The interplay of larval age and particle size regulates micro-polystyrene biodegradation and development of Tenebrio molitor L.

Microplastics, tiny plastic fragments from 1 μm to 5 mm, are widespread globally, even in remote environments. Due to their small sizes, they are easily ingested by organisms and contaminate the food chain. Recently, the biodegradation of some recalcitrant plastics by larva of Tenebrio molitor L. (mealworm) has been reported. However, the effects of microplastic feeding on them are limited. In our study, we selected rigid micro-polystyrene (MPS) as the model plastic to investigate the influences of particle size and larval age on plastic consumption and degradation, and the effects of microplastic feeding on the survival and development of mealworms at different larval ages. The smaller the microplastic fragment was, the more plastics the mealworms consumed, though there was a limit on particle size. Mealworms of three-month-old had the highest consumption rate. Both depolymerization and modification on the functional groups were only observed in frass excreted by three-month old mealworms. Additionally, mealworms cofed with wheat bran and MPS of this age had comparable mortality, larval growing curve and pupation distribution as the control group with wheat bran. Our results demonstrated that mealworms in this larval stage had the greatest resistance to high doses of microplastic feeding. We suggested that microplastic waste could be provided to three-month old mealworms as half replacement of bran diet to result in the greatest plastic consumption and degradation.