A dozen yellow aspen leaves, each with a single trapezoid of green still vibrant on the blade, fluttered to the ground. I’d just been sharing one of my favorite fall stories with sixth graders from Washburn Elementary School on a field trip to their school forest. They’d each picked up a yellow and green leaf and examined it closely.
We saw how a tiny moth larvae lives inside the leaf petiole, just below the blade, and nibbles on the green chlorophyll. Although the moth is not native to North America, the trees are not significantly harmed by these small acts of late-season parasitism. Non-native species aren’t always so benign.
I was about to move on down the trail when a couple of students started exclaiming about a tree trunk. A chestnut-colored husk hung there in a bit of a brownish web; tufts of golden hairs erupted from the segments. It looked like an empty pupa, where an insect had metamorphosed and emerged as an adult. “It’s one of those invasive things,” someone exclaimed. Emerald ash borer popped into my head, but these were aspens not ashes. “No, the moth!” a girl clarified. “And look, that’s their eggs!”
Squinting up to where she pointed, I saw a fuzzy, tan-colored lump on the pale tree. “Looks like a fungus to me,” I guessed, before waving everyone down the trail.
Later, I uploaded the snapshot of the pupal case to iNaturalist. When I saw the top ID suggestion, I smacked my forehead. The mystery and the egg mass were from spongy moths—the terribly invasive insect previously called a gypsy moth—just as the two girls had been trying to tell me. “Do you know how the students knew about the spongy moth? Have you been talking about them in class?” I asked the teachers in an email. Personally, I’ve been ignoring the headlines about them, which is why I didn’t recognize these two life stages.
“Two of my students have these moths all over trees at their houses,” wrote back Ms. Van Der Puy. “They climbed up into a tree and found what they are calling a “nest,” proceeded to poke it, which cause ‘tons of eggs to fall out.’ They were super intrigued by this, so they went in and looked it up. And THAT is how they knew about the spongy moth.”
I asked Ms. Van Der Puy to tell the girls that I’d learned something from them on the field trip. Then I called Paul Cigan, Plant Pest & Disease Specialist for the Wisconsin DNR.
Paul gladly took my call, because the northern parts of Douglas, Bayfield, and Ashland counties are experiencing a severe outbreak of spongy moths. These two girls’ homes are in the epicenter! And it’s not over yet.
The adult moths are not the problem. They don’t feed at all. The female sits on a tree wafting pheromones into the breeze. Those scent chemicals are so strong, that if you squash her with your shoe, you’ll attract a horde of male moths, too.
A male flies to her, they mate, and she lays a pile of eggs with a spongy consistency. It’s appropriate that the insects are named for their egg stage, because that’s how they spend 75% of their life cycle. Those eggs can endure negative 20 degrees Fahrenheit, and like so many critters in the subnivean zone, they are insulated by the snowpack when they are attached near the base of the tree. Milder temperatures near Lake Superior also increase survival.
In May, as new leaves unfold, caterpillars hatch from the eggs and begin feeding. For five to six weeks they eat. And eat. And strip every leaf down to its veins. While spongy moth caterpillars can eat more than 300 species of deciduous and evergreen trees, oaks and aspens are their main targets in northern Wisconsin. Healthy trees can survive, but this summer ended with a drought, so more trees might succumb to the stress of having all their leaves nibbled off.
Next spring the caterpillars will emerge anew, for what Paul hopes will be the last year of a major infestation. Then there will be little left to eat, and the caterpillars themselves will start falling prey to more dangers. A cool, wet spring would be helpful, Paul told me. That weather means more caterpillars die of fungal and viral infections. Who would have thought we’d be cheering for a virus?
In addition, non-stinging, parasitic wasps—introduced in 1908 to help control the moths—lay their own eggs in the moths’ egg masses. Wasp larvae eat the moth eggs.
Predators on the moths’ various life stages include a long list of beetles, flies, stinkbugs, spiders, harvestmen, ants, chipmunks, shrews, voles, skunks, raccoons, squirrels, cuckoos, orioles, robins, crows, nuthatches, blue jays, and even my beloved chickadees peck at egg masses.
Spongy moth caterpillars are covered in irritating hairs, and larger ones hide at the base of trees during the day. This makes them challenging to eat, but deer mice will skin and gut them before feasting.
Unfortunately, natural predators aren’t able to control spongy moths on their own. At this time of year, you can help by scraping the eggs masses off your trees and putting every last one into soapy water and then the trash.
If you need help identifying spongy moths on your trees, I know of a couple sixth graders who are already experts!
In addition, the University of Wisconsin Extension has handy references on their website, and experts are available to give advice at the Spongy Moth Hotline, (800) 642-MOTH, or email@example.com.
Emily’s award-winning second book, Natural Connections: Dreaming of an Elfin Skimmer, is now available to purchase at www.cablemuseum.org/books and at your local independent bookstore, too.
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