First published: The Victoria Naturalist, Vol. 82.1 (July/August 2025);
Updated here: July 29,2025
It was spring exam time at college. Pausing from studying for a few minutes, I was looking out my bedroom window, when I spotted a mass of caterpillars on the neighbors’ Douglas Fir tree. It had been their Christmas tree several years before and they’d planted it out front, near the corner of their yard. Every year I admired its beautiful, new, bright-green growth and marveled at how quickly it was getting tall. At this stage it was probably approaching fifteen feet, so it still had its lower branches, some hanging into our yard.
I told my mother what I’d seen and she promptly found loppers and collected the terminal two feet of the head-height branch loaded with caterpillars, bringing it to our backyard. My mother had a draconian view of any and all caterpillars. I remember bursting into tears, when I was very young, after complying with her sergeant-major order to stomp on my newly found caterpillar. What are you crying about? I wanted to keep it in a jar. Oh. I’m sorry.
Before she could kill the several hundred Douglas Fir caterpillars she let me clip some twigs, to put a couple dozen in a large peanut butter jar. She was a woman on a mission though, and didn’t pause for long. Holding the infested branch out at shoulder height she waved flaming sheets of newspaper beneath, so the tall flames rose through the branch, curling the needles and killing the rest of the caterpillars.
My rescued caterpillars were like pets for the next weeks. Every day I tended them, noting their growth and observing their behavior. I would come home from writing an exam, with that feeling of tired release when its finally over, and sit at my desk with the jar. I’d bring freshly cut branches from the tree, then holding the chewed branch next to the new branch, I’d help the caterpillars climb onto the new, then slip them into a clean jar. After watching the caterpillars for a while I’d clean the just vacated jar, washing away the caterpillar frass and leaf bits. This little routine was the perfect interlude between writing an exam and taking up books to study for the next.
As the caterpillars grew I watched some shed their skins, emerging just the same, but bigger. I was getting quite used to seeing this as they were growing until one day I was horrified when something different happened. I thought I was watching another shedding event, but instead the head of the caterpillar opened like a door and a white worm emerged. Leaving the shell of the now empty caterpillar behind, it crawled around using a peristaltic motion, bunching up, reaching forward, pulling up its rear, bunching up again, going in circles around the bottom of the jar. I had no idea what I was looking at, but it was revolting. Eventually the worm was still and became an oval that turned a dark reddish brown. Over several days more of the white worms appeared and hardened into dark brown shapes. I removed the brown things to a different clean jar. The remaining caterpillars pupated, creating cocoons in the first jar. Now I had two jars: one with cocoons, and one with brown things.
Exams ended and I started a full-time summer job on campus, one of several students helping an agriculture professor who was writing a book. Home from work one day I was surprised to discover sharp-edged black flies in the jar with the brown things – some of the brown things had new exit holes.
Ah ha! I was beginning to see. The white things were fly maggots and new adult flies had emerged from pupation. Next day I called the forestry entomology office on campus and asked if anyone wanted my jars of insects. Yes, please! I took the two jars, one with cocoons, the other with pupae and flies, and delivered them to the lab in question, before biking across campus to work.
A few days later I received a phone call at work telling me the moth species, and the flies, which were known parasites on those caterpillars. I didn’t remember the species names for long, but the concept stayed with me.
Now, years later, I have a cherry tree in my backyard, easy to see from the kitchen sink; it’s looking pretty ragged right now, as caterpillars are feasting on its leaves. But so too, birds are feasting on the caterpillars. For days I’ve been delighting in the parade of Yellow-rumped warblers, males and females, busy finding green caterpillars, large and small, and gulping them down. Yesterday, Savannah sparrows joined the crowd. And this morning, while watching more yellow-rumps, an entirely yellow warbler, best guess is a Wilson’s, popped into my binoculars’ view.
We know these spring migrants fly at night, stopping over during the day for the essential business of fueling up for the next stage. I watched a female yellow-rump perched up high, feathers fluffed, preening. Next to filling her belly, feather maintenance probably is critical for migration. The birds I’m seeing today may have arrived overnight, and yesterday’s birds may have already shipped out.
And so what about this cherry tree? Every year it is hit very hard, then it grows new leaves and looks fine again. Watching the birds I see they find a variety of caterpillars, occasionally a big fat green one, but also many small green caterpillars, most of which are winter moth, the one responsible for much of the damage.
They are small, green “inchworms” with legs at front and back but none in the middle, so they walk by inching along, pulling up the rear so the middle loops up high, then stretching the front end forward again.
Every year I wonder if I should do something about them. Every year I wonder if I’m a Bad Person to let this go on. My neighbors may be unhappy, as the caterpillars could spread, descending on silk threads to be blown by a breeze into their trees. Vast numbers rappelling into the unknown simply end up on the ground below, where crows casually swagger about picking them up, one after another. But the damaged trees look bad. One day I was standing on the sidewalk out front talking with a friend when a man driving a minivan went past, hollering out the open passenger window “the green worms are bad this year!” I tried to call back “they feed the birds!” but he was gone.
Winter moth caterpillars occur in such tree-denuding numbers because they are not native here. Unlike the Douglas Fir caterpillars, this ecosystem has no specialized predators that eat winter moth caterpillars.
In Europe, where they are native, there are parasitic predators specially adapted to preying on them. A particular wasp inserts its eggs directly into the caterpillars, and a particular fly lays its eggs on leaves the caterpillars are eating. Once those eggs are inside, either injected or ingested, they hatch and the larvae grow, eating the caterpillars from the inside, eventually killing them. In their native ecosystem this natural control means they don’t reach such tree-denuding populations. But now, here, where they were accidentally introduced – there is no specialized control – and they overwhelm the system.
And so what about this cherry tree? Winter moth numbers can be reduced by wrapping tree trunks with sticky tape to catch the flightless females as they climb the tree after emerging from pupating in the soil. Preventing new females from getting up into the tree can break the population cycle and limit their damage. I even bought the materials so that I could wrap the cherry’s trunk. But I couldn’t do it. Not yet.
Because birds eat caterpillars – soft bodied caterpillars are the best possible food for birds – and every year our cherry tree becomes a hot spot of activity during spring migration. This tree supports dozens, maybe hundreds, of migrating songbirds every spring. And the chickadees that nest every year in the box on a pole under the tree are constantly hopping throughout its branches, then flying back to the box where we hear them feeding noisy babies. As the babies grow the parents increase their feeding trips, so the sound of begging chickadee babies is almost constant, until suddenly they’ve fledged and the box is quiet again.
Many birds, such as chickadees, must have lots of soft-bodied caterpillars to feed a nest full of babies; there are estimates that to raise a brood of chickadees, just to the point of the babies venturing out of the nest, a chickadee pair will collect 6,000 to 9,000 caterpillars. Once out, the parents continue feeding them caterpillars as they learn to hunt for themselves. Supporting the migrants and the nesters is too important.
Clearly, winter moths are out of balance in this ecosystem, but there is more out of balance here than just that. Populations of other insects are too low, with even lower populations of the predatory insects that eat other insects, including caterpillars. Gardens planted with all the usual non-native plants, the kinds that fill the garden shops, are starving the ecosystems around us at the most foundational level. Because non-native plants do not support insects, particularly caterpillars, the way native plants do, the sun’s energy they capture through photosynthesis does not flow through the interconnecting trophic layers that make a functioning ecosystem.
Without native plants, and all the caterpillars that grow on them, populations of all the creatures that eat those caterpillars are diminished too. And so the myriad feed-back loops that would control insect numbers are broken; we have too few native caterpillars, too few caterpillar predators, and way too many winter moth caterpillars.
I’m adding native plants to my garden, hoping to jump-start a working ecosystem in my yard. But still I’m wrestling with the question of winter moth. Below the infested cherry tree I’ve planted native hawthorns, which support so many different caterpillars (98 species!) they are a “keystone” plant. But the winter moth caterpillars parachute in or rappel down onto the hawthorns, eating the growing tips, particularly their tallest leaders. In past I hosed, and hosed, and hosed the little trees, trying to remove caterpillars. But this year, for my first time ever, I resorted to BtK spray which, while non-toxic to us, kills all caterpillars. With unsteady hands I mixed the solution in the sprayer, pumped up the pressure, and sprayed mist onto the little trees. It feels very strange to be spraying a solution that kills caterpillars in a garden specially designed to support caterpillars. I tried to steer clear of a group of tent caterpillars on one branch, as I have no beef with them, but sadly the spray drifted and killed them as well. I did not want that.
A mix of native flowers I’m putting beneath the cherry will, I hope, support a broad range of insects that in turn will support insect predators. I don’t see a clear path that takes me from this unbalanced garden ecosystem to the caterpillar- and bird-rich garden that I want. Maybe, as my garden is filled with caterpillar-friendly plants, it will be okay to wrap the cherry trunk with ugly sticky tape, while still attracting the migrating birds. We’ll know we’re on the right track when more birds stop over to refuel and when more stay to nest.
In the The Little Prince the flower tells the prince she does not want the protection of the glass globe anymore, because “I must endure the presence of two or three caterpillars if I wish to become acquainted with the butterflies.”
And now we know that we must endure the presence of caterpillars if we wish to be acquainted with functioning ecosystems that support birds, and humans.
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*caterpillar photo at top by Reinhold Möller, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=131825842: Reinhold Möller
