Could a Pest of the Honey Industry Clean up Plastic Waste?

In 12 hours, 100 wax moth caterpillars had reduced a mass of plastic by 92 milligrams, the fastest plastic degradation process known so far.

Wax moth caterpillars in a Petri dish. Credit: César Hernández/CSIC

Wax moth caterpillars in a Petri dish. Credit: César Hernández/CSIC

Update (September 20, 2017): Three scientists from Johannes Gutenberg University Mainz, Germany, say the study doesn’t show clinching evidence that wax moths digested polyethylene and excreted ethylene glycol. They point out the original study did not have adequate controls to rule out other influencing factors. They followed the same protocol but rubbed egg yolk and ground pork on polyethylene film instead of mashed moth caterpillars. Under the infrared spectrometer, fats and oils from these products clinging to the film gave similar readings as the original study. However, they don’t rule out the possibility that the worms may be digesting plastic. Much more rigorous experiments are needed to prove it. This was published in the same journal, Current Biology, on August 7, 2017.

May 1, 2017: Federica Bertocchini, a molecular biologist from the University of Cantabria, Spain, tends bees in her spare time. Several months ago, when she took her honeycomb panels out of storage, they were infested with the caterpillars of the honeycomb wax moth (Galleria mellonella), a common parasite of beehives.

Brownish-grey wax moths inveigle their way into beehives and lay up to 600 round pinkish eggs in batches. About three to five days later, white caterpillars hatch and devour wax, bee grub, pollen and bees’ faeces. If not contained, they can destroy hives.

After cleaning the panels, Bertocchini threw the caterpillars into a shopping bag and sealed it. When she returned to the room a few hours later, she found the larvae everywhere. She examined the bag and found it riddled with holes. Polyethylene bags such as the one she used are one of the toughest plastics – and yet the caterpillars had eaten their way out of it. But unlike the average beekeeper, she wasn’t irritated with the many worms crawling all over the room.

“My reaction was of happiness,” Bertocchini told The Wire. “It was very exciting.”

By chance she had stumbled upon the amazing ability of these little worms to digest plastic, potentially a game changer in biodegrading this indestructible substance.

The global production of plastic topped 300 million tonnes in 2015. Of these, the packaging industry uses nearly 80 million tonnes of polyethylene, the most common plastic. By 2018, production is expected to touch 100 million tonnes. Each year, we use five trillion polyethylene shopping bags, and we discard most of them after use. The rate at which they degrade can’t keep up with our rate of usage. Instead, they break into smaller and smaller fragments but don’t decay.

Plastic after being biodegraded by 10 wax moth caterpillars for 30 minutes. Credit: César Hernández/CSIC

Plastic after being biodegraded by 10 wax moth caterpillars for 30 minutes. Credit: César Hernández/CSIC

Low-density polyethylene takes at least 100 years to degrade and the toughest may take 400 years. Plastics overflow from landfills, clog our rivers and swirl in great garbage patches in the oceans. Many find their way into the guts of wild animals – whales, turtles, elephants are just a few of the many species that die from it. Scientists around the world are working to find ways to degrade plastic. And the wax moth caterpillars may just be the most promising so far.

“When I saw [the moth-eaten plastic bag], it clicked right away,” says Bertocchini. “We immediately decided to pursue the research.”

In the lab, Bertocchini and her colleagues Paolo Bombelli and Christopher Howe, of the University of Cambridge, UK, observed what 100 wax moth larvae could do to a typical shopping bag. In 40 minutes, the caterpillars chewed holes through it at the rate of two holes per worm an hour. In 12 hours, they had reduced the mass of plastic by 92 milligrams, the fastest plastic degradation process known so far.

“There are few previous reports about biodegradation of polyethylene,” Wolfgang Zimmermann, a biochemist at Leipzig University, Germany, told The Wire.

Most research on biological forms of degradation focus on bacteria like Nocardia asteroides and fungi such as Penicillium simplicissimum. Last year, Japanese biochemists isolated a bacteria called Ideonella sakaiensis outside a plastic recycling plant. It produces two enzymes – PETase and MHETase – to break polyethylene terephthalate (commonly known by its abbreviation PET) into terephthalic acid and ethylene glycol. But they were much slower than the wax moth caterpillars, digesting only 0.13 milligrams a day, and PET is a simpler plastic and easier to disintegrate than polyethylene.

The caterpillars of wax moths aren’t unique in eating plastic. The larvae of Indian mealmoth (Plodia interpunctella) also chomp it up. Chinese researchers isolated bacterial strains from its gut. However, over a four-week incubation period, these bacteria succeeded in making pits and cavities of 0.3 to 0.4 micrometers in depth, too slow to be of practical use.

Plastic is obviously not the wax moth caterpillars’ normal diet. But they chewed through it with as much élan as when they bite into beeswax.

“Wax is a polymer, a sort of ‘natural plastic’, and has a chemical structure not dissimilar to polyethylene,” Bertocchini said in a press release.

To rule out the possibility of the caterpillars’ jaws mechanically abrading the plastic, the researchers squashed the worms and smeared the goo on a polyethylene sheet. In 14 hours, by just being in physical contact, 13% of the plastic had corroded – that’s 0.23 micrograms per square centimetre per hour.

A wax moth caterpillar chewing a hole through plastic. Polythene debris can be seen attached to the caterpillar. Credit: César Hernández/CSIC

A wax moth caterpillar chewing a hole through plastic. Polythene debris can be seen attached to the caterpillar. Credit: César Hernández/CSIC

On conducting a spectroscopic analysis, the team identified the residue of ethylene glycol not only on the treated bag but also around the edges of holes eaten by the larvae. The larvae seem to break polyethylene down into ethylene glycol.

“The caterpillar produces something that breaks the chemical bond, perhaps in its salivary glands or a symbiotic bacteria in its gut,” said Bombelli. “The next steps for us will be to try and identify the molecular processes in this reaction and see if we can isolate the enzyme responsible.”

Although the researchers don’t know what substance produced by the caterpillars reacted with polyethylene, they suspect the process of digesting beeswax and polyethylene may be similar. However, the wax moths aren’t going to be freeing the world from its enormous plastic problem anytime soon.

“It is not clear how the degradation of the polyethylene occurred,” says Zimmermann. “It is likely that enzymes from the gut microflora of the larvae were responsible for the observed effects, but this has not been shown. The biochemical characterisation of the treated polyethylene and the breakdown products is only preliminary and not complete.”

“The next step is to detect, isolate, and produce this enzyme in vitro on an industrial scale,” explains Bertocchini. Unleashing a known beehive pest on waste dumps may hammer an already beleaguered honey industry. The team plans to find a biotechnological way to solve the enormous intractable problem of plastic waste.

“In case the enzymes involved in polyethylene degradation can be characterised and further developed, Bertocchini’s report could make an important contribution towards this goal,” says Zimmermann.

Bertocchini normally studies the embryonic development of animals, but she wasn’t daunted by taking on a research project so far outside her regular field of work. “Although Paolo and Chris follow different lines of investigation, we had been talking about this issue of plastic degradation for quite some time,” she says. A bigger challenge was financing the research.

The wax moths already hold one record for awesomeness. They can hear ultrasonic frequencies of up to 300 kHz, the most sensitive in the animal kingdom.

Even as beekeepers try to find ways to avoid falling victim to wax moths, it could be truly transformative if the insects’ ability to digest plastic and transform it in a form harmless to the environment becomes practical.

The study was published in the journal Current Biology on April 24, 2017.

Janaki Lenin is the author of My Husband and Other Animals. She lives in a forest with snake-man Rom Whitaker and tweets at @janakilenin.