Polythene-digesting caterpillars

26 April 2017
Posted by Chris Smith.

Caterpillars that can munch their way through plastic bags have been described by scientists in Cambridge.

Worldwide, production of polythene and other plastics has risen exponentially over the last 50 years and now exceeds 80 million tonnes per year.

Only 25% of this gets recycled, and a further 36% is burned for energy. That means that nearly 40% - or 32 million tonnes per year - ends up in landfill. 

This is a problem because polythene is notoriously stable and does not readily break down, meaning that it is destined to persist in the environment for extremely long periods of time.

But has University of Cambridge scientist Paolo Bombelli stumbled upon a natural way to get rid of the stuff?

When he and his colleagues Chris Howe and Federica Bertocchini confined some wax worms, Galleria mellonella - which are are the larvae of the wax moth - in a plastic bag they were surprised when the creatures ate their way to freedom shortly after.

Subsequent experiments, published this week in Current Biology, showed that the caterpillars can break down the polythene in the bag material, producing an average of 2 caterpillar-sized holes per worm every hour.  A plastic bag exposed to 100 of the larvae lost about 100mg over a 12 hour period.

Chemical tests on the degraded bag material show that the wax worms are converting the plastic into ethylene glycol, a component of antifreeze, although it's not clear yet exactly how they are doing it. Ground up caterpillars also had the same effect, suggesting that the bacteria within their intestines might be the real biochemical brains behind the feat.

Regardless of how, the reason why the worms are able to perform this feat is probably explained by their ecology: mother wax worm moths lay their eggs in beehives. When the young hatch they feed on a diet of beeswax, which is chemically quite similar to the bonds that join together the carbon atoms in a piece of polythene. If the mechanism by which this is achieved can be identified, it may be possible to copy and scale it to produce an industrially viable way to deal with plastic waste.

At the moment, using the caterpillars themselves is not an option because they are just not hungry enough. At a couple of milligrams per caterpillar per day, it would take billions of them just to deal with the plastic waste from one relatively small country let alone the world. And then there's the other sting in the tail: these insects target the same honeybees upon which we depend to pollinate our crops. Bees are already in trouble without us rearing an army of their arch-enemies, albeit with the aim of confining them in plastic-digestion facilities. If the moths staged a break-out it could spell disaster for the bee population.

Instead, the best approach will most likely be to resort to bacteria. If scientists can discover how the caterpillars are digesting the plastic, and unpick the players in the relevant biochemical pathway, then it may be possible to use genetic engineering techniques to endow easy-to-grow microbes with the same abilities. These could be cultured in large fermenters and fed on plastic waste.

However, that may not actually be the best outcome. As some commentators have highlighted, the carbon released by the process will probably be in the form of carbon dioxide, which will contribute to global warming. So we might as well burn the plastic in the first place and at least get some energy out of it in the process...

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