The issue of microplastics in the environment is becoming a global challenge with serious consequences for organisms. Agriculture is an important source of microplastics in terrestrial ecosystems, especially because of mulching films, which are often ploughed into the soil after use. Since little data is available on the long-term impacts of microplastics on terrestrial invertebrates, we investigated the effects of microplastics from conventional and biodegradable mulching films on the development of the mealworm beetle Tenebrio molitor. The experiments were first conducted for 8 weeks in the soil, but due to excessive mortality in the control groups, we carried out further experiments on wheat bran, where we exposed larvae of mealworms for 12 weeks. Afterwards, we exposed their offspring (2nd generation) to microplastics for another 9 weeks. In both types of exposure, we exposed mealworms to six different concentrations of two types of microplastics (low-density polyethylene or LDPE and polybutylene adipate-co-terephthalate or PBAT). In all experiments, we monitored the occurrence of pupae and adult beetles, the growth and molting of larvae, and the mortality of organisms. In experiments on bran, we did not observe any effect on the mortality of mealworms, but on the contrary, we observed effects on their molting, growth and the occurrence of bugs and beetles. These changes were most pronounced at the highest concentration of LDPE (5 % dry weight of microplastics / dry weight of bran), where we detected reduced larval growth and molting in the first generation. With PBAT (5 % w/w), we observed a reduced occurrence of pupae and adult beetles in the 1st generation. A reduced number of moults and an increased growth of larvae were detected in the second generation with PBAT (5 % m/m). The results show that both conventional and biodegradable microplastics cause changes in the development of T. molitor, especially at the highest concentration.
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