Will hungry caterpillars reduce our plastic problem?

Worldwide, we still use over a trillion plastic bags every year. And one of their constituent materials, polyethylene, can be very difficult for nature to degrade. Katie Haylor investigated a new, unlikely ally in the fight to reduce our waste.

Katie - The plastic carrier you took home from your last shopping trip could be sitting on a landfill site for years and some don’t even get there. A plastic bag can end up in the natural environment threatening wildlife and damaging ecosystems. So how can polyethylene bags be quickly broken down? Surprising research from scientists at Cambridge University and the Universidad de Cantabria suggests that a humble worm could be the answer. I spoke to Cambridge biochemist Chris Howe…

Chris - The wax moth is a moth that is a serious pest inside beehives. Specifically we’ve been looking at the caterpillar - sometimes known as the wax worm - the caterpillar of the wax moth. They are a serious pest inside beehives - they will break down beeswax. But, on the other hand, one of the more interesting, more exciting things that we found that they can do is that they can actually also break down plastics, so we found they can break down the plastic polyethylene.

Katie - This is plastic bags, right?

Chris - This is plastic bags. A lot of the experiments that we did, we did with plastic bags that we got from a supermarket in the marketplace.

Katie - This is not just any old plastic bag?

Chris - It’s just any old plastic bags, yeah.

Katie - No points for guessing which supermarket! How did this come about - was this in the lab?

Chris - Well, this was a discovery by Fredericka Bertacchini, who is the lead author on the project. She’s also a keen beekeeper as well as being a developmental biologist. She was cleaning out her beehives for the winter and they had some of these wax worms in them and she picked the out and put them in a plastic bag. And then came back a few days later and discovered that actually they’d made holes in the plastic bag and they’d escaped. As a good scientists she thought that’s interesting, I want to know how they managed to do that and if we can exploit it.

One of the experiments we did involved putting about a hundred wax worms on a plastic bag and leaving it for about 12 hours, they managed to break down about a hundred milligrams of plastic. This particular plastic is actually very difficult to break down so to get any breakdown at all is really quite significant.

Chemically it’s made of a whole string of carbon atoms end to end with hydrogen atoms atoms attached to them. But that carbon-carbon, carbon-carbon backbone is really quite stable and quite difficult to breakdown.

Katie - And it’s that you think these moths are targeting?

Chris - Absolutely. That’s what we think they’re breaking down. One of the things that’s important is that the worms are actually chemically breaking down the plastic and not just chewing it up. So we made a homogenate of the caterpillars…

Katie - By homogenate you mean you squish them together?

Chris - We squish, yes. Technical term - we squish them to make a kind of caterpillar - I can’t say that no caterpillars were harmed during the course of these experiments. We squished them in a pestle and mortar, spread some of the caterpillar puree on top of the plastic and then left it for a while, and then measured the amount of plastic that had been broken down. That showed us that it must be some kind of chemical process rather than live caterpillars physically chomping away at the plastic.

Then another approach was to use a microscopy technique called atomic force microscopy to look at the surface of the plastic. You can see that the plastic has become much more corroded, if you like, as a result of the treatment with the caterpillar puree.

Katie - So these carbon-carbon bonds are being broken - what is being produced?

Chris - One of them, we think, may be a compound called ethylene glycol, which is a chemical that contains two carbon atoms in it and it also the chemical that is better known as antifreeze, but there are certainly other things being produced as well.

Katie - You can’t see them so how do you know that they are being produced?

Chris - For that we used a chemical technique that’s a kind of spectroscopy to look at the ability of particular compounds to absorb light of particular wavelengths. It gives  you a kind of signature of particular compounds.

Katie - Why do you think these caterpillars are breaking these bonds? What’s in it for them?

Chris - The plastic is actually chemically quite similar to the beeswax. They’ve evolved to be able to break down the beeswax. We don’t actually know if it’s the caterpillars themselves or if it’s the bacterial living in the guts of the caterpillar and that’s one of the things that we need to find out. But, because they are able to break down beeswax, that means that they’ve also become able to break down this plastic.

Katie - What does this all mean for recycling? Are we going to be seeing caterpillar farms next to every recycling centre?

Chris - One thing that it definitely doesn’t mean is that we should be able to carry on using plastic in a reckless way. It’s going to be a long time before this approach, if indeed it ever does, become applicable for breaking down plastic.

One of the things that we need to do is to find out more about how the caterpillars are chemically breaking down the plastic. We think it must be an enzyme that’s involved and if we can isolate the enzyme then we can understand more about its mechanism. We can also, we hope, get hold of the gene, or genes, if it’s more than one enzyme that contain the information for making it. Then what we might do is put that gene into bacteria that we can grow up very easily at large scale and that would give you a large vat, if you like, of bacteria that are then able to break down the plastic.

Katie - So there’s not going to be beehives and caterpillar farms all over the cities?

Chris - I don’t think so. But I’m happy to be proved wrong if that’s how it turns out.