Spruce Budworm - Video

September 2013


Meet the spruce budworm, one of the most damaging native pests of spruce and fir trees in Canada. Found in the boreal, Great Lakes and Acadian forest regions of Canada, mature larvae measure between 20 to 25 mm in length and have a shiny dark-brown head and a reddish-brown body with white or yellow spots.

Populations of the spruce budworm are a normal and important part of healthy forest ecosystems in Canada and cause little damage at endemic levels. However, a major outbreak, which occurs about every 30 to 40 years, can last for several years, defoliating tens of millions of hectares of trees in the process. During these outbreaks, the budworm can kill the tops of the trees in about 3 years and entire trees in about 5 years.

(Louis de Grandpré): When a spruce budworm outbreak affects trees, it can result in reduced tree growth, because when the budworm eats the leaves of trees, the trees don’t grow as well. Using the reduced growth, we are able to reconstruct the history of spruce budworm outbreaks and we can often go back 300 to 400 years. These studies show that periodicity exists in the cycle of outbreaks. In other words, every 30 to 40 years, we see an increase in the spruce budworm population, which leads to reduced growth and tree mortality.

Signs of budworm damage are often most noticeable in the crowns of trees where the foliage appears reddish-brown.

From early July to early August, adult spruce budworm moths deposit their eggs on the needles of spruce and balsam fir trees. Within two weeks, the first stage larvae emerge and moult. After spending the winter in their silken hibernacula, the larvae emerge again in the spring and moult several more times as they feed on new shoots. In summer, the adult moths emerge from their pupae to continue the cycle.

Spruce budworm populations are generally regulated by factors such as natural predators that keep them at low densities between outbreaks.

(Véronique Martel): Among the natural enemies of the spruce budworm, of course we have predators like birds or spiders but the most important are parasitoids. So, most of them will attack the larvae after the winter, so during spring after the diapause, so they will attack larger larvae. And then we will have pupal parasitoids, so these are the parasitoids that will lay their egg in the pupae of the spruce budworm. So we have like these different species that will attack different stages of the spruce budworm. And we also have egg parasitoids that will attack the eggs so they lay their eggs inside the eggs and will kill the egg when they come out.

In permanent research plots in forests near Baie Comeau, Quebec, Natural Resources Canada researchers are conducting biological experiments. These are aimed at understanding the processes involved in outbreak dynamics and forest response to help manage spruce budworm populations, as well as to monitor the insect behaviour and ecology in the boreal forests. More studies are taking place in the Lower St. Lawrence region of Quebec, with a view to using early-intervention strategies to test the possibility of stopping the outbreak by treating apparent epicentres with insecticides and pheromones. The hypothesis is that populations could be kept relatively low in order to prevent an outbreak. Natural enemies, migratory behaviour, reproduction failure are all studied factors in that multidisciplinary research project.

(Deepa Pureswaran): One of our big research projects focuses on the impact that the insect will have on the northern boreal forest where black spruce is the dominant species. So we’re looking at how the insect defoliates trees…how the insect defoliates stands that are dominated by balsam fir or versus those dominated by black spruce. And we’re also looking with a multidisciplinary research team; we’re looking to see how the ecosystem responds to this disturbance in the northern boreal forest.

There’s another project that’s focusing on early intervention strategies and at what point in the outbreak its best to implement these strategies in order to control the populations and maintain them at endemic densities where they have trouble growing. So the spruce budworm at endemic population densities has difficulty growing so the populations have trouble rising from this deep endemic state. And so we’re interested in knowing what the factors are that keep them in this low density state and also the factors that contribute to population growth and the rise in these populations that contribute to outbreaks.