The Magical Mystery Tour of Tent Caterpillars

When I saw the first signs of tent caterpillars outside our flat, I kept the sighting to myself.

We have a neighbor, a home owner just up the hill who screams at crows — and who dead-heads her plants to the point of denuding them. I knew if she saw this tiny tent on the fir tree, she’d make short work of the squirmers inside.

This spring we had what’s called an “outbreak” of tent caterpillars in Seattle — Malacosoma californicum — a cyclical occurrence of every six to ten years where these white, diaphanous tents drape across branches of alder, apple, ash, birch, cherry, cottonwood, willow, fruit trees, and roses. The tents are shelter, shade, and molting sites for the vivid larvae inside. The caterpillars venture out in the day to feed on foliage, then return each night to hunker down in their fortress. The black speckles are frass, or caterpillar waste which accumulates in their temporary home.

As they feed, they turn leaves into lattice, finishing each cluster of green before moving up the branches to the next.

As damaging as this looks, the consensus of information is that trees in good health will survive these short spurts of caterpillar activity. Tent caterpillars may eat up to 20 percent or so of leaves during their four to six weeks of activity, but trees are adapted to rejuvenation.

Department of Natural Resources manager Karen Ripley, quoted in Small Forest Landowner News, explained that “the trees are not being devastated. Healthy trees are unlikely to be damaged, even if all their leaves are consumed. Deciduous trees are ‘used to’ replacing their foliage each year and will even produce a second crop of foliage in the next few weeks. The plants growing beneath the caterpillar laden trees are getting extra exposure to sunlight and a shower of nutrients in the feces and leaf fragments that drop from above.”

That’s why I was particularly annoyed when I came upon this note in a local arts publication:

“Every six to ten years — including now — populations of these voracious defoliators explode. The fuzzy, inch-long orangey bugs get their name from the nests they spin, where they amass in the hundreds. Word from Fish & Wildlife Dept: Kill on sight.”

I wasn’t able to find the “kill on sight” information at the Washington Department of Fish and Wildlife website, and have to assume it came from a referenced source. The USDA, when discussing forest damage by tent caterpillars says, “because western tent caterpillar is more of a nuisance than a damaging pest, infestations normally are allowed to run their course without intervention.”

Their documentation goes on to say:

A large complex of natural enemy species feeds upon western tent caterpillar and, together with unfavorable weather and foliage depletion, regulates population size. High populations usually collapse within a year or two due to natural controls, although outbreaks in the Southwest have lasted longer and caused significant aspen mortality.”

The Washington Department of Natural Resources adds that “even the trees reduce the digestibility and nutrient content of their leaves in response to caterpillar activity.”

I saw Facebook posts with tent caterpillar photos captioned with “ewwwww” and “creepy.” I also saw loads of misinformation suggesting that these caterpillars cause total deforestation and plague — with people clipping, burning, and poisoning the tents and larvae. This reduces a potential food source for birds that feed on caterpillars and also puts those species at risk where poisons are used.

That was earlier this spring and what people didn’t seem to realize was that in a matter of weeks, these hungry caterpillars would stop eating and seek individual cocooning sites, sequestering themselves for mothdom. Then, they’d emerge as brown tent caterpillar moths and lay eggs which would overwinter until next spring to repeat the cycle.

After that, they will be gone, reappearing again after many years for their magical mystery tour. I might be the only one in my neighborhood remotely interested in observing and photographing these once-a-decade visitors. I look forward to their return. And when those silky masses burst out of the branches of my local trees, no one else will know — if I can help it. 🙂