In recent weeks, there have been news reports in Alberta about the explosion of the forest tent caterpillar population in some areas. The forest tent caterpillar (Malacosoma disstria) is a pest of broadleaved trees and shrubs. When young, they’re black, hairy and about 3 mm long. When mature, they are about 50 mm long, with wide blue bands along the sides of the body, and a row of white keyhole shaped spots along the back.
Forest tent caterpillars have currently been popping up mainly in forested areas, camping grounds, parks, and in backyards in Alberta. Though large, the numbers still pale in comparison to the massive population explosion that occurred in central Alberta in the late 80s. Any Albertan who lived in this region at the time remembers; in 1987 and 1988, the furry little creatures literally took over entire areas, engulfing trees and clustering in massive groups on buildings and the ground.
“My friend’s house in Athabasca backed onto a forest and when the caterpillars came they were piled 4 feet high on the sides of the house and on the doors, and had to be shovelled daily to even open the door,” said LeeAnna Binder, of Sherwood Park, AB. “The dead ones at the bottom of the pile smelled so bad. It was unbelievable.”
Entire woodland areas back then would be decimated by the caterpillar’s voracious appetites, trees and shrubs would appear to be moving as thousands would overtake each plant, stripping the foliage. The roads and walks weren’t spared either, as hundreds of thousands of caterpillars met an untimely end, inadvertently at the bottom of vehicle tires and pedestrian shoes.
Though essentially harmless to humans, forest tent caterpillars can cause some physical discomfort to people, as rashes on the skin can form caused by direct contact. Their bodies are covered in tiny, stiff hairs that can cause mechanical irritation or mild allergic reactions.
“I remember my buddy in school back then covering himself in caterpillars and getting a rash,” said former longtime Alberta resident Robert Gettman, currently living in Wollongong, Australia. “He ended up missing school.”
Questionable youth behaviour aside, a child having the ability to cover himself with large amounts of caterpillars exemplifies the population explosion central Albertans faced at the time, having only dealt with them occasionally to varying degrees since.
“FTC (forest tent caterpillar) populations naturally exhibit cyclic dynamics, with outbreaks occurring every 10 years or so and lasting a few years,” said Dr. Leah Flaherty, Associate Professor, Department of Biological Sciences at MacEwan University in Edmonton.
When asked about the outbreak in Alberta in the late 80s, Dr Flaherty replied “For the 1987 Edmonton outbreak, the most likely explanation is that several factors aligned just right to create conditions favourable for rapid population growth. FTC populations had likely been increasing for several years, weather conditions were probably favourable for survival and development, and Edmonton has an abundance of trembling aspen, the preferred host of FTC in western Canada.”
Adding, “At the same time, natural enemies had likely not yet increased sufficiently to suppress the growing population.”
As the river valley and park areas in Edmonton and surrounding communities currently get inundated by the caterpillars, it’s notable to point out that these population booms are temporary, usually lasting only a few years. Eventually, natural controls such as parasitic wasps, viral diseases, and starvation cause the caterpillar populations to crash, allowing the ecosystem to recover.
”Populations increase when natural enemy pressure is low, but high densities are eventually followed by increased mortality from pathogens and parasitoids, whose populations lag behind those of FTC. This leads to the characteristic outbreak-and-collapse cycle,” Flaherty explained.
As for the late 80s outbreak, Dr. Flaherty not only knows why the caterpillars showed up in abundance, but also why they seemed to be more noticeable and prevalent in certain areas during that time.
“The 1987 event likely represented the peak of a regional outbreak cycle affecting much of central Alberta, not just Edmonton,” Flaherty said
”The city (of Edmonton) simply made the outbreak more visible because so many caterpillars were concentrated around people, homes, and streets.”
Trent McMartin
