Tent caterpillars swarm once a decade, then vanish overnight

They appear in sudden waves, carpeting roads and crawling up walls, leaving silky tents in their wake. Then, just as abruptly, they vanish. Western tent caterpillars, with their striking orange-and-black bodies and hairy frames, are infamous during their peak outbreaks – and almost forgotten during their quiet years.

For Judith Myers, a professor in the Faculties of Science and Land and Food Systems at the University of British Columbia, these insects have been a lifelong research focus.

Her recently published study draws on five decades of work, offering surprising insights into their behavior, population cycles – and their resilience to climate change.

An unwelcome natural spectacle

Western tent caterpillars live across British Columbia, with dense populations on Vancouver Island and the southern Gulf Islands. Their range stretches as far as Manitoba to the east and California to the south. They favor the leaves of red alder and fruit trees.

Eggs hatch in April, and the young larvae gather inside silken shelters, or “tents,” which provide warmth and protection.

By early June, they leave these tents in search of vertical surfaces where they can pupate. They cluster in large numbers on fences, walls, and buildings. While this display fascinates some, many find it revolting.

In high densities, the caterpillars can strip fruit trees bare. On Salt Spring Island in 2012, a severe outbreak was so disruptive it forced the cancellation of the apple festival. Some tourists to the Gulf Islands have even cut their trips short upon encountering the swarms.

They are also inedible to most animals, and there has been one reported case of a horse becoming severely ill after consuming larvae.

Five decades with caterpillars

Myers has always been interested in what causes animal populations to rise and fall. She began her work in British Columbia in 1972 and soon encountered the caterpillars on Mandarte Island, where her husband was studying song sparrows.

Weekends spent in the field deepened her curiosity, leading her to focus on how these insects’ numbers boom and crash over time.

Her early fascination turned into a long-running investigation into the natural forces that govern their cycles. One of the key factors, she and other researchers discovered, is a virus specific to the species which drives population declines in much the same way seen in other moths.

Lessons from tent caterpillars

Working with Dr. Jenny Cory from Simon Fraser University, Myers found that outbreaks occur simultaneously on islands and the mainland. The team was intrigued to learn that some female moths travel tens of miles to lay eggs in areas where the species had previously disappeared.

Perhaps most unexpected is the species’ lack of response to a changing climate. Over 50 years, scientists have not seen any effect of global warming on such insect populations.

The caterpillars’ behavior offers a clue: they bask in sunlight during cooler weather and retreat under their tents during hot spells – a strategy that appears to shield them from temperature extremes.

This adaptability led Myers to reflect on whether humans might take a lesson from them. She wonders if our own global population growth is stripping the planet’s resources in much the same way caterpillars defoliate their host trees.

Cycles of caterpillar swarms

The last significant outbreak came in 2023, particularly on Galiano Island and other Gulf Islands, as well as Westham Island in the Lower Mainland.

By this year, the population had collapsed, with only one tent recorded in Myers’s Galiano study area. Such crashes are part of the natural cycle, with numbers expected to build slowly over six to eight years before peaking again.

The predictability of these cycles means outbreaks can be anticipated. Researchers often forecast them up to three years in advance, identifying early signs each April when small larvae and new tents appear in fruit trees.

Keeping outbreaks in check

While the sight of caterpillars covering fences and trees can be unsettling, Myers stresses that nature provides its own check. The same virus that drives their declines typically spreads as populations grow dense, eventually bringing outbreaks to an end.

In the meantime, fruit tree owners can take action early in the season. Removing egg masses in winter or tents in spring can reduce damage. Commercial growers often turn to BTK, a microbial insecticide that is most effective before caterpillars have done significant harm.

Western tent caterpillars may come and go in waves, but their story – shaped by mobility, adaptability, and natural controls – remains one of nature’s most intricate and enduring cycles.

The study is published in the Journal of Animal Ecology.

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