2022 is shaping up to be a big year for western tent caterpillar. You may recognize this native critter, best known for its fondness for the leaves of many of our fruit trees including apples and plums. Tent caterpillars are the larval form of the western tent caterpillar moth.

Life Cycle

Tent caterpillars only live for one calendar year. They start off as groups of eggs laid together in a single egg mass on trees and shrubs. Here on Mayne Island, we have observed them most often on thinner branches (1-3cm diameter) of their favorite host trees such as red alder, apple, or plum. During the winter, they shelter in their eggs while growing into fully formed (though tiny) caterpillars. Come April, the tiny caterpillar family emerges and begins munching on the leaves of their host trees. While eating, they work together to build tents of silk in the tree’s branches to protect them. This helps them continue to eat undisturbed by predators and bad weather. Once they have eaten the leaves near their nest and grown into full-size caterpillars, they disperse from their family group to look for suitable places to pupate. After finding a sheltered place, they build themselves a tightly wound cocoon so they can transform into a moth. Pupating takes two to three weeks, and once they transform into a fully formed moth they emerge in late July or early August to mate and lay eggs, starting the cycle all over again.

Egg masses can be picked off of fruit trees by hand during the winter before the hatch in early April.

Agricultural Impact

In most years, there aren’t too many of these caterpillars around and their impact on local fruit trees and native deciduous trees is minimal. However, occasionally their populations can grow so large that they eat a large portion or all the leaves on a given tree. In 2012, the renowned Salt Spring Island Apple Festival was cancelled due to a combination of poor spring weather and unusually high damage from western tent caterpillars that caused a crop failure for many apple varieties. Ten years later 2022 is shaping up to be another big year for the western tent caterpillar.

Recently hatched colonies of western tent caterpillar stay together and can be removed by hand.

Recommendation for Fruit Tree Growers

Though the western tent caterpillar is a native species and a valued part of our local ecosystem, it can be a significant agricultural pest for fruit growers in years when populations are high. We recommend anyone with apple or plum trees take a close look at their trees as soon as possible and remove by hand any of the small tents they find. Some local growers have been reporting as many as 50 or more nests per fruit tree this spring, which could result in a significant impact on fruit production this year. The small tents can be easily picked off by hand if caught early enough, with even large trees being managed within an hour or two. If left unmanaged, the nests will grow and have a more significant impact on the tree.

Conservancy staff first noticed the increase in population during the winter when they found hundreds of egg masses laid on the red alder seedlings planted at one of our habitat restoration sites. Staff picked off all the egg masses they could find to prevent stressing the young trees. Once the trees are mature, they will be more resilient and can provide food and habitat for the western tent caterpillar and a wide range of other native species.

Photo credit: Washington State University

The Natural Balance

The western tent caterpillar’s main predator, and the one typically thought to be responsible for reining in population explosions, is a native species of parasitoid fly that lays its eggs on the caterpillar’s head. When the eggs hatch, the fly larvae begin to eat the caterpillar alive. So next time you curse the western tent caterpillar for eating your apple tree, you can find comfort knowing the fate that may lie in wait for it. Look for the white egg masses on caterpillar heads later this summer. You may hear that this predator – prey relationship cycles every seven year but the truth is that nature doesn’t stick to a schedule. These relationships are complicated and influenced by other organisms within the environment as well as weather in any given year.

More info about western tent caterpillar

Vancouver Sun article about 2012 Salt Spring Apple Festival Cancellation


Suzanne Stirling · May 26, 2024 at 9:14 am

I think 2024 is a parasitoid fly year! I have photos from Pender Island of lots of eggs on caterpillars.

    Rob Underhill · June 27, 2024 at 9:38 am

    Yes, lots in 2023 as well. I was surprised the population of tent caterpillar did not crash more between 2023 and 2024.

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