A massive outbreak of forest tent caterpillars is defoliating trees across Alberta, with millions of insects currently feeding on leaves in areas such as Edmonton’s river valley.. The event, which recurs roughly every decade in northern Prairie regions, is a natural phenomenon driven by factors including the health of the caterpillars, their natuarl enemies, temperature, moisture, and tree performance.

Decennial Decimation : 70% of Northern Alberta’s Defoliation Attributed to Caterpillars

Alberta Forestry data from 2025 indicate that 70 percent of defolliation in the province’s north was due to these caterpillars, according to the report.. The report notes that the caterpillars lay 100 to 300 eggs on tree canopies before winter; the eggs hatch in spring and begin voracious feeding for four to five weeks.

Municipal Management: Red Deer and Olds Avoid Large‑Scale Pesticides

Municipalities like Red Deer and Olds report increased sightings but are not conducting large‑scale pesticide treatments; instead, crews may physically remove or squish caterpillars where practical, the report says.. The report also highlights that the caterpillars’ silk trals can make roads slippery and that their hairs may cause skin irritation or allergic reactions in sensitive individuals.

Ecological Upside: Weak Trees Removed, Stronger Trees Left Standing

Ken Fry, an entomology instructor at Olds College of Agriculture & Technology, notes that while the caterpillars are not his favorite insect, they hold a personal significance from his childhood, when his father allowed him to climb trees to remove egg masses before they hatched.. fry emphasizes that this defoliation benefits the forest by removing weak trees and leaving stronger ones, according to the report.

What Happens After the Outbreak? Moth Emergence and Temporary Bare Trees

The report notes that by early July, the moths will emerge, hover around porch lights, and then perish. The cycle is a normal, recurring part of a healthy forest ecosystem, and while the sight of bare trees may seem dramatic, it is temporary and generally not a cause for long‑term concern.