One of our favorite games at Kentucky Blue Grass is the classic: “What’s that caterpillar?!” When people discover these many-legged crawlers feeding on their trees, it can feel like a full-blown invasion.
Fortunately, most caterpillars found in Alberta are unlikely to cause enough damage to kill a healthy tree. However, problems can arise if a tree is already weakened, if the infestation is particularly severe, or if the same tree is heavily defoliated over consecutive years. In most cases, tree caterpillars are primarily an aesthetic nuisance, damaging foliage or leaving behind large, unsightly silk nests.
In this post, we’ll cover the most common caterpillars in Alberta that can cause issues, how to identify them, and what you can do to manage them.
Tent Caterpillar
Tent caterpillars are among the most noticeable caterpillar species due to their tendency to gather in large silk nests built in trees. They primarily target trembling aspen but can also be found on birch, maple, cotoneaster shrubs, and fruit trees. While they prefer certain hosts, they will feed on almost any foliage if preferred options are scarce.
Tent caterpillars overwinter as rings of eggs laid on twigs, covered in a fine, brown, foam-like material. In spring, they hatch around the same time their host trees begin to bud. The larvae quickly start feeding on fresh growth and construct silk nests where they gather for protection. As they grow, they consume increasing amounts of foliage, often stripping entire leaves and leaving parts of the canopy bare.
After about six weeks, the larvae leave the nest to find sheltered locations where they spin cocoons and pupate. Adult moths emerge roughly ten days later and spend the remainder of their short lives mating and laying eggs before the onset of winter.
There are several species of tent caterpillars, but the three most common are forest, eastern, and western tent caterpillars. Forest tent caterpillars are typically blue with black stripes and white spots along their backs. Eastern tent caterpillars are dark brown, with a lighter brown stripe down the back and a very hairy appearance. Western tent caterpillars are brown with a paler underside and feature yellow and blue spots along their backs.
Tent caterpillar populations tend to follow a cyclical pattern, peaking roughly every ten years. During these cycles, about two years experience very high populations, while the remaining years see relatively low numbers.
What can be done
Egg clusters can be found during the host tree’s dormant season (after it has shed its leaves) and can be gently scraped off the branches. Because many parasitic insects and predators rely on tent caterpillars as a food source, these egg masses can also be relocated away from the host plant. This ensures that newly hatched larvae have no access to food while still allowing natural control agents to thrive.
Larvae can be physically removed and disposed of. Their silk nests may be destroyed using a stick, a strong spray of water, or, in some cases, by pruning out the affected branches. Once the nests are disrupted and the caterpillars are knocked to the ground, many are unable to return to the tree and will not survive.
When physical control methods are not practical, contact insecticides can be used to manage infestations. However, these products only kill insects they come into direct contact with and do not prevent adult moths from returning to lay new eggs.
A systemic insecticide, such as TreeAzin, can also be applied to help make host trees more resistant to infestation.
Leaf Roller
In Alberta all aspen trees tend to have some leaf roller present. The tell-tale signs of these tree caterpillars are the characteristic rolled up leaves on the trees. While some target aspens, others target birch, willow, ash, or poplar. Leaf roller caterpillars come in many varieties but live similar lives, their eggs are placed within a bud to over winter and emerge in the early spring when the leaves start to grow. They quickly us their silks to roll a leaf up, using it for protection while they eat. When they get big enough they will roll up a new leaf to pupate inside. The adults emerge as moths to mate and lay legs with almost all the leaf roller species having a single generation per year.
What can be done
While the damage caused by the leaf roller can be unsightly it barely effects the tree’s health. There are no effective treatments for leaf roller as they are protected from contact insecticide sprays and systemic treatments are ineffective. They do have several natural predators like other insects and birds.
Satin Moth
Satin moths primarily target poplar species in Alberta but will also feed on willow and oak when their preferred hosts are less available. The larvae emerge from their eggs in the spring, coinciding with bud break, and feed and grow until June.
The larvae are dark, hairy caterpillars with large white spots along their backs and smaller orange spots on either side. Adult moths are bright white, with wings that have a distinctive satin-like sheen.
In severe infestations, satin moth larvae can completely defoliate trees. However, they rarely kill their hosts unless defoliation occurs repeatedly over several consecutive years.
What can be done
Satin moths have several natural parasitic predators that target them along with fungal and viral ailments that control their population. In the spring the larvae can be crushed if found or sprayed off the host with a strong spray of water.
Fall Webworm
Fall webworms are easily identified by the large, unsightly web tents they construct in the outer canopy of their host trees in late summer. They emerge from their cocoons in late July, mate, and then lay eggs on the undersides of leaves. The adult moths are typically white, with a furry thorax and brown spots on their wings.
The eggs hatch within about two weeks, and the larvae spend the next six to eight weeks building their nests and feeding on surrounding leaves. These larvae are hairy, greenish-brown caterpillars marked with dark brown and green patterns along their backs.
As they mature, the larvae drop to the ground and burrow into the soil, where they form cocoons and overwinter. Fall webworms target a wide variety of deciduous plants in Alberta, but they rarely cause significant damage because their feeding stage occurs late in the season. However, they are often considered an aesthetic nuisance due to the large web nests they leave behind.
What can be done
The easiest way to control the webworms population is to prune their nests out of the host and destroy them. This should be done before they go to ground in September.
Spiny Ash Sawfly
Spiny ash sawflies specifically target ash trees, feeding on and skeletonizing their leaves throughout the summer before moving to the ground to overwinter. Adult sawflies emerge from cocoons in the soil in late May, where they begin mating and laying eggs in the soft, newly developed leaves.
The eggs hatch after about two weeks, and the larvae spend the following weeks feeding and growing. Adult sawflies resemble dark-colored wasps with clear wings. The larvae begin as smooth, green caterpillars with black heads but develop their distinctive Y-shaped spines as they mature.
They feed in groups, consuming leaf tissue until early July, when they descend from the tree and burrow into the soil to form cocoons for the winter.
While spiny ash sawflies rarely cause enough damage to threaten the overall health of a tree, they can significantly impact its appearance. In urban settings, they may partially defoliate mature trees and completely defoliate smaller, younger trees, reducing their aesthetic value.
What can be done
At present, spiny ash sawfly populations typically cause minimal damage to ash trees, and no widely established control methods are required. As with many caterpillar-like pests, the larvae can be physically removed and destroyed when found, although eliminating all individuals is often impractical.
Contact insecticides may be applied to help reduce larval populations when necessary. Additionally, in early spring, the ground around the base of the tree can be covered with plastic or landscape fabric to help limit the emergence of adult sawflies from the soil.
Note – Sawflies are a common pest to trees and there are many types. Other types such as the Yellow headed spruce sawfly target spruce trees. Learn about them in our post dedicated just to them! https://kentuckylandscape.ca/sad-spruce-sawfly/
Pear Slug
Pear slugs are named for their slug-like appearance, although they are actually the larval stage of a sawfly. In Alberta, they primarily target hawthorn but can also be found on cotoneaster, saskatoon, pin cherry, and choke cherry trees.
They overwinter in cocoons in the soil near the base of their host plants. In spring, they pupate and emerge as adult sawflies in June or July. The adults resemble small black flies and spend a short time mating before laying eggs within the leaves of their host.
The eggs hatch after a couple of weeks, and the slug-like larvae begin feeding on the leaves, skeletonizing them throughout the summer. Under ideal conditions, pear slugs may produce a second generation in a single season, increasing the extent of damage into the fall.
What can be done
In the case of a minor infestation the leaves with slugs on them can be removed and destroyed, destroying the insects at the same time. Pear slug is easily controlled with contact insect controls like Insecticidal soap, neem oil, or contact insecticide sprays.
In Conclusion
Defoliating insects, such as tree caterpillars, share several common characteristics. They represent just one stage in a larger life cycle, and it is only their destructive larval stage that causes significant damage. If larval populations are kept under control, the overall impact is usually minimal, and host trees typically recover with only minor aesthetic damage.
However, repeated defoliation over several years can lead to reduced growth, causing trees to become stunted or malformed. In cases of severe, consecutive defoliation, trees may eventually decline.
Most of these insects are naturally regulated by environmental factors, including parasitic wasps, predators, fungi, and bacterial infections. As a result, control measures are generally only recommended in severe infestations or when it is important to maintain the appearance of ornamental trees.
Written References
Hugh Philip & Ernest Mengerson, University of Alberta, “Insect Pests of the Prairies”
W.G.H Ives & H.R Wong, Government of Canada Canadian Forestry Service, “Tree and shrub insects of the prairie provinces”
Websites
https://entnemdept.ufl.edu/creatures/trees/forest_tent_caterpillar.htm
https://tidcf.nrcan.gc.ca/en/insects/factsheet/11020
https://tidcf.nrcan.gc.ca/en/insects/factsheet/9504
https://www.forestpests.org/vermont/satinmoth.html
https://d1ied5g1xfgpx8.cloudfront.net/pdfs/31782.pdf
https://tidcf.nrcan.gc.ca/en/insects/factsheet/7688
https://www.gov.mb.ca/agriculture/crops/insects/pear-slug.html