Do you remember that study a while back that found that some caterpillars digest plastic? Well, not all that glitters is gold. Although many cheered that this could be the solution to our plastic problem, it’s actually not so sure yet. Some German researchers found that the evidence provided doesn’t hold up under further examination.

The original study found that the moth Galleria mellonella’s larvae break down polyethylene (plastic) as it goes through their digestive system. The two ways that it could be broken down are either mechanically or with a special enzyme. The idea of an enzyme is alluring because it could potentially be created artificially on a large-scale to bio-degrade plastic. Plastic is a huge problem because almost no living creatures can break the bonds, so it just accumulates. We now have so much of it on Earth and it won’t go away.

One of the larvae on a plastic bag. Image credits: César Hernández/CSIC.

When the researchers mashed up the larvae and rubbed them on plastic bags, holes still appeared in the bags. They used spectroscopic analysis to show that the chemical bonds in the plastic were actually breaking, and being bio-degraded. They concluded that the caterpillars use a special enzyme to break down the plastic.

Now, researchers from the Johannes Gutenberg University in Mainz, Germany argue in a correspondence published in the same journal that the study provides insufficient proof that the caterpillars produce an enzyme that breaks down plastic. The German group was unconvinced because ethylene glycol signals were missing. This substance should appear because it is a byproduct of polyethylene degradation. The original study took the presence of one infrared absorption as proof for its presence. However, these researchers thought that this signal could be from residual protein contamination on the plastic.

They did an experiment of their own and rubbed plastic bags with a mixture of egg yolk and ground pork (protein). The infrared absorption was the same as in the larvae study.

“While the biodegradation of mostly inert artificial polymers is definitely a very interesting research field, we must respectfully disagree with the methodology and conclusions from this paper,” the German team wrote in this correspondence.

In any case, further studies are needed to prove if the larvae have a plastic dissolving enzyme. One method to use would be to add carbon labels to the plastic to track the digestion of it. However, we shouldn’t rely on a “magic” solution to our plastic problem, we just need to stop using so much plastic.

Journal reference: Weber, C., Pusch, S. & Opatz, T. (2017) Polyethylene bio-degradation by caterpillars? Current Biology 27, R744–R745.