests. Exotic insects and diseases introduced into Canada generally have no natural predators or parasites here.

What losses do insects and diseases cause?

Many insects and diseases do not harm trees or forests. However, some pests, whether present at an endemic or an epidemic level, may destroy huge swaths of forest and timber resources. For example, between 1982 and 1987, insects and diseases destroyed more than 16 million cubic metres of wood per year in Quebec (Hall 1994). Most of these losses were caused by the spruce budworm (10 million m³), decay (2.6 million m³), maple dieback (2.7 million m³) and hypoxylon canker (636,000 m³).

Timber losses are not the only consequence of the presence of insects and diseases. Some insect outbreaks help to regenerate the forest or change its structure, by creating openings in the canopy, for example. The size of these gaps often determines which tree species can grow within them. When the forest's structure or composition changes, this also has an effect on the animals that inhabit it. The most drastic changes are usually caused by exotic pests.

What is an exotic pest?

An exotic pest is an insect or disease that is not native to any part of Canada. These are pests that have been brought in from other temperate countries or regions, often inadvertently through travel or trade, and have successfully established themselves here as parasites of tree species that are indigenous to Canada. The wood used to stow or pack commodities in containers for transport by ship or aircraft has provided passage to Canada for a large number of exotic pest species. For this reason, Canada, like many other countries, has strict regulations on international trade.

More than 30 exotic pest species have already been introduced into Canada since the late 19th century, including chestnut blight (disease), white pine blister rust (disease), the gypsy moth (insect), Dutch elm disease (disease and insect), beech bark disease (insect and disease), the balsam woolly aphid (insect), the European strain of scleroderris canker (disease), walnut canker (disease) and the pine shoot beetle (insect).

What makes exotic pest species so dangerous?

Tree species that are present in a region, country or continent have evolved over thousands of years into their present form. Meanwhile, the indigenous insects and diseases associated with these "host" trees have evolved along with them, and so have diseases, parasites and predators of these various insects and diseases. After evolving together for many years, all of these ecosystem components have reached a state of equilibrium.

The adaptation of trees occurs on a scale of centuries. By contrast to insects and fungi that produce one or two generations every year, trees begin to reproduce at an age varying between 15 and 60 years, depending on the species. After that, a tree produces seeds every year or at intervals of 2, 3 or 5 years. It takes trees longer than herbaceous plants, insects and fungi to adapt to changes.

The arrival of an exotic pest species disrupts the ecosystem balance that has evolved over hundreds of years. Trees lack defense mechanisms against these new pests and although they may be able to develop means of resistance, this process takes a lot of time. Exotic pests, on the other hand, quickly adapt to their new hosts and may therefore wreak considerable havoc. The introduction of new, very different pests always poses a major threat to tree species and forests as a whole.

What kind of damage do exotic pests cause?

Several exotic pests have caused irreparable damage to our forests. Chestnut blight, which was introduced in the early 20th century, wiped out chestnut trees in the southern tip of Ontario. Dutch elm disease, first observed in Canada in 1944, destroyed 600,000 elms in the St. Lawrence Valley between 1944 and 1960. The gypsy moth caused moderate to severe defoliation of 6,400 km2 of forest in Quebec and Ontario between 1982 and 1987. The annual timber losses attributable to this insect total 325,000 m³.

White pine blister rust was first detected in Canada in 1917. As early as 1920, Quebec forest authorities were thinking about reducing the volume of white pine grown in plantations. This disease causes annual timber losses of 84,000 m³ in the Maritimes. Following the introduction of the European strain of scleroderris canker, in 1978, eastern white pine became less attractive to forest companies wanting to produce sawn lumber. As a result, Quebec does not produce enough pine to supply its own industry. Between 1990 and 1997, Quebec had to import 3.3 million m³ of white and red pine from New England.

The introduction of exotic pests can displace indigenous fauna and flora and threaten the natural biodiversity of our forests. Old-growth forests like Muir Wood and Domaine Joly are examples of threatened forest ecosystems.

Exotic pests pose a threat to our economy as well. The countries that buy our wood products sometimes impose costly phytosanitary treatments as a condition of purchase to ensure that pests are not introduced into their country in our products. These measures lead to a loss of competitiveness in our industry, often associated with losses of markets and jobs.

Do the insects and diseases that affect forest trees also affect urban trees?

Yes, some insect species that are forest pests also attack trees in the urban environment. However, as a rule, the insect species and diseases that are considered forest pests are not considered urban pests. In forests, pests that reduce timber quality or destroy trees before they can be harvested are of greatest concern. By contrast, in urban environments, the focus is on insects and diseases that can reduce the lifespan or ornamental value of trees.

Diseases and insects that attack the trunk or branches of trees are more of a concern than those that attack the foliage, particularly with deciduous species. Trees growing in an urban environment often sustain injuries through unfortunate encounters with equipment such as lawnmowers and snow removal machines, as well as pruning tools. These injuries provide a route of entry for many microorganisms, which can eventually kill trees by damaging their cambium.

Insects and diseases that affect foliage and can diminish the ornamental value of trees are a worry for many property owners. However, these pests rarely pose a threat to the survival of deciduous trees, and in most cases, the tree will produce new leaves and regain its beauty

In urban environments, is it always necessary to take action to control an insect or a disease?

Some tree diseases and insects occur in urban settings, but they seldom cause serious damage. In urban environments, prevention is the most important step. It is important, first of all, to choose a planting site with the right conditions for the species of interest. Next, the tree will need to be protected against mechanical injury and be given proper care to maintain its vigour. Urban trees become vulnerable to insects and diseases primarily when their vigour declines as a result of exposure to stress. Trees are especially vulnerable in the first few years after they are planted.

Insects and decay- and canker-causing organisms that attack the trunk and branches are the biggest threat to trees and necessitate control measures. That is why it is so important to prevent injuries that could provide easy access for these types of pests.

The type of treatment required depends on the severity of the attack and generally requires consulting an expert. The solution may involve fertilizing a tree that is suffering from nutrient deficiency, using tree surgery techniques to treat an injury or applying some kind of treatment to control an insect or a disease. There are ways to suppress insects and diseases that do not involve using chemical pesticides and that are more environmentally sound. Whatever the case may be, it is important to consult specialists to determine when and how to take action.

Who is responsible for protecting forests from insect and disease damage?

Several public agencies deal with issues related to forest insects and diseases, although their roles and powers differ.

Public forest land belongs to the provinces and forest management is a provincial responsibility. Monitoring insect populations, diseases and the resultant damage is one of the components of forest management. In Quebec, for example, the Direction de la conservation des forêts of the ministère des Ressources naturelles is responsible for monitoring insect populations and diseases in forests, plantations and nurseries. The Société de protection des forêts contre les insectes et maladies is in charge of applying treatments against forest pests. The ministère de l'Agriculture, des Pêcheries et de l'Alimentation (MAPAQ) administers the regulations pertaining to ornamental plants, including ornamental trees and nursery products.

The inspection and enforcement of regulations on imported products, including the application of measures to control insects and diseases and to prohibit the entry of certain commodities to Canada, is a federal responsibility. The Canadian Food Inspection Agency (CFIA) has been entrusted with this task. It assesses the risk that international trade poses for forest resources and the risk related to the presence of pests near Canada. It also issues guidelines on the importation of goods to Canada and on the circulation of risky products within the country.

Natural Resources Canada's Canadian Forest Service oversees forest management on federal lands, but its chief task consists in conducting research in various fields related to sustainable forest development. Many of the studies deal with insects and diseases that cause problems in forests, and some focus on exotic pests.