by Celine Kagan
Maybe “storm” is a too strong word, but I don’t think “s#*t drizzle” is a thing. At least not yet.
Have you been hearing what sounds like the gentle pitter-patter of rain drops, every time you step outside? What you’re hearing is falling frass (the technical term for caterpillar poop), which has been raining down in pockets of Columbia county since May. I’m writing to you on June 24th, which is just about the apex of this nasty storm. Things will get quiet soon, but, sadly, it’s only the quiet before next spring’s s#* storm.
The spongy (formerly gypsy) moth (Lymantria dispar), currently in caterpillar form, is the invasive critter responsible for all the frass, leaf litter, and disturbingly defoliated trees you might be noticing around you. Many Columbia County residents are experiencing plague-level infestations of spongy moth caterpillars, some for the first year, some for a second or third year running. (I’m in year two.)
Desperate, dismayed, disheartened, and disgusted, I decided to learn what a layperson can about what’s happening and what we can expect. This is me sharing that information with you. Ready? Let’s do it.
Spongy moths came to the US in 1869, from Europe, initially brought ON PURPOSE to Massachusetts for silk production. (PS: They don’t make silk.) As the story goes, a few spongy moths escaped from the lab where they were being held and quickly started wreaking havoc. Initially, active measures were taken by local and state governments to keep the population under control, but those efforts were largely abandoned in the 1970’s. The 1990’s saw the formation of Slow the Spread, a non-profit organization that is working with the USDA to resume efforts in controlling this voracious, invasive insect.
Identifying spongy moth caterpillars is easy. They have five pairs of blue dots followed by six pairs of red dots along their very hairy backs. Please don’t touch, as these hairs can cause rashes for humans. These caterpillars are hungry-hungry indeed, devouring trees’ leaves and needles, in what feels like a blink of an eye. When too many find themselves feasting on the same hapless tree, the younger (and lighter) of the caterpillars will spin a silken thread (the only silk they will every make) from which they’ll dangle in the air until blown by the wind to another, less crowded tree. This mode of transportation, called ballooning, is (slowly) spreading spongy moths south and west across the country, as well as north into Canada, at a rate of roughly 12 miles/year. Once full grown and covered in protective hair spikes, the caterpillars will move freely along the ground to find food, particularly if they’ve fallen off what they were previously eating, or to set up in interesting places to pupate into moths.
After about 6-8 weeks of relentless eating and pooping, the caterpillars undergo their metamorphosis. This process takes 10-14 days, after which they live in moth form for about four weeks. During that time they do nothing but mate, with males flying around looking for flightless females to fertilize. Because the female moths can’t fly, they lay their eggs in the same location where they emerged from their pupa. Once a female has mated, she makes a beige, spongy (hence the name!) egg sack, which she can affix to whatever surface she happens to be on. She then dies leaving her eggs to hatch in the spring Each sack contains 500-1000 eggs. (Check wheel wells and hubcaps, if driving into a spongy moth-free zone from an infested one.)
Once the spongy moth has migrated into an area, pathogens, parasites, and predators usually keep population numbers in check. It’s common, however, for population explosions to occur periodically, which is what’s happening now. The time span between outbreaks can be as short as 5 years, but is on average more like 10-15. Typically, an area experiencing a caterpillar code-red will be under siege for 2-4 consecutive years. During that time, the two main mechanisms that keep spongy moths at manageable levels during “normal” years eventually kick back in, take effect, and bring the numbers down. These beacons of hope are a fungal pathogen called Entomophaga maimaiga, which requires spring rainfall, and a virus called Nuclear Polyhedrosis, which the caterpillars pass to one another. Starvation can also play an important role in knocking the population back, but this is at the expense of our trees. Certain States, including Illinois, Wisconsin, Pennsylvania, and New Jersey, take matters into their own hands and use aerial spraying in severely impacted or vulnerable woodland areas. Bacillus thuringiensis kurstaki, commonly known as Btk, is the pesticide most commonly used. Unfortunately, Btk kills than just spongy moth caterpillars, but also other insects in the Lepidoptera classification that are in a larval stage and consume it. Control methods that interfere with the mating cycle are also used, in addition to pesticides.
Besides the ick-factor, frass can be dangerous. To say “slippery when wet” is an understatement. Surfaces, like roads, sidewalks, decks, and walkways, if covered with wet frass, can be a serious safety hazard. (Just ask my friend Rob, whose truck nearly skidded off a frass covered road!) It's also near impossible to wash it off any surface it's landed on. (Please leave info in the comments section below, if something has worked for you.) But, the real and present danger here is to our beloved trees and forests, and the ecosystems they support. A healthy broadleaf tree can withstand 2-3 years of even heavy defoliation during the spring and summer months. After that, the damage can be irreversible and the tree could die. Conifer trees, which are also caterpillar food, are unlikely to recover after just one season of defoliation. Most in danger are oaks, aspens, willows, apple, crabapple, white birch, witch hazel, pine, and spruce.
There are some DIY approaches to controlling spongy moths, like wrapping strips of burlap around tree trunks or scraping off egg masses to prevent hatching, but the effectiveness of these methods is very limited. I spoke with Jeff Decker, ISA Board Certified Master Arborist with Green Cross, about the current infestation and what we can do. “It is going to be over soon and [your] trees are likely going to be ok,” he assured me, although he did point out that conifers face a dicier road ahead. Jeff has received calls about spongy moths throughout his career, but last spring there was a very significant increase in the number of baffled and concerned landowners contacting him. While he anticipates another bad spring in 2024, the population should curb itself after that. While spraying with Btk when the caterpillars are younger or Spinosad when they are mature are options in certain scenarios, Jeff encourages all of us to think about proactive tree care and fertilization to ensure the resilience and longevity of our woody friends.: A healthy tree can better withstand any number or stressors, of which spongy moths are only one. Jeff acknowledges that spongy moths are “gross,” but he’s quite confident we can ride out this storm. “A homeowner can just sit tight through it. Sorry to say, but when the hurricane comes through, you can either move or you can board up your windows and come back out when it’s over.”
By the time you read this, the poop rain may very well be over and our broadleaf trees on the road to recovery. Personally, I can’t take too much solace in this. Rather, I’m already bracing myself for next year’s onslaught. In the meantime, I will do my best to stay informed, assess my options, monitor the health of the trees in my care, and try-try-try to keep the invasive plants now thriving in the sun-drenched understory from taking over. (That is my gentle way of encouraging you all to do the same.)
Sources and Resources:
The Great Gypsy Moth War; A History of the First Campaign in Massachusetts to Eradicate the Gypsy Moth, 1890-1901, by Robert Spear https://www.umasspress.com/9781558494794/the-great-gypsy-moth-war/