Class: Insecta
Order: Lepidoptera
Family: Lasiocampidae

Host plants: broad-leaved deciduous hardwood trees

Adult size: moth wing span = 1-1.75 in (2.5-4.5cm)
Larva size: length at 5th instar = 2.0 in (5.0cm)

Description & Life Cycle: Young larvae emerge in spring coinciding with the leaves unfurling on hardwood trees. Timing is dependant on regional weather. Just an eighth of an inch long (3mm) and almost completely black, they are distinguishable by the hairs covering their length.

For the next 5 – 6 weeks, the larvae eat their way through 5 instar stages, moulting out of their skin as they outgrow it. With each successive shedding, bright blue markings appear down both sides and slowly get larger, as do the white footprint-like marks down their centre back. By the end of the 5th instar stage, the caterpillars measure about 2 inches (5.0cm) long.

When they are ready to pupate at the end of the 5 or 6 weeks, the caterpillars spin cocoons of pale silk covered in a yellow powdery dust, usually curling up leaf. But they will use any convenient, out-of-the-way spot including up under the eave of the house or tool shed. In 10 – 14 days the adult moth emerges.

Adult moths have stout, buff-coloured bodies. The wings are yellow-brown to orange-brown in colour with two darker lines in the middle of the forewing and a darker band decorating the hindwing. Females tend to be a lighter colour than the males. Wingspan ranges from 1.0 – 1.75 inches (2.5 – 4.5cm).

The moths do not feed…living a scant one to ten days. Their sole purpose is to mate. Once a female is fertilized, she lays 100 to 350 eggs in a one inch (2.5cm) mass on a small diameter twig and encases them in spumaline – a frothy, glue-like substance which hardens and turns a glossy, dark brown colour. This protects the eggs from small predatory insects.

Within three weeks, the embryos have developed into larvae. At this stage, they will diapause through the winter inside the eggs.

Special Notes: Malacosoma disstria are native to North America, more commonly found in the eastern regions of the continent and as far west of the interior region of British Columbia, Canada south into California.

Forest tent caterpillars do not form a tent such as those one sees encasing the western tent caterpillar, Malacosoma californicum and northern tent caterpillar, Malacosoma californicum pluviale. You usually spot the tents of the latter two species at the tips of the tree branches, whereas the forest tent caterpillar is found further along the branch towards the trunk. They like to congregate in a silken mat to rest or moult. During the early instar stages, they are usually found in the very upper reaches of a tree. In the latter stages, they are normally lower down in the branches or on the trunk.

Large outbreaks of forest tent caterpillars typically happen roughly every seven to fourteen years and can last for two to three years in a row.

The adult moths are nocturnal and highly attracted to lights.

It has been discovered that the adult moths can be dispersed over a much larger range if weather conditions are right. One such incidence occurred in Alberta in 1964 when a cold front came down from the north and carried the newly emerged moths from the Edmonton area south as far as Calgary and surrounding communities. This is a distance of about 300 km (185 miles).

Unless it is a severe outbreak in numbers, Malacosoma disstria rarely cause the death of a tree. While whole masses of them coating the branches and/or tree trunk look unsightly…not to mention the denuding of leaves on the branches…unless the tree is already in distress from other factors such as drought, it will recover. Unfortunately, if the infestation happens to be on your fruit trees, it may be at the expense of your crop.

Natural Controls: Predators include frogs, small mammals and a large number of different bird species.

There are also a large number of insect predators including bees, wasps, flies, ants, beetles and earwigs that will prey on the forest tent caterpillar throughout various stages of its life span.

During large outbreaks, numerous diseases are known to develop amongst colonies of Malacosoma disstria which can be directly attributed to controlling their numbers. One such disease is the nuclear polyhedrus virus (NPV). Often, an infected dead caterpillar will be seen hanging in place from a branch by its mid-section…its body forming an inverted V-shape.

Caution: The yellow powdery dust on the cocoons may cause an allergic reaction in some people.

In our Zone 7a garden: We have been finding forest tent caterpillars in our garden for the last three years now. This is apparently unusual for them to be as far west as Vancouver Island, according to Dr. Linda Gilkeson, an entomologist friend on Salt Spring Island. But they have somehow found Black Creek as one neighbour for sure has this caterpillar species on a number of their trees, including cherry and damson plum.

The caterpillars emerge in late April or early May. This year (2013), we spotted a mass exodus from our spartan apple tree on May 19th. On May 24th, John saw another colony doing the “ant army” nose-to-butt pheromone trail up the trunk of our Italian prune plum tree where they have congregated in three masses of bodies in different parts of the tree to moult.

For more information on the distribution of the forest tent caterpillar in BC, follow this link: http://linnet.geog.ubc.ca/efauna/Atlas/Atlas.aspx?sciname=Malacosoma%20disstria

Posted on May 28, 2013; more photos added May 30, 2013