Forest Tent Caterpillars

Yes, those large black clumps of wriggling things you see in the crooks of your trees, or clustered on the trunk or branches are Forest Tent Caterpillars.  They grow into approximately 5 centimetre long caterpillars with a striking blue stripe, and what appears to be brown fur. Other than the fact that they can really make your tree unattractive in a short period by eating all the leaves, they aren't that harmful.  Their food source is the leaves of the trees they hatch on – mainly aspen, sometimes willows; and in urban areas, cherry, oak, birch and ash trees may be attacked. They are not classified as a Pest under the Agricultural Pest Act, therefore, the County of Grande Prairie does not pursue control measures for them.

What To Look For


If you have a few trees, spray the caterpillars with strong streams of water from your garden hose until they fall to the ground. Once on the ground, squash them so that you can ensure they are not able to breed or return to your trees. If you are a bit squeamish, gather them up in a bag and put them in the garbage, ensuring they cannot escape. If you have a large number of trees, this may not be an effective solution. There is an option to use a bacterial agent called B.t.k. (Bacillus thuringiensis var kurstaki) to spray your trees. This will kill the larvae. Talk to your local pesticide dealer to determine availability.

Although your tree may look like it's dead with no leaves, be assured that aspens (and most other trees) can survive a few years of these attacks without too much concern.  The biggest issue is largely aesthetics.  Most often the forest tent caterpillar will persist for between 4 and 6 years, but will eventually die out from the action of natural enemies.

For more information please call 780-532-9727.