2023 has set a record in the Pacific Northwest islands for tent caterpillars. In the nearly 40 years I’ve lived here I’ve never seen so many, not even close. Islanders were variously amazed, horrified and disgusted this spring as armies of tent cats rampaged through foliage, stripping trees bare. I’d assumed everyone in the Northwest was having the same population boom, so it was a surprise to read this year was nothing special outside the islands (Guemes, Whidbey, and the San Juans). For folks missing their own tent cat invasion, here’s an account of what we’ve seen this year.
Our spring was much cooler than normal, with pollinator action so minimal our fruit trees mostly didn’t set fruit this year. But in the beginning of May we started getting more normal temps. That woke up the tent cat eggs. Very quickly tents appeared on all their favorite trees: native crabapples and cherries, willows, alders, orchard fruit trees.
Many thousands of tents, and uncountable millions of tent cats! So many, they swarmed out into plants they usually leave alone, like rose bushes, blackberry brambles, strawberry plants, even oak and madrona trees, stripping them bare too. People were reporting tent cats invading their houses and clothing and beds and cars. It was apocalyptic.
Here’s a 51-second video of a few tents and cats on a native Bitter Cherry tree I pass every day on my walk. Note how some cats are waving their heads around — that’s a defense strategy against predators.
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These are Western Tent Caterpillars, Malacosoma californicum, native to the western North America (there’s an eastern counterpart). Their annual life cycle is simple: eggs hatch in spring, larvae feed for six weeks until early summer then build cocoons and hatch out as moths, who mate immediately and lay eggs by midsummer. That’s it, one generation per year. Populations peak every 10 years or so, with a three-year outbreak followed by negligible numbers.
Why did we get so many this year? There are several likely contributing reasons according to entomologists.
1. Prolonged drought over the past few years. DNR entomologist Glenn Kohler said “Drought conditions will create smaller leaves with a greater sugar concentration….These caterpillars are getting a more nutritious food source right now.” Spring precipitation has been below normal on the island every year since 2015, according to my CoCoRaHS data, and for many of those years summer has been abnormally dry too.
2. Lack of viruses that usually control numbers. The primary pathogen driving the boom/bust cycle is nuclear polyhedrosis virus (NPV), which is inhibited by exposure to sunlight. In areas where trees have been cleared, like pastures and residential development, the tent cats were much more abundant (the part of the island where I live is more wooded than the village and we had many fewer tents). Researchers at University of Alberta studying Eastern tent cats found fragmented forests have longer outbreaks than continuous forest canopy.
3. Lack of predators. Tent cats are eaten by spiders, ants, yellow jackets, assassin bugs and birds, but only when larvae are small. Later instars have too many long irritating hairs. If insect predators aren’t around to feed on the cats when they are small, they won’t at all, and the cold spring we had this year delayed insect proliferation.
However, even a bad tent cat year like 2023 is not a catastrophe, by any means. This moth species is native, and has always had an important role in our western ecosystems. Per Washington State University:
Western tent caterpillar has a beneficial role in nature in that defoliated trees may become more efficient in their physiological processes and compensate by producing more leaves. Defoliation also allows light and water to reach the forest floor so the understory plants benefit. At the same time all the digested leafy material rains down as thousands of little caterpillar poops. When water moistens them, the microbes can feast, returning nutrients back to the plants.
Trees and shrubs also grow new foliage once the tent cats pupate. That’s already happening. The few trees that are killed in a major outbreak are young or sickly, or have been stressed over multiple years of drought. That could well become an issue as climate change continues to shift our seasons, as it has already begun to do.
In spite of the massive defoliation, they didn’t have much impact on native flowers and fruits, at least in my neighborhood.
Tent caterpillars provide food for other animals. Besides the insects and birds who eat them as larvae, the adult moths are food later in the summer.
To some extent they feed birds and their fledglings, but mainly the moths are a boon to our local bats, who are nursing their pups in mid to late summer.
A lot of humans go all Rambo attempting to eradicate tents in their backyard fruit trees, chopping off major branches or blasting them with a flame thrower. The reality is those efforts aren’t very effective, however satisfying they might feel, since the caterpillars roam widely in search of food and there are millions more out there. What really knocks back the tent cats is the NP virus that spreads rapidly in their crowded colonies. By June a great many of the tents were not only filled with frass but had dead caterpillars hanging from them, killed by the NP virus.
Yet another natural population control is parasitoid insects, in our area mainly tachinid flies. These lay an egg on a caterpillar’s head, and when the egg hatches, the fly larva burrows into the tent cat consuming its body from the inside. Ultimately the insect that emerges from the tent cat’s cocoon is a fly not a moth. I was seeing a lot of tent cats with white dots on their heads in May and June.
So, tent caterpillars are not our enemies. They have a role in our local ecosystems, natural controls on their population, and a biology that’s actually pretty amazing. Some fascinating facts about tent cats:
—> they can’t digest their food unless the ambient temperature is at least 60°. That means they can consume as much food as they want but the nutrients can’t be accessed unless it’s warm out. For that reason they build their tents where they get maximum sun, out in the open where they can bask in sunlight during the day. Trees in sunny spots (like along the shore or roadsides) were defoliated while the same kinds of trees just a short distance away but in shade were not. The cats will also congregate in a tight mass to raise their temperature.
—> they leave the tent at night to feed, laying down an “exploratory” pheromone trail so they can retrace their steps back to the tent in morning. If a cat finds an abundant food source and eats as much as it wants, it will lay down a “recruitment” pheromone trail on its way back so its buddies in the tent can follow the trail to find the good food source.
—> only about half the food they eat is digestible so cats generate a lot of frass. As the season goes on, their tents fill up with frass. Tent cats go through 5-6 instars, with the last instar being by far the largest. It’s estimated that 80% of the food eaten and frass generated comes from the final instar.
—> when they are ready to pupate, the cats descend from their trees and march en masse to suitable vegetation. They like grasses but in a big outbreak year they’ll congregate in thickets too. This year there were so many crossing the road in some spots they formed a solid moving carpet, and got squished in such huge numbers the road became slippery to walk on.
—> after pupating for two weeks, moths emerge and mate immediately. The females emit a pheromone attracting males, and after mating she lays her eggs on a branch, and dies. She lives as a moth for a day; the males live for a week.
The tent cat frenzy is over for the year now. By all indications 2023 was the peak of the current outbreak. There will probably be a lot of tent cats next spring but nothing like we saw this year. Then it will get quiet for another decade. However as spring droughts get worse in the Pacific Northwest with climate change, we can expect to see more outbreaks like this year’s in future.
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Sunny and dry the Pacific Northwest islands. Temp in high 60s, low 70s later. Light breeze.
What’s up in nature in your neighborhood?
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