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A laboratory-based study was conducted to investigate the consumption and degradation of expanded polystyrene (EPS) by superworms (darkling beetle larvae, Zophobas morio; heterotypic synonym Zophobas atratus). Superworms fed on one of three diets (wheat bran control, EPS blocks, or EPS S-shaped chips) following a 60-second pretreatment with UV radiation or no UV radiation exposure. Loss-of-mass measurements were conducted every week for 4 weeks. Nutrients (nitrogen and phosphorus) and EPS degradation products in frass were determined as well as the superworm gut microbiome composition. The average loss of mass after 28 days was 34.7% for EPS blocks and 25.6% for S-shaped chips. Small quantities of nitrogen and phosphorus were found in the frass of superworms that fed on the EPS diet, but they were lower than those from wheat bran-fed superworms. Twenty-one EPS degradation products were identified in the frass of superworms on an EPS diet, including short-chain carboxylic acids, polyols, amino acids, metabolites of amino acids, and phosphoric acids. Pseudomonas aeruginosa(polystyrene degrader) and Stenotrophomonassp. (nitrogen fixer and phosphorus solubilization promoter) were identified from the guts of superworms that fed on an EPS diet. Overall, superworms have the potential to deal with plastic waste degradation problems.