What you're seeing is the Forest Tent Caterpillar (Malacosoma disstria), and Alberta is currently in a cyclical population peak that is producing one of the most dramatic outbreaks in a decade.
What Are These Caterpillars?
The forest tent caterpillar is a native North American species, and what you're describing matches it perfectly. Fully mature larvae are approximately 2 inches (5 cm) long, with a dark brownish-black body covered in fine, silky hairs. Their most distinctive markings are light blue stripes running along the sides of the body, with a row of white keyhole- or footprint-shaped spots down their backs. Despite the name, they do not actually build tent-like webs — instead, they spin silken threads along which they travel and congregate in silken mats on trunks and branches.[1][2][3][4][5][6][7]
The 2026 Outbreak
Alberta is at the apex of a roughly 10-to-12-year population cycle this year. The outbreak spans the entire province — municipalities including Red Deer, Olds, Devon, Calgary, Drayton Valley, Pigeon Lake, and Yellowhead County have all issued public notices. In central Alberta in particular, Red Deer has noted populations are "higher than usual" and crews are actively monitoring parks and trails. Alberta Forestry data from 2025 found that forest tent caterpillars were responsible for 70% of all defoliation in Alberta's north that year, setting the stage for this year's even larger surge. Yellowhead County notes this is currently Year 2 of the outbreak, meaning populations could persist for another year or more.[8][9][10][11][12][1]
Favourite Trees and Feeding Habits
Their preferred food is trembling aspen and balsam poplar — the dominant deciduous trees of central Alberta — but they will also attack green ash, mayday, birch, apple, cotoneaster shrubs, and other deciduous trees when food runs short. Eggs hatch in spring just as buds open, and the caterpillars feed voraciously for about 5–8 weeks. In the fifth larval stage, a single caterpillar can consume up to seven leaves per day. When a tree is fully stripped, entire colonies migrate together in a line following silk trails to find new food sources.[11][13][3][14]
Timeline: What Happens Next
By late June/early July, the caterpillars will be at maximum size and will begin spinning whitish cocoons inside folded leaves, on fences, decks, and building eaves. After a pupal stage of about 10 days, they emerge as tan-coloured furry moths, which then hover around porch lights and lay new egg bands (100–300 eggs per band) on tree branches before dying. The most intense caterpillar activity in your area should be subsiding by the second week of July.[8][15][9][3][5][16]
Will Your Trees Recover?
Almost certainly yes — healthy deciduous trees almost always recover and can produce a second flush of leaves within the same season. However, trees stressed by drought, repeated multi-year defoliation, or other disease are more vulnerable; persistent feeding over two or three consecutive years can cause twig and branch dieback. Given that Alberta is in Year 2 of this outbreak, it's worth keeping an eye on tree health, especially if your area has had moisture stress.[15][17][10]
Natural Controls
The caterpillars have a vast array of natural enemies that will ultimately bring the population down:[16]
- Over 60 bird species feed on caterpillars and moths (though many birds avoid them due to irritating body hairs)
- 28 insect predator species and 127 insect parasites attack eggs and pupae
- The flesh fly (Sarcophaga) is the most effective parasite, laying live maggots on cocoons that burrow inside and kill the pupae[3]
- Bats, mice, frogs, squirrels, skunks, and even bears consume them[16]
- Nuclear polyhedrosis virus (NPV) — a naturally occurring disease — builds up in dense populations and causes population crashes[18]
- Late spring frosts and high summer temperatures can also cause mass mortality[3]
What You Can Do
Municipalities across central Alberta are not recommending large-scale pesticide treatment because the outbreak is natural and temporary. However, for individual trees on your property:[10][11]
- Handpick or knock off caterpillars with a strong jet of water; they drown easily in a bucket[16]
- Wrap tree trunks with duct tape or foil coated with Vaseline or Tanglefoot to block migration to unaffected trees[16]
- Bacillus thuringiensis kurstaki (Btk) — a naturally occurring bacterial insecticide — can be sprayed on leaves and kills caterpillars that eat them; works best on small early-stage larvae[19][16]
- In the fall, remove egg bands from the lower twiggy ends of branches — each band contains ~300 eggs[14][16]
- Don't fertilize stressed or defoliated trees, as lush nitrogen-driven growth combined with defoliation adds further stress[16]
The hairs are worth noting for you personally: while the caterpillars don't bite or sting, their fine hairs can cause mild skin irritation or allergic reactions in sensitive individuals if handled directly. Given your outdoor environmental work, bare-hand contact is best avoided.[9][10]
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- https://www.yhcounty.ca/county-office/county-news/post/forest-tent-caterpillars
- https://www.caterpillaridentification.org/information.php?primary_name=forest-tent-caterpillar
- https://www.gov.mb.ca/nrnd/forest/pubs/forest_lands/health/forest_tent_caterpillar_brochure.pdf
- https://www.orkin.com/pests/moths/forest-tent-caterpillars
- https://www.dnr.state.mn.us/treecare/forest_health/ftc/id.html
- https://www.canr.msu.edu/ipm/diseases/forest_tent_caterpillar
- https://dnr.wisconsin.gov/topic/foresthealth/foresttent
- https://ladiescorner.ca/2026/06/13/caterpillar-surge-hits-pigeon-lake/
- https://montreal.citynews.ca/2026/06/01/just-be-amazed-alberta-is-seeing-a-cyclical-outbreak-of-caterpillars/
- https://www.olds.ca/news-and-notices/posts/municipal-mini-forest-tent-caterpillars-in-olds/
- https://www.reddeer.ca/whats-happening/news-room/forest-tent-caterpillar-activity-increasing-in-red-deer.html
- https://www.youtube.com/watch?v=i4d8o8vgLlU
- https://tidcf.nrcan.gc.ca/en/insects/factsheet/9374
- https://www.parklandcounty.com/news/posts/forest-tent-caterpillars-in-the-county/
- https://ground.news/article/forest-tent-caterpillars-feast-in-calgary-as-outbreak-spikes-population-across-alberta
- https://gardening.usask.ca/documents/grow-with-us-pdfs/foresttentcaterpillar_website2025.pdf
- https://www.play1037.ca/2025/06/10/15393/
- https://sfmn.ualberta.ca/sfmn/wp-content/uploads/sites/83/2018/09/PR_2000-31.pdf?ver=2016-02-25-091419-697
- https://www.reddeer.ca/media/reddeerca/recreation-and-culture/parks-and-trails/FOREST-TENT-CATERPILLAR.pdf
- https://birchfumigators.ca/blog/tent-caterpillars-2026-and-the-spring-of-the-swarm/
- https://www.ctvnews.ca/edmonton/video/2026/06/13/alberta-campgrounds-overrun-by-caterpillars/
- https://natural-resources.canada.ca/forests-forestry/insects-disturbances/forest-tent-caterpillar
- https://www.instagram.com/p/DY70RXLDJiV/
- https://www.cbc.ca/news/canada/calgary/forest-tent-caterpillar-moth-larvae-9.7218166
- https://kentuckylandscape.ca/caterpillars-in-alberta-trees/
- https://treefruit.wsu.edu/crop-protection/opm/tent-caterpillar/
- https://content.ces.ncsu.edu/forest-tent-caterpillar
- https://dec.ny.gov/nature/animals-fish-plants/tent-caterpillars
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