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Forest Tent Caterpillars

Forest Tent CaterpillarForest tent caterpillars (Malacosomadisstria) defoliate trees, and do cause them stress, particularly if the outbreak lasts for more than three years. Typically the caterpillars defoliate aspen, oak, ash, maple and white birch. Trees are weakened by repeated defoliation, which makes them more susceptible to stresses such as drought or other pests. However, the trees generally do not die. Population outbreaks occur every 10-12 years in the boreal forest and last about three years. In Thunder Bay, we expect a peak in the population in 2016, 2017, and 2018.

Forest tent caterpillars do not make tents, but rather weave a silky sheet where they lie together during molting.

Once the caterpillar outbreak slows down, expect another pest: friendly flies. They are native parasites that attack forest tent caterpillars and follow their populations. As the fly population increases, the caterpillar population decreases. Friendly flies do not bite humans.


  • Scrape off and discard overwintering egg masses and tear the protective tents out by hand before the larvae start to feed.
  • Control caterpillar movement and restrict access to feeding areas with Sticky Tree Bands or Tanglefoot Pest Barrier. However, the band should only be on the tree for a short period of time to avoid constricting the tree.
  • On privately owned trees one could choose to apply Bacillus thuringiensis, var. kurstaki (Bt-k) to the leaves to kill feeding caterpillars.

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