Canteen waste as food for black soldier fly larvae: risk of heavy metals accumulation? Variability during one year of rearing

Worldwide, about 244 million t/yr of waste are produced by canteens and restaurant, of which 4.7 million t/yr are generated at the Italian level. Canteen waste (CW) could be used for feeding larvae of insect to obtain new marketable products (e.g. protein meal, frass and chitin). Black Soldier Fly Larvae (BSFL) are voracious and polyphagous species, and CW is a complete food for them. Furthermore, BSFL are capable to bioaccumulate some heavy metals thus allowing to obtain safe organic frass, but the bioaccumulation in larvae intended for feed purpose needs to be evaluated. The CW was sampled once a month and the sampling was carried out for a year from a local canteen, located in the northern part of Italy. The feeding was conducted in quadruplicate in plastic containers (12 l) and 2,000 g of CW and 2,000 5 days-old larvae were allocated in each box. Each trial ended after 7 days and growth performances were evaluated. The results reported an average dry matter final larval biomass, along the year, of 83.8 ± 46.05 g and an average dry matter frass quantity equal to 199.9 ± 140.32 g. The percentage of dry matter for initial diet was 21.7 ± 14.10, for final larval biomass was 34.3 ± 7.39%, and for the residue of 62.2 ± 17.01%. The initial diet, BSFL before and after feeding, and the residue were sampled and oven dried. Elemental analyses were performed with a mass spectrometer in triplicate for evaluating arsenic, lead, cadmium, and mercury quantities within CW, residues and BSFL before and after feeding. This trial certifies the ability of BSFL to accumulate (BAF > 1) arsenic (average of 0.165 ± 0.1480 ppm), lead (average of 0.238 ± 0.1562 ppm), cadmium (average of 0.077 ± 0.0367 ppm), and mercury (average of 0.008 ± 0.0043 ppm) during feeding, and it proves that the bioaccumulation complies with European legal limits on the amount of heavy metals in larvae.