Determination of microplastics in reared black soldier fly larvae (Hermetia illucens) using polarised light optical microscopy

Microplastics have become ubiquitous in the environment and are increasingly found in a variety of matrices, including in animal feed. At the same time, the presence of plastics from packaging materials in animal feed is still prohibited in the European Union. This is a major barrier preventing certain organic waste streams from being repurposed as feed. However, issues associated with the presence of plastic particles in feed for ‘conventional’ livestock such as poultry and pigs (e.g. accumulation in stomach/other tissues) may not pose a problem to reared insects such as black soldier fly larvae (BSFL). In the hypothetical absence of such transfer, insects reared on feed contaminated with plastics could potentially be repurposed as feed. Such reuse would be a major opportunity for a more circular food chain. The aim of this exploratory study was to develop a method to determine the potential transfer of microplastics from the substrate to biomass of reared BSFL. Tested plastics consisted of low-density polyethylene (LDPE) of three different sizes, and polyethylene terephthalate (PET); to mimic the types of synthetic polymers commonly used for food packaging. The results showed no significant effects of dietary exposure of tested (micro-)plastics on larval yield and survival. We benchmarked fluorescence microscopy through Nile Red staining against a newly developed technique utilizing polarised light optical microscopy to identify plastic particles in BSFL. Tested LDPE particles were virtually absent from the post-exposure larvae. PET was recovered from the larvae, although it is unclear what the exact mechanism for this transfer was. More research is needed to validate these experimental findings with plastics of different types and particle sizes.