Yes, those large black clumps of wriggling things you see in the crooks of yours trees, or clustered on the trunk or branches, are Forest Tent Caterpillars. They grow around 5 centimetres long and have a striking blue strip and what appears to be brown fur.
Are they harmful?
While they can make your trees unattractive 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. In urban areas they may attack cherry, oak, birch and ash trees.
Do they damage trees?
Although your tree may look like it’s dead with no leaves, don’t worry! Aspens (and most other trees) can survive a few years of these attacks. The biggest issue is aesthetics. Most often the forest tent caterpillar will persist for 4 to 6 years but will die out from natural enemies.
Are they pests?
No, they are not a regulated pest under the Agricultural Pest Act
What to look for
Larvae
Adults
Prevention
You can 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 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. Just make sure that they cannot escape from your bag.
What if I have many trees?
If you have many trees, you may want to use a bacterial agent called Btk (Bacillus thuringiensis var kurstaki) to spray your trees. This will kill the larvae. Talk to your local pesticide dealer to see if they have Btk available.
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