FILE: An adult male spongy moth.
U.S. Department of AgricultureUp close, it seems harmless — cute, even. With its rabbit ear-shaped antennae, fuzzy body and soft, mottled wings, the 2-inch-long insect looks like it emerged from a Guillermo del Toro or Hayao Miyazaki film.
But don’t be fooled, scientists warn: According to new research, these tiny creatures, which thrive in the Northeastern U.S., could soon wreak havoc on the West Coast’s forests due to global warming — and maps indicate that they’ve already invaded the far reaches of Southern and Northern California.
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According to a Jan. 6 study published in Nature, rising temperatures are expected to bolster populations of Lymantria dispar, also known as the spongy moth: an adorable — but insatiable — pest that feeds on over 300 species of trees and shrubs. Known to cause millions of dollars in damages, the beige-colored insects have already invaded the Northeast, and scientists worry that as they continue to spread to western corners of the U.S., California’s millions of acres of oak forests will become increasingly susceptible. That’s partly because the heat is expected to hinder the bug’s main rival — an “insect-eating” fungal disease called Entomophaga maimaiga — signaling the dawn of a new era for the spongy moth empire.
A botched Victorian-era experiment
Native to Europe, Asia and Africa, these creatures were first introduced to the U.S. by scientist Leopold Trouvelot, who brought them from France to Massachusetts for breeding experiments in 1869. However, according to the United States Forest Service, his project to harvest silk-spinning moths failed, and some of his caterpillars escaped, causing them to “balloon” and spread across the country via air currents. Since Trouvelot’s mistake, they’ve settled in the Northeast, where millions of them emerge each spring to devour forests and urban foliage. For three decades, though, E. maimaiga has kept them at bay — and academics believe it’s still the best line of biodefense against these destructive moths.
The fungal pathogen, which was introduced to the U.S. from Japan around 1910, somehow appeared in the East Coast 79 years later, where it began to specifically kill off Lymantria dispar, leaving their cadavers in its wake. According to Cornell University, data from 1996 indicates that this mysterious fungus ultimately helped defoliation decline by 85% in several states compared to the year prior.
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However, it needs cool climates in order to grow, and as global temperatures continue to rise, it appears that spongy moths are evading their enemy and expanding their reach: On Dec. 28, the California Department of Food and Agriculture announced that 16 spongy moths were trapped in Calabasas — the upscale community in Los Angeles County — between October 2023 and July 2024, leading to an emergency proclamation to decimate them before they cause significant economic and environmental damage. According to the government agency, spongy moth populations in Calabasas are due to be fully treated by September 2026, but these dates could be extended. This also appears to be the first time that multiple moths were trapped all the way down in Southern California, their website shows.
Overall, booming spongy moth populations could “have a very bad effect on California forests,” which have millions of acres of oak woodland, their main food source, Greg Dwyer, one of the study authors and a professor of ecology and evolution at the University of Chicago, told SFGATE.
Dwyer said that when there was a spongy moth outbreak in Michigan, hundreds of square miles of oak forests were left bare. Once the bugs finish feasting, they move on to other targets, like beech trees or even cherries. These trees need leaves in order to thrive, he explained, and invasions prevent them from properly photosynthesizing. If they’re defoliated for two or three years in a row, they’ll usually die.
‘It’s pretty gross’
In the past, this has led to millions of dollars of damages, Dwyer said, but spongy moths also have another economic impact: They decimate property values in the suburbs.
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According to Dwyer, spongy moth outbreaks can even depress neighborhood house prices. “Nobody wants to buy a house in an area where the trees don’t have any leaves, and they leave a terrible mess behind,” he said.
“Their poop is mostly kind of just squishy, cellulose-y kind of stuff, like ground up celery or something,” he continued, explaining that while it doesn’t smell, it accumulates in unsightly piles that would make any real estate agent squirm. “It’s pretty gross. The insects will be all over the houses and people’s cars and whatnot, so that’s pretty disgusting too, and that reduces property values.”
Though it’s unclear just how big their populations will become and when, he suspects that California will ultimately become more susceptible to spongy moth invasions in the coming years, and that state officials will be “freaking out” over how many moths they’ll find in their traps. Killing them is no easy feat, either. Trees must be wrapped in burlap to catch larvae, which are then supposed to be drowned in soapy water or stuffed in the freezer — but the real solution is to simply wait for natural controls, like E. maimaiga, to fight them off, Dwyer explained.
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Unfortunately, study authors predict that global warming will significantly weaken the fungal pathogen, opening the floodgates for the maligned insects to take over.
“The thing that always goes through my head is that people think that climate change is just polar bears in the Arctic,” he said, “but it’s also spongy moths in the backyard of hedge fund managers in Connecticut.”
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