The Very Hungry Plastic-Eating Caterpillar

Could waxworms—or something inside them—help degrade the planet’s mountains of plastic waste?

A waxworm chews a hole through a plastic bag.  (Federica Bertocchini )

When she’s not working in her lab at Spain’s IBBTEC institute, Federica Bertocchini keeps bees. One day, when she looked at her hives, she found them infested with caterpillars called waxworms. These insects are the bane of beekeepers because they voraciously devour the wax that bees use to build their honeycombs. Bertocchini picked out the pests and put them in a plastic bag, while she cleaned out the hives. And when she returned to the bag, she found it full of holes.

The waxworms had eaten their way out.

Bertocchini doesn’t study insects, waxworms, or plastic—she focuses on the early development of animal embryos. But you can’t keep a good scientist away from an interesting question, and the perforated bags posed an obvious one: Were the waxworms actually digesting the plastic?

Plastics make for good packaging—and even better pollutants—because they are so hard to degrade. They’re very long chain-like molecules with lots of powerful carbon-to-carbon bonds, which can’t be easily broken by bacteria, fungi, and other organisms that normally decay organic matter. So when plastic enters the environment, it usually stays there.

Other scientists have also discovered plastic-busting species, including various fungi and bacteria. Last year, a Japanese team identified a previously unknown bacterium that can degrade PET. And in 2014, Chinese scientists suggested that two species of bacteria from the guts of Indian mealmoths, a type of waxworm, can degrade polyethylene (PE)—the world’s most common plastics.

In both cases, the microbes did their work over the course of weeks or months, which is impressive since plastics can last for decades without decay. But Bertocchini saw that her waxworms—from a different species called the greater wax moth—were working much faster. When she put them in a polyethylene shopping bag, holes would appear within 40 minutes. After a few hours, the bag would be a shredded mess.

To check that the insects were truly digesting the plastic, rather than just chewing holes in the bags, Bertocchini mushed them up and applied the resulting paste to polyethylene. After half a day, around 13 percent of the plastic had disappeared. Even a waxworm smoothie can destroy polyethylene.

Finally, Bertocchini teamed up with biochemists Paolo Bombelli and Christopher Howe at the University of Cambridge to analyze the chemical composition of the plastic as it reacts to the waxworm paste. By looking at how the polyethylene absorbs or reflects infrared radiation, they showed that some of the substance is converted into ethylene glycol—a sign that it was genuinely being degraded.

Why should waxworms have the ability to digest plastic so quickly? Bertocchini think it’s a coincidence. The carbon-carbon bonds that are found in polyethylene are also present in the wax that the caterpillars eat. By evolving to digest the latter, they may have inadvertently gained the ability to degrade the former.

People produced 311 million tons of plastic in 2014, and that figure is set to double in the next 20 years. Around 40 percent of that consists of polyethylene, in the form of plastic bags, containers, and other products, much of which ends up in landfills. Could the waxworms help to break down that mountain of persistent trash?

Ramani Narayan from Michigan State University, who looks for ways of degrading plastics, isn’t convinced. He says the evidence that the waxworm paste was producing ethylene glycol is “tenuous at best,” and could be explained by other chemical changes. And even if Bertocchini is right about the degradation, Narayan says that it’s unlikely to have practical use. An army of bag-chewing caterpillars might consume a lot of plastic, but they would also end up releasing small fragments or microplastics into the environment, which can “pick up toxins like a sponge, transport these toxins up the food chain, and can cause harm to the environment and human health,” he says. “Biodegradation isn’t a magical solution to plastics waste management.”

Susan Selke, director of the Michigan State University School of Packaging, adds that caterpillars wouldn’t survive in the oxygen-free conditions within landfills, and it’s unclear if they would go for plastics over, say, other sources of food nearby. “There’s a long way between demonstrating that biodegradation can occur and the development of a system that provides benefit from biodegradation,” she says.

But, Bertocchini says, “the idea would be to not use the worms.” Instead, she wants to identify the enzymes that they’re using to degrade polyethylene. “Maybe we can find the molecule and produce it at high-scale rather than using a million worms in a plastic bag.”

Jennifer DeBruyn from the University of Tennessee, who is also looking for plastic-degrading microbes, is convinced that the waxworms really are digesting polyethylene, and wants to know whether it’s the insects themselves or the bacteria in their guts that are producing PE-busting enzymes. “If I had to guess, I would suspect the bacteria,” she says.

Either way, “the hunt for organisms that can degrade plastics is on,” she adds. “Currently, there are no alternatives—right now we don’t have a good solution for dealing with the plastics that are piling up on our planet.”