As the weather warmed earlier this spring, we started spending more time outdoors, enjoying our parks and gardens, admiring the flowers and watching them grow. But amongst the blossoms and the fruit, you may have noticed something else in the trees that is a much less welcome sight: Tents. Growing around the tips of tree branches, alive with rapidly growing larvae we would soon see dropping from the treetops on silken strands, and on the move on sidewalks, walls and patio railings.

They’re living things we love to hate: Tent Caterpillars. Considered a pest, a nuisance, a destructive intruder we have a duty to eradicate. Some years they experience a population boom and seem to be everywhere, growing out of control. Then within just a few weeks we reach late summer and almost overnight they seem to just disappear, forgotten until next year. But how much do we really know about these little misunderstood creatures?

Of the 26 known species of tent caterpillar worldwide, six are native to North America, and three are commonly found in Canada: The Forest Tent Caterpillar, the Eastern Tent Caterpillar, and the one we know and love here in Greater Victoria – Malacosoma californicum, the Western Tent Caterpillar. They are identified as “reddish brown on top and pale underneath. They have a row of blue spots on their backs, with orange spots mixed in between. The adult moths are orange-brown with yellow lines on the wings” (Government of Canada)
            The life cycle of the Western Tent Caterpillar begins in early fall when a female moth lays between 100-350 eggs in a band around a tree branch, encased in a hard material which protects them against predators and weather. The eggs hatch soon after into larvae who remain dormant through the winter, and become active when the weather gets warmer in springtime when they go on the search for food.
            Tent caterpillars are very social creatures. One surviving egg mass will stay together during its 4-8 week caterpillar stage. They produce silk which is spun into a large tent they all live in together when not out foraging for food. The tent offers protection from weather and predators, but they also live in a large group together for warmth and social interaction. Sometimes several egg masses will join together to form a larger colony.
            The Western tent caterpillars build their tent around the tip of a tree branch and in Greater Victoria you may commonly find them in oak and apple trees. They also favour willow, poplar, plum and cherry trees, compared to eastern tent caterpillars which prefer different species of trees and build their tent in the fork of larger tree branches. Forest tent caterpillars build a wide mat around a tree trunk.
            Tent caterpillar eggs are about 1mm (one quarter inch) but mature caterpillars grow to be two inches long. In order for this to happen they need to eat, and they do – a lot! As they do much of this in spring when new leaves are forming on trees, the visibility of their eating is seen by humans as mass destruction. They also travel when they get older looking for suitable places to build their cocoon, which is why they can be found on roads, sidewalks, patios and crawling up building walls. Because of their noticeable presence, the caterpillars can be considered a nuisance, and in high population years even an infestation.

The Truth

But are they really that bad? True, caterpillars cause huge tree defoliation, but they don’t actually harm the tree trunk or root system. Tent caterpillars rely on the trees for the survival of their species into the next year, it would serve them no benefit to kill or make them sick. Healthy trees can re-leaf later in the season with tougher leaves that are less appetizing to predators. The amount of leaves falling this time of year are the proof of how many were growing through the summer months.

Western tent caterpillars build their cocoon around August and only spend a few weeks in it before emerging as the adult moth. They only live for a few more weeks, during which time their most important purpose is to mate and lay their eggs. The moths are much less noticeable then the caterpillars which is why we tend to forget about them. Moths reach the end of their life cycle in early fall, and die off soon after egg laying.

Every ten years or so, there is a mass population boom of tent caterpillars which often continues for two or three consecutive years. This could be weather related. Warmer winters offer the dormant larvae a better chance of surviving to spring. Other environmental factors can be favourable to tent caterpillar survival and development but be less favourable to the predators who keep their populations under control.
These boom years is when people tend to be most concerned about tent caterpillars, but it is not necessary for humans to take tent caterpillar population control into their own hands. While many people feel like it is part of responsible yard management to destroy or remove the tents, nature is able to take care of things and overpopulation can be naturally brought back under control in a few years.
Overpopulation is to the caterpillar’s detriment as they overeat available resources, leading to food shortages and starvation in later years. Cold winters can kill off larvae so they don’t reach mature caterpillar stage. And both the caterpillars and moths are food sources for birds, bats, insects and other predators. While there are pesticides designed and recommended for tent caterpillar removal, it is very important to consult an expert source for guidance to ensure other plants or animals are not harmed in the process.

Tent caterpillars are a native species, meaning they are part of the native ecosystem. While not always evident to people trying to raise non-native plants – for instance roses, hydrangeas and rhododendrons – believed to be “at risk” by these perceived pests, it is important to remember that every native species plays an important part in the survival of the ecosystem. Western tent caterpillars serve their role, and the best thing humans can do is leave them and their predators alone so the ecosystem can continue to thrive for years to come.

Written by Darcy Harnadek, HAT volunteer.