The forest tent caterpillar (Malacosoma disstria) is a common pest that feeds on broadleaved trees and shrubs.
Young caterpillars are about 3 mm long, black, and hairy. Mature caterpillars can reach up to 50 mm.
They are identified by wide blue stripes along their sides and a row of white, keyhole-shaped spots down their back.
Caterpillars often cluster together on tree trunks and branches when not feeding.
Eggs are laid in bands around twigs and small branches.
Although called "tent" caterpillars, this species doesn’t build true tents. Instead, they may spin light silk mats to rest on.
These caterpillars typically surge in population every 10 years in Alberta. Their preferred food sources include trembling aspen and poplar, but they also feed on birch, ash, fruit trees, cotoneaster shrubs, and—during outbreaks—even garden plants.
In most cases, no control measures are needed. Healthy trees can tolerate light to moderate defoliation, and most recover fully. Outbreaks are usually short-lived, as natural predators such as birds and ground beetles help reduce their numbers.
You can manage tent caterpillars on your property by taking the following steps:
Remove egg bands by scraping them off branches with a dull blade. Place them away from trees where predators can find them.
Remove caterpillar clusters by hosing them off or placing them in a bucket of water and disposing of them once dead.
Destroy cocoons and silk mats whenever found. Seal them in a bag for disposal.
Spread awareness by learning about these pests and sharing information with neighbours.
Use insecticides only as a last resort, and always follow label directions. Consult a gardening expert if you’re unsure.
Please note: The Town of Devon does not remove or control tent caterpillars on private or Town-owned lands.
References:
https://www.edmonton.ca/programs_services/pests/forest-tent-caterpillars
https://www.canada.ca/en/health-canada/services/pest-control-tips/tent-caterpillars.html
For advice or more information on how to manage tent caterpillars, contact Devon's Parks Department at 780-987-8316.