Total gas and methane emissions of black soldier fly Hermetia illucens grown on different organic wastes

Edible insects have been suggested as an alternative source of protein for animal feeding. The production of insect meals should have a lower environmental impact as compared to other productions such as beef or dairy products. However, few information is available in literature with special regards to Green House gases emissions. An experiment was conducted to quantify emission of gas and methane by larvae of the black soldier fly (BSF) Hermetia illucens (Diptera Stratiomyidae) grown on different organic wastes. The experimental substrates were: a standard hen diet (control), okara, maize grain dry distillers and brewers spent grain mixed with trub (derived from brewing process). An in vitro method derived by evaluation of feed for ruminants using a semiautomatic pressure system was adapted for this purpose. Twenty BSF larvae were positioned in duplicate into serum bottles and added with the experimental substrates (diets) ad libitum. For each sample two blanks (i.e. substrates without larvae) were added. Headspace pressure was recorded after 24 hours of incubation using a digital manometer (model 840082, Sper Scientific, Scottsdale, AZ, USA). The gas pressure data were converted to moles of gas using the ideal gas law. A fixed-volume sample of gas was also collected for subsequent methane analysis using gas-tight syringes fitted with needles through the bottle top. The gas composition of the headspace was determined by micro GC gas chromatograph (Agilent Technologies, Santa Clara, CA, USA). An external standard mixture of CO2 and CH4 was used for instrument calibration. For all substrates, no detectable traces of methane were determined in the analysed air samples. There were differences on total gas (ml of Gas/g of incubated dry matter) produced by BSF larvae incubated with the different substrates and corrected for the blank gas production. Particularly, total gas productions were higher for larvae incubated with hen diet and brewers mixed with trub, intermediate for okara and lower for larvae incubated with distiller grains. The method proposed seems promising to estimate gas and CH4 productions of BSF larvae in a simply and fast way. The differences in gas production between BSF larvae fed with the different substrates could be useful to better define the optimal diets for this species