Wax moth caterpillar spit could be used to break down plastic waste

Wax worms could provide a new way to combat the plastic pollution crisis.

European researchers have found that the saliva of wax moth larvae is capable of breaking down the most common type of plastic, polyethylene. They identified two enzymes that can break apart the plastic's long polymers into smaller chains.

Dr Federica Bertocchini, who co-authored a paper revealing the discovery in Nature Communications, says that these molecules could one day be applied on an industrial scale to degrade plastic waste.

'This research has opened up several new paths that need to be deeply investigated before these enzymes could be used to tackle plastic pollution.'

'However, as the enzymes can be produced in the lab, it is promising for any future applications. This could include use in waste management facilities to degrade collected plastic, and we could imagine its use in the home further down the line.'

'Before we get to this point, however, we need to know more about these enzymes, including the byproducts of plastic oxidation.' 

What are wax worms?

Wax worms are the caterpillars of multiple wax moth species such as Galleria mellonella. They are just a couple of millimetres long at hatching but grow up to three centimetres before transitioning to the next stage of their life cycle.

Wax worms are a pest of beehives, as they tunnel and chew through beeswax to feed on pollen and the shed skins of bees, and it was Federica's interest in beekeeping that initially led to her discovering the potential of wax worms to break down plastic.

'Beekeeping has been my hobby for many years,' says Federica. 'At the end of the season, beekeepers usually take some empty hives into a storage room to put them back into the field in the spring.'

'In 2012, I did this and found that my stored honeycombs were plagued with wax worms. I cleaned the honeycombs and put the worms into a polyethylene bag, only to discover a short time later that the bag was riddled with holes.'

After carrying out initial studies, Federica found that the polyethylene had been oxidised, rather than being chewed through. G. mellonella's gut microbes had already been identified as having the ability to degrade polyethylene, but their results suggested that that a different mechanism was at work.

The breakdown was much more rapid than polyethylene being broken down by bacteria and fungi, which can take from weeks to months to achieve partial breakdown. In many cases, it also requires the plastic to be pre-treated with UV light or high temperatures.

As polyethylene accounts for around 40% of all plastics, faster and more efficient ways to degrade it could be a significant advance in dealing with plastic pollution. 

How do wax worms degrade plastic?

Following previous research that revealed polyethylene film could be broken down by wax worm saliva, the researchers analysed the secretion to identify the proteins within it, identifying a number of potential targets.

After producing pure solutions of each protein, the researchers tested their individual abilities on the plastic. They found that two enzymes, which the scientists named Demetra and Ceres after the ancient Greek and Roman goddesses of agriculture, were effective at breaking the plastic polymers down into smaller fragments.

'We're not sure why the wax worms produce these enzymes,' Federica says. 'It could be that similarities between beeswax and polyethylene allow the larvae to break down both with the same enzymes.'

'Alternatively, the worm might be able to break down plastic additives that help it maintain its shape and structure. Many additives have an aromatic structure similar to compounds that plants produce to defend themselves from insects, and lepidopterans like the wax worms must have ways to neutralise these.'

In the future, the mass production of enzymes similar to Demetra and Ceres could be the first stage in degrading plastic waste. By breaking the inert polymers apart and adding in additional elements, such as oxygen, the fragments can then be more easily broken down by a variety of microbes. 

The resulting products, which depends on the type of starting plastic, could then be used to make new materials or be metabolised further by the microbes themselves and so removing them from the system.

The researchers now hope to continue investigating these enzymes to find out how they work, and the steps that are necessary to break down polyethylene. They also hope to find similar proteins, of which little is currently known.

'Enzymes with sequence similarities with Demetra and Ceres could be tested in the future,' Federica adds. 'It is also possible that other insect larvae which can degrade polyethylene and polystyrene may possess similar enzymes, offering a range of potential pathways to degrade these materials.'