cals.cornell.edu
moth egg mass.
Frass: noun. The excrement of insect larvae.
Insects are excreting all the time, but it usually doesn’t negatively impact our human lives. When we started to see little caterpillars (aka larvae) on our porch in late April, we took a magnifying glass to one. It was gray and bristly, with five pairs of blue dots and six pairs of red. Uh-oh.
We flashed back to a hike years ago when we heard the distinct sound of spongy moth (formerly known as gypsy moth) caterpillars chewing coming from all the oak trees in the nature preserve. The mass munching was eerie and alien. We were trying not to step on all those hairy caterpillars. The oak tree leaves skeletonized in real time, only their veins remaining, sadly twisting in the breeze. Stinky brown frass was all over the place. Super gross. But also fascinating; I like insects. Entomology was one of my favorite courses in college— and one with the most long-term relevance in my life.
When spongy moth (Lymantria dispar) larvae started descending from trees on silken threads around our place—a process called ballooning, whereby the wind distributes the young caterpillars beyond their point of hatching from eggs—we were in denial. “Maybe it’s just a few, it’ll be no big deal.” American Robins, Common Grackles, and Gray Catbirds collected little caterpillars to feed their young; the birds will take care of it!
Alas, no. Once the munching began, frass rained upon us for weeks, finding its way into every crevice imaginable. Hikes along the rail trail became precarious as the spongy moth frass mixed with rain to create a slick surface. More than once I pulled a small caterpillar from my cleavage. Ewww. My husband and I usually enjoy “driveway birding” in the spring. Not this year.
According to ecologist Clive Jones in “What You Need to Know about Spongy Moths: A Q&A” on caryinstitute.org, this was a peak outbreak year for our region (the Hudson Valley). Jones says, “Spongy moth caterpillars have been recorded to eat the leaves of more than 300 species of woody plants. But they have preferences. If oaks are unavailable, then they will eat leaves of trees that are lower on their preference list.”
In our yard and neighborhood, the red oaks (Quercus rubra) and pin oaks (Q. palustris) were hit hardest, including the mature pin oak right over our little house’s main entrance. The trees that were unaffected included sugar maple (Acer saccarhum), slippery elm (Ulmus rubra), tupelo (Nyssa sylvatica), and conifers. (Here, I am imagining Steve Harvey on “Family Feud” asking “What is the best solution to any ecological problem?” and someone hitting the buzzer: “Species diversity!” and DING, that is number 1—and the families lose their minds.)
“If your trees are moderately to fully defoliated, they will very likely re-flush new leaves by the end of July. If there is a summer drought, make sure those trees have adequate water. Add mulch if needed to help retain soil moisture. Most healthy trees will survive two, sometimes three, years of defoliation but additional stress—drought, other pests, diseases— increases the risk of mortality.”
—Clive Jones, ecologist, from “What You Need to Know about Spongy Moths: A Q&A” on caryinstitute.org
Though pitiful-looking, the oaks around us valiantly regrew a sparse canopy. We could see the traffic much more clearly over the ridge than we do in typical canopy years. The frass, pupal cases, and caterpillar parts remained a big part of life for a while longer, a reminder of the chaos. The barnyard-like smell persisted. Divots in driveways seem to be permanently sealed with concretized spongy moth caterpillar frass.
When this all started, we of course looked online to See What We Could Do. My favorite resource on this was the Mass Audubon (massaudubon.org) page about spongy moths. It outlines why hand-picking the caterpillars, applying chemicals, or rigging traps are not the solution. What does work: making sure you have species diversity in your yard, favoring the species that the spongy moths don’t. Biological controls like a gorgeous beetle, Calosoma sycophanta, have also been helpful.
The other thing that helps is to scout for spongy moth eggs masses in fall and winter. Once you start looking, you will be amazed at how many of these buff-colored masses are around. You scrape them off and boil them or put them in soapy water. Of course, those little caterpillars will still blow in from other places, courtesy of the ballooning described above.
But for me, in the midst of fretting about the oaks, about the mess, Mass Audubon put the spongy moth situation in perspective beautifully:
While a disheartening sight, the long-term effect of the phenomenon is not as disastrous as some commonly assume and may in some ways be beneficial. Thinning of forests by spongy moths may produce a healthier, more diverse, and perhaps a more spongy moth–resistant stand of trees.
Moderate defoliation benefits forest wildlife by stimulating understory growth of shrubs and berry-producing thickets. The larval frass (droppings) fertilize the soil, the larvae provide food for birds and mammals, and the skeletal remains of trees that succumb provide habitat for wildlife, thus promoting diversity in the forest ecosystem.
Well, then.
Accepting that the spongy moths have some long-term benefit to the overall ecology—that was the best treatment. Now back to scrubbing the bottoms of my shoes.
From gutters to the crevices of our cars, the spongy moth mess is ongoing. Why do I find it so unsettling? I’ve farmed, made compost daily for whole summers, harvested armfuls of kelp from lakes to put in said compost, brushed many a species of insect off my skin and clothes. When I was a professional gardener, all sorts of things fell out of my clothes at the end of the day. So why did I feel unhinged in the midst of the spongy mothnado?
It’s the concentration of the thing. The sheer number of caterpillars and their frass, and the smell of the latter as it accumulates, is just a drag. It interferes with our enjoyment of nearby nature, our favorite pastime.
But also. The pandemic has had lasting effects on me. After all that hypervigilance about germs, I have a hard time relaxing amidst messes of all kinds. I imagine many of you can relate. More thorough handwashing is a productive outcome from that time, but other behaviors and reactions are not so proving so adaptive. My quiet freakouts are compounded by dust allergies that went bonkers after a bad respiratory infection that I’m still getting a handle on with medication and shots. Somewhere along the way, messes and sickness have become linked in my subconscious.
Outbreaks are localized and go in cycles lasting an average of two years before the spongy moth populations collapse due to two naturally occurring diseases that build up and take out the caterpillars. If we’re lucky, we’re spared an outbreak of the 2024 intensity for at least another ten years. But even if they come back in such full force in 2025, it will be unpleasant, but at least I’ll know what to expect. And maybe that will be my impetus to get back into therapy.
Michelle Sutton is a horticulturist, writer, and editor.
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