Plastics are common synthetic materials that have been abundantly present as pollutants in natural ecosystems for the past few decades. Thus scientists have investigated the capability of plastic digestion by insects. Here we compare the effectiveness of biodegradation of the specific polymers: expanded polystyrene (EPS), polyvinyl chloride (PVC), low-density polyethylene (LDPE) and polypropylene (PP) altogether with above variants of plastics with microelements and vitamins by the mealworm - the larval form of the beetle Tenebrio molitor - and larvae of the beetle Zophobas morio, known as superworms. Z. morio beetles on all diets were able to complete their life cycle from larvae through pupae and imago, gaining 19 % and 22 % in mass on LDPE and EPS; 8 % and 7 % on PVC and PP. Mealworms (T. molitor) reared on polymers had minimal weight gain, gaining 2 % on LDPE and EPS, and a slight reduction in mass was observed when reared on PP and PVC. Not all specimens of T. molitor were able to pupate and transform to the adult stage. The results suggest that larvae of Z. morio can eat and degrade some types of plastic compounds more effectively than T. molitor. The changes in microbial gut communities were compared between these two species. The highest mass gain for Z. morio is associated with higher diversity in gut microbia and it was more diverse than that of T. molitor. Citrobacter freundii, a bacterium recognized for its ability to degrade long-chain polymers, linear hydrocarbons and cyclic hydrocarbons, was found in the microflora of Z. morio. The results confirm that superworms can survive on polymer feed. Moreover, this diet supplemented with microelements and vitamins increases the number of bacterial species and the diversity in the microbial gut.