The forest tent caterpillar (Malacosoma disstria) is a native insect common across the prairie provinces. The caterpillars hatch from eggs into their larval stage in late May.

Forest tent caterpillars are 1-5cm long and black/dark brown/grey in colour with blue and yellow stripes. The caterpillars have long setae, giving them a furry look.

Plants Affected

Symptoms

Forest tent caterpillars can defoliate branches or even whole trees if populations are high.  Look for webbing on the trunk and ragged branches with no leaves.

The caterpillars like to gather together in large groups on the lower trunks of trees overnight and can often be found there in the early morning.

Outbreak populations occur approximately every ten years but any region may not experience an outbreak if late frosts or disease increase mortality.

Control

On smaller trees, control may be achieved by handpicking the caterpillars as they cluster on the trunk/branches early in the morning. Healthy trees seldom die from infestations of tent caterpillars.

Forest tent caterpillars are vulnerable to picking at all stages.  Remove egg bands from twigs in the winter months.  Pick and squish larvae and cocoons throughout the summer.  Adults can be found resting on buildings or concrete early in the morning.

Where trees are too large for hand picking of caterpillars, or for trees under heavy infestation, a properly timed application of a Btk insecticide (Bacillus thuringiensis var. kurstaki) should prevent branch defoliation.

Forest tent caterpillar cocoon. In outbreaks, these cocoons will cover many outdoor surfaces.
Caterpillars on a tree
Late stage forest tent caterpillar; almost ready to pupate into a moth
Egg-band of the F.T Caterpillar
Caterpillars at outbreak
Adult moth and her egg band
Webbing and outgrown larval skins indicate there are caterpillars in this tree
Defoliation damage and webbing