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June 17, 2026 - Caterpillar Chaos

Increased Forest Tent Caterpillar populations are hard to miss this June.

Between days of much-needed rain, the trees and streets come alive with hordes of forest tent caterpillars (FTC). Whether they are clumping in masses, crawling slowly on the concrete, or falling from a tree onto your shoulder, residents are becoming increasingly familiar with these critters.

Forest tent caterpillars (Malacosoma disstria) are easily recognized by their blue and black body, white “keyhole” shaped dots and fuzzy appearance. They tend to gather in dense clusters for warmth and protection, which can be a jarring site if you haven’t seen them before.

While this is not the first year we are seeing forest tent caterpillars in the county, we have seen populations become more widespread. Outbreaks of these tree pests are cyclical, occurring approximately every 10 years. Population increases can last three to five years before they naturally decline.

Although population outbreaks can be visually dramatic, FTC are native defoliators. This means that they have an important role to play as nutrient cyclers in the regeneration of Alberta’s forests.

Aside from the larvae (caterpillar) population themselves being a physical and visual nuisance, the primary concern with FTC is the defoliation of deciduous trees. The larvae are typically active from May to July, during which time they feed heavily on the leaves of broad-leaved trees like trembling aspens and balsam poplars.

A short drive north of Rocky on Hwy 22 paints a picture of FTC feeding, with noticeable patches of trees bare of leaves. Feeding damage can range from a light thinning of treetops to trees completely stripped of their leaves. Fortunately, most healthy trees can tolerate the larval feeding and will push out a second flush of leaves later on in the season.

Many will be happy to hear that the defoliation from FTC will soon be coming to an end for the season. Larval feeding will begin to decline as the FTC begins to enter a new stage of growth within the next couple of weeks.

FTC will pupate between June and July in white silken pupal cocoons spun between leaves, undergoing transformation into adult moths. These brown moths emerge in mid to late summer and focus on reproduction, laying eggs that will overwinter and hatch the following spring. 

For ornamental or high-value trees on your property, caterpillars can be removed by hand picking or with pressurized water. Unfortunately for most, FTC populations are too vast and widespread for physical removal to really be effective. Additionally, removing egg masses in the fall and early spring can help lower populations on your property following year.

Chemical control options may be available, however they are not generally recommended due to the limited long-term effectiveness and potential impact to beneficial insects. This is why letting the cyclical population spike naturally correct via natural predators, diseases, and parasites is the general approach with these outbreaks.

Forest tent caterpillars, although a nuisance, are a natural part of our forest ecosystems. For more information about the forest tent caterpillar, visit the Clearwater County website or reach out to the Agriculture & Community Services department at 403-845-4444.