Moth larva is not a "plastic eater"?

Mainz researchers doubt biochemical degradation of polyethylene by caterpillars

The larvae of Wachsmotto undoubtedly like to nibble on plastic - but do you digest it too? © César Hernández / CSIC
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Rough conclusion? Researchers doubt the results of a recent study on supposedly plasticizing caterpillars. Thus, in their spectroscopic analyzes, the authors have provided no clear evidence that polyethylene is actually digested by the caterpillars. Control experiments with minced meat and egg yolk suggest that what the authors have measured may not have been plastic degradation products.

The news about plastic-eating caterpillars went through the media worldwide in April: Scientists led by Federica Bertocchini from the University of Santander in Spain had reported that the wax moth larvae can break down the plastic polyethylene (PE) - one of the most persistent plastic variants ever.

About 40 percent of the plastic produced in Europe consists of this polymer. Polyethylene is in plastic bags, foils, plastic packaging and plastic coatings of all kinds, is biologically but hardly degradable. As a result, vast amounts of this plastic go down as waste in lakes, rivers and seas or fill entire landfills. Could the little caterpillars of the Art Galleria mellonella really help solve this problem in the future?

Digested - or just crushed?

Till Opatz and his colleagues from Johannes Gutenberg University in Mainz are skeptical. After the Spanish group of authors found that the caterpillars ate holes in shopping bags, the researchers wanted to know more.

They asked whether this is actually a biochemical digestion process by enzymes or bacteria in the digestive organs of the caterpillars - or merely a mechanical comminution by their chewing tools. In the second case, the caterpillars would excrete the plastic only in smaller pieces, but chemically unchanged again. display

Missing signal pattern

To clarify this, the scientists tried to understand the experiments of Bertocchini's team. In the original study, the Spaniards had studied the impact of killed and crushed larvae on a polyethylene surface. They wanted to have found evidence that the PE is decomposed into ethylene glycol fragments by contact with the caterpillars and their digestive enzymes. This would speak for a biochemical degradation.

However, Opatz and his colleagues doubt that the researchers actually detected ethylene glycol in their spectroscopic and microscopic studies. Control experiments showed that certain signals, which are important for the clear detection of ethylene glycol, are missing in the published spectra. The Spaniards would have been able to detect this with the help of such control experiments, but apparently they did not succeed.

Minced meat under the infrared spectrometer Carina Weber / Stefan Pusch

The minced meat indicator

But what did Bertocchini's team measure? Opatz and his colleagues say that in what the scientists interpret as signals of biochemical degradation products, it could in fact be a signal pattern that causes any animal protein-fat mixture. To prove this, the Mainz egg yolks and minced meat analyzed analogously to the published test protocol.

The result: The signal patterns were almost congruent with the signals that supposedly prove the biochemical degradation process. For the researchers it is clear: it was not plastic degradation products that were detected, but rather caterpillar remains. These remnants, according to the team, would explain most of the other readings casually.

"At least very questionable"

However, the biochemical degradation of the plastic by the caterpillars has not been definitively refuted. Without further supportive results, however, the work published as a sensational report appears at least very questionable and can actually no longer have a stock, according to a statement by the Mainz University.

Whether the larvae of the plastic moth can help in the future in the dismantling of Plastikm ll or not, thus remains open for now. (Current Biology, 2017; doi: 10.1016 / j.cub.2017.07.004)

(Johannes Gutenberg University Mainz, 30.08.2017 - DAL)