In Defense of Clothes Moths, Marvels of Evolution

Jan 27, 2024

Though they cause headaches for dining on your wardrobe, webbing clothes moths (Tineola bisselliella) are unique creatures with fascinating specialized biology. They can eat hair and metabolize their own water. They can chew through plastic and digest mercury. And that's not all. An entomologist studying these moths’ makes a case for appreciating their evolutionary feats. (Photo by Andrey Ponomarev via iNaturalist, CC BY-NC 4.0)

By Isabel Novick

Isabel Novick

The insect order Lepidoptera, derived from Latin and Greek for "scaly wing," may sound alien, but in fact consists of our familiar friends and foes: butterflies and moths. Lepidopterans are characterized by the presence of four scale-covered wings and make up the second-largest order of insects in the world (the largest being Coleoptera, the beetles), with over 160,000 described species. Out of that number, butterflies make up only about 5 percent. In fact, moths originated first, diverging from their ancestors about 300 million years ago, with butterflies branching off later, about 98 million years ago. In other words, butterflies are just day-flying moths with a better reputation. So, why are actual moths treated with such disdain?

Moths are innovative, crafty, and able to capitalize on unfilled niches. They’re not malicious in their demolition of landscaping, crops, or clothes; they’re only operating exactly as they evolved to. The reasons we dislike moths—being persistent, destructive, difficult to eradicate, and less colorful than butterflies—are the same reasons they’ve been able to survive and thrive for so long. Of course, that doesn't mean that some species are not truly devastating to local ecosystems or that battling these moths isn't a huge challenge. From grape berry moths (Paralobesia viteana) in vineyards to diamondback moths (Plutella xylostella) in broccoli crops to spongy moths (Lymantria dispar) in forests, many moth species have broken the hearts of homeowners, farmers, and conservation biologists alike.

Then there's the Big Cheese, the moth that jumps to mind perhaps more than any other when someone says, "I have a moth problem"—the webbing clothes moth (Tineola bisselliella), which seems to cause grief wherever it goes. And it goes everywhere. Though its distribution isn't fully understood, I have personally worked with samples from Canada, Mexico, Chile, Nigeria, Australia, France, South Korea, Palestine, and more. It is hypothesized that these moths originated in Africa and expanded their range by hitchhiking on 19th century sailing ships. They are now a globally distributed and economically important pest species, causing up to $1 billion in damage per year in the United States alone. Clothes moths belong to a unique and ancient lineage, the fungus moth family, Tineidae, that predates other, more recognizable species like luna moths or atlas moths.

To those who are lucky enough to be unfamiliar with webbing clothes moths, let me clue you in. They begin their lives each as a pearlescent egg about 0.3 millimeters wide, smaller than the period at the end of this sentence. From there, they hatch into tiny, hungry, cream-colored caterpillars that spin silk as they munch. When they are ready to pupate, they are about as long as a grain of rice, and about the same color. They sneakily construct their cocoons of silk and bits of substrate (i.e., whatever they’re feeding on), making them hard to spot, and they only truly make themselves known upon emergence as full-grown adults. Adult moths are not much bigger than their larvae and are a soft, shimmery champagne color. As adults, they do not eat (and don't even have mouths), and they only live for a few weeks with the sole purpose of finding a mate. Unlike other moths, they are light-averse, are poor flyers (they prefer, strangely, to scurry by foot), and are attracted to potential larval habitats like furs or old rugs.

I understand these images likely don't conjure feelings of fascination or awe, but have you ever paused to consider how insanely cool clothes moths are? Let's look closer. These moth larvae eat hair. They eat skin. Hair, skin, horns, and hooves are all made of keratin, a protein that is infamously difficult to digest. What kind of organism can digest and metabolize keratin like this, let alone what kind of lepidopteran? It is unclear how they can process keratin—it's possible that they have a microbe in their guts that produces a helpful digestive enzyme, but the process is still shrouded in mystery. However they do it, all they need are keratin or detritus, a little vitamin B, and that's it. Wool hats, rugs, and upholstery are all-you-can-eat restaurants to a clothes moth.

Their strange eating habits are only the tip of the iceberg. These moths, unfortunately for those with infestations, have other behaviors that contribute to their indestructibility. They can metabolize their own water as a byproduct of keratin digestion, so access to water is not a dealbreaker for survival. What kind of organism can create its own water? This moth has evolved to be an efficient, dynamic, super-survival machine. They are incredibly temperature tolerant, with the ability to survive as eggs or larvae for several days at broiling temperatures as high as 95 degrees F and as far below freezing as 5 degrees F. They are attracted to the smell of woolens, and once established, send pheromonal signals to nearby moths to invite them to party. To add to their tank-like nature, webbing clothes moths can digest toxic metals like arsenic, mercury, and lead. They have no problem metabolizing synthetic materials or chewing through soft plastics. They have even been found on mummified human remains and have been around long enough to be mentioned in the Bible.

While webbing clothes moths are certainly one of the most widespread pests within the family, other tineids have remarkable habits, too. One genus within the Tineidae family, Ceratophaga, eats the shells of dead giant tortoises. Once it's time to pupate, they burrow down into the earth below and later emerge as winged adults, ready to produce a horde of tortoise-shell-eating babies. Another common tineid pest species, the casemaking clothes moth (Tinea pellionella), has an eccentric larval habit. As they feed, caterpillars spin themselves up in a tube of silk, pieces of food, and tiny poops, and then carry that on top of themselves wherever they go. Why the outlandish fashion choice? It's possible that it helps to regulate larval water loss, but when all is said and done, we’re really not sure. Oinophila v-flava, another tineid cousin, is a fungus-lover with a taste for the finer things. Larvae are found in wine corks in such abundance that their name, Oinophila, translates to "wine-lover."

It is apparent that tineids have adapted to fit nearly every niche, with many of them preferring human-created environments. Who wouldn't? Our homes maintain a consistent temperature and humidity, there are very few predators, and we pack our living spaces with all kinds of delicious keratinaceous products. This incredible feat of evolutionary acclimatization has made clothes moths so well-suited to our homes that they are rarely found outdoors anymore. They are classified as a "synanthropic" species, defined as an organism that benefits from, and has thus adapted to, human establishment, like German cockroaches or head lice. In fact, clothes moths have been with us since before we were human, probably tagging alongside our primate ancestors during the construction of night-time sleeping nests (Martin et al. 2015). Just like that, they found their niche and have since stopped looking. The mechanism that allowed them to colonize, and eventually require, our spaces is still unclear. However, it seems likely that it was related to their dietary choices. Webbing clothes moths are facultative keratinophages, meaning they can eat and digest keratin when they want to and can instead opt for fungus and other organic garbage when it is available. This dietary flexibility is characteristic of other synanthropic species (looking at you, raccoons) and may just be the key to their world domination.

The digestive powers of the larva of the webbing clothes moth (Tineola bisselliella) mean materials like wool hats, rugs, and upholstery are an all-you-can-eat buffer for the tiny caterpillars. Such substances are chock full of keratin, a protein found in hair, skin, horns, and hooves and that is infamously difficult to digest. It is still unclear how webbing clothes moth larvae can process keratin—it's possible that they have a microbe in their guts that produces a helpful digestive enzyme. (Photo by Andrey Ponomarev via iNaturalist, CC BY-NC 4.0)

We as humans have a love/hate relationship with the order Lepidoptera. Both butterflies and moths provide invaluable ecosystem services as pollinators and as a food source for birds and other insects. However, both butterflies and moths have the potential to be destructive invasive species, like the butterfly Pieris rapae (sometimes known as the cabbage white butterfly) or the webbing clothes moth. But one of these insects inspires feelings of joy and peace and the other frustration and disgust, even violation. Why do these two so closely related creatures evoke such opposing reactions? Why do we feel such disdain for the moth? Is it just because of the harm done to our possessions? Or is it because the moth inhabits our space as an uninvited guest?

Ultimately, we as humans are the final arbiter on what makes a pest. Despite the communal inhabitation of all plants, animals, and fungi of the earth, we as humans have made our global authority clear. Ultimately, if we examine the webbing clothes moth through a human-centric lens, of course we deem it a pest. But, in defense of this little brown moth, if we can remove ourselves from the equation and observe it from an evolutionary standpoint, it, too, may inspire feelings of awe, wonder, and admiration.

Isabel Novick is a Ph.D. candidate at Boston University studying the evolution of clothes moths. Email: [email protected].

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