Bioaccumulation of heavy metals in the black soldier fly, Hermetia illucens and effects on its life cycle

In developing countries, effective waste management strategies are constrained by high collection costs and lack of adequate treatment and disposal options. The organic fraction in particular, which accounts for more than 50% of the waste production, constitutes a great, yet mostly neglected, reuse potential. Concomitantly, the demand for alternative protein sources by the livestock feed industry is sharply increasing. A technology that effectively transforms organic waste into valuable feed is therefore a timely option. Larvae of the non-pest black soldier fly, Hermetia illucens L. (Diptera: Stratiomyidae), may be used to reduce the mass of organic waste significantly. Concurrently, larval feeding converts organic waste into prepupae (last larval stage) which is high in protein. In combination with a viable market, this potential animal feed may cover the waste collection costs and thus promote innovative, small-scale entrepreneurs to establish a profitable business niche. Organic waste, however, often contains persistent pollutants, such as heavy metals, that may accumulate in the larvae and prepupae of black soldier flies and consequently in the food chain. In this study, we fed black soldier fly larvae chicken feed spiked with heavy metals (cadmium, lead and zinc at three concentrations each) to examine the extent of metal accumulation in the different life stages and the effect of heavy metal concentration in the feed on the life cycle determinants of the flies. The cadmium accumulation factor in prepupae (metal concentration in the body divided by metal concentration in the food) ranged between 2.32 and 2.94; however, the lead concentration remained well below its initial concentration in the feed. The bioaccumulation factor of zinc in prepupae decreased with increasing zinc concentration in the feed (from 0.97 to 0.39). None of the three heavy metal elements had significant effects on the life cycle determinants (prepupal weight, development time, sex ratio).