Everything you need to know about the forest tent caterpillar, a native forest pest
By: David Dutkiewicz, ISC Insect Diagnostician
Over the past number of years, the march of the forest tent caterpillar (Malacosoma disstria) has been ramping up. This species, also referred to sometimes as “army worms”, can become a major pest in Canadian forests. But rest assured – forest tent caterpillars are not an invasive species! Invasive forest pests usually cause rapid tree mortality due to a lack of host tree defences. While forest tent caterpillars can cause major disturbance in the forest canopy, their relationship with forests has been evolving for hundreds of years allowing, in most cases, minor impacts to the forest as a whole.
In 2018, the Ontario MNRF mapped approximately 1.1 million hectares of forest disturbance due to the forest tent caterpillar. Trees were stripped of their leaves in early summer, leaving large areas of forest with no foliage. You may also have seen them around your yard and stuck to your car.
Forest tent caterpillars are easily distinguished from other similar species by the white shoe print like marking trailing along the back. Some like to refer to these markings as “footprints through the forest”. Forest tent caterpillars are unique in that they do not build a tent structure for protection – instead, they prefer to form large clumps and roam around the forest looking for leaves. However, they often end up on the side of houses, cars, mailboxes, and backyard sheds, too.
There have been periodic pest reports of the forest tent caterpillar dating back to 1835, and they reach outbreak status approximately every 10 years. These caterpillars can be found on all broadleaf trees, with a preference for aspen and maple trees. Forest tent caterpillars are native to Canada and have predators that help control the pest population during an outbreak. The friendly fly (Sarcophaga aldrichi), which is native to North America and strongly resembles a common house fly except for three black stripes along its back, are parasitic and will find the cocoons of the forest tent caterpillar to lay their eggs on them. The fly maggots will hatch and eat the developing pupa before dropping to the soil to overwinter. Populations of this fly naturally increase in response to forest tent caterpillar outbreaks and signal to scientists and other forestry specialists that the end of an outbreak is approaching. Fortunately, in 2018 there were multiple reports and samples sent from the public to the Invasive Species Centre insect diagnostic lab of the friendly fly – which will hopefully mean the end of this 3-year forest tent caterpillar outbreak.
It is also interesting to note that there are two other species of tent caterpillar native to Canada that can be found in prairie and central provinces: the Northern and Eastern tent caterpillar. These caterpillars are similar in size to the forest tent caterpillar, but their biology differs slightly and they are often seen as not creating significant impacts on forest ecosystems.
Northern tent caterpillars (Malacosoma californica) can usually be found in tent-like structures on cherry trees, but are also known to infest birch, aspen, and apple trees. The distinguishing feature of this caterpillar is the mottled orange and black pattern on its back. Northern tent caterpillars become most noticeable in mid to late June, beginning to pupate towards the end of July. This pest is usually found roadside in “ditch trees”, along forest edges, and in garden ornamental trees (alerting homeowners to their presence by causing localized damage to trees).
Eastern tent caterpillars (Malacosoma americana) have similar biology to the northern tent caterpillar as they are most often found feeding on cherry trees (and occasionally apple). Eastern tent caterpillars are distinguished from other tent caterpillars by a very distinctive white strip down the length of their backs. They are often found in large silk tents in tree notches where branches shoot off. They are usually found with 1-2 tents per tree, but in some areas, there can be several tents per tree. These caterpillars usually stop feeding around early July when pupation occurs, at which point they are seen as white/yellow dusty silk cases under mailboxes, along house fascia, and in the cracks of tree bark. This pest is also found along roadsides and in ornamental trees.
So, while you’re out enjoying the beautiful weather this summer, don’t panic when you see an abundance of tent caterpillars. These critters are not an invasive species, and the cycle of their forest monopoly is almost over – for now!
(1/2: Forest tent caterpillar; David Dutkiewicz)
(3: Northern tent caterpillar; NRCan, 2015)
(4: Eastern tent caterpillar; Ric Bessin, University of Kentucky Entomology, 2004)