Why these hairy caterpillars swarm every decade – then vanish without a trace

Western tent caterpillars might not be on your mind every year, but during their peak outbreaks, they’re impossible to ignore—hairy larvae wriggling across roads and swarms of caterpillars climbing houses to form yellow silken cocoons.

They’re certainly on the mind of Dr. Judith Myers, professor emerita in the faculties of science and land and food systems, who has spent five decades studying this native moth species and their boom-and-bust population cycles.

In this Q&A, she discusses her journey and findings from  a recently published study, including the caterpillars’ surprising resistance to climate change.

Where are Western tent caterpillars?

These hairy, orange-black caterpillars occur across B.C., especially Vancouver Island and the southern Gulf Islands, ranging as far as Manitoba and California. They mostly feed on the leaves of red alder and fruit trees. Eggs hatch in April and the larvae stay together, building a silken ‘tent’ for warmth and shelter, hence their name. In early June, the larvae leave their tents to find vertical surfaces and safer places off the ground to pupate. This can lead to dense clusters on fences, walls, and houses — a sight many people find revolting. After a year or two at high density, population numbers drop and tent caterpillars are temporarily forgotten. 

In large numbers, tent caterpillars can defoliate fruit trees. On Salt Spring Island, a severe outbreak in 2012 led to the cancellation of the apple festival. Tourists visiting the Gulf Islands have been known to cut their visit short in response to caterpillar outbreaks. And they’re apparently no good to eat, as they’re thought to have caused severe illness to a horse that accidentally ingested larvae.

What drew you to study these insects for five decades?

I've always been interested in what causes animal populations to rise and fall. After I moved to British Columbia in 1972, my interest grew when my husband and I were courting. He worked on Mandarte Island studying song sparrows, and I often joined him on the weekends. While he worked on sparrows, I observed the caterpillars—a fascination from a graduate seminar I gave years earlier. Over time, I became captivated by how their cycles interact with natural controls like viral disease.

What have we learned about these fuzzy critters?

Over the years, myself and other researchers have confirmed that a virus specific to these caterpillars was driving the populations’ cyclicdeclines, something seen in some other moth species as well. 

My research partner and SFU professor Dr. Jenny Cory and I observed that outbreaks occur simultaneously across islands and the mainland. We were surprised to find that as populations increase, some female moths must fly tens of kilometres to lay their eggs in areas where populations previously went extinct. 

Interestingly, over 50 years, we haven’t seen any effect of global warming on these insect populations. They’re highly adapted to the environment — basking in the sun when it’s cool and sheltering under tents when it’s hot. 

Perhaps we humans can learn about our own adaptability from tent caterpillars -  are we too defoliating our own “trees” with a booming global population which the Earth can’t support? 

When can we expect our hairy overlords to descend again?

The last major outbreak was in 2023 on Galiano and other islands including Westham Island in the lower mainland of BC. This year, we found just one tent in our study area on Galiano. That crash is typical, but tent caterpillars will begin to increase gradually over the next six to eight years to reach another outbreak. 

Outbreaks can be predicted, and their damage controlled: 

• As numbers begin increasing, we can often forecast an outbreak three years ahead. This is the time to start checking in April for small, hairy larvae and silky tents in fruit trees
• To protect your backyard, early intervention helps. If you see egg masses in winter or tiny larvae forming tents in spring, you can remove them before they damage trees
• Commercial fruit growers often use Btk, a safe and effective microbial insecticide. But it works best if applied early, before the caterpillars cause significant damage
• Remember not to panic: the virus acts as a natural control as the caterpillars become overcrowded