About the Forest Tent Caterpillar
The forest tent caterpillar (Malacosoma disstria) is native to North America and is the most widespread defoliator (leaf-eater) of deciduous trees on the continent.
Outbreaks of forest tent caterpillar occur in Ontario approximately every 10 to12 years and last anywhere from three to six years in a given location. The last forest tent caterpillar outbreak peaked in 2000/2001.
The good
- Outbreaks of forest tent caterpillars can provide benefits to some plants and animals.
Many insects, birds, and mammals rely on forest tent caterpillars as a source of food and are able to increase their populations during forest tent caterpillar outbreaks. Forest tent caterpillars also provide nutrients for forest plant life. Caterpillar droppings (frass) are rich in nutrients - especially nitrogen, which fertilizes the forest soil.
Forest tent caterpillar outbreaks also help drive forest succession. Removing leaves from the overstory (larger, taller trees that shade the forest floor), allows more sunlight to reach the understory (smaller trees that grow beneath the overstory). This promotes the growth of the smaller tree species.
and the bad…
- Forest tent caterpillar outbreaks can also have negative effects on other parts of the ecosystem. Severe forest leaf loss alone can drastically change the entire ecosystem. Leaves provide shade and excessive leaf loss allows sunlight to penetrate down to the forest floor. This can cause the understory temperature to rise, warming streams, and reducing habitat for some organisms. Soil temperatures can also rise, causing drought stress to the understory plants and trees, and increasing the risk of forest fires.
Status of forest tent caterpillar in Ontario
In Ontario, the last major forest tent caterpillar outbreak began in the late 1990s and peaked in 2001. It covered more than 13 million hectares. Back-to-back outbreaks in northeastern Ontario resulted in severe and widespread aspen mortality affecting more than 618,379 hectares of forest. Aspen trees in the northeast were still relatively weak because of the short recovery time between outbreaks.
How are forest tent caterpillar outbreaks managed in Ontario?
Insect populations in Ontario are monitored each year through ground and aerial surveys as part of the province’s Forest Health Monitoring Program. Forest health technical staff record and report on forest health conditions and the occurrence of events related to living things, such as insects and diseases, and non-biological events, such as severe weather conditions. When an insect outbreak occurs in Crown forests, the Ontario Ministry of Natural Resources forms a planning committee of ministry staff, representatives from the forest industry, and local citizens committees. The committee examines management options to address the problem, which can include:
- taking no action
- implementing a control program
- application of an insecticide), and/or
- carrying out salvage, accelerated, or redirected tree harvesting.
Because forest tent caterpillars are native to Ontario, natural control factors will typically reduce population densities. Outbreaks are therefore usually allowed to run their course. However, management programs can reduce the impacts of this forest pest on high-value hardwood forests.
Outlook for forest tent caterpillar infestations
Periodic outbreaks of forest tent caterpillars will likely continue in Ontario. Historical outbreak patterns and the increasing occurrence of larvae in northern Ontario suggest that a full outbreak can be expected in the north in the next few years.
