Effects of swine manure mixed with circulating fluidized bed fly ash on black soldier fly (Diptera: Stratiomyidae) larvae and larval frass.

Abstract

Black soldier fly larvae (BSFL) were reared on mixtures of swine manure and circulating fluidized bed fly ash (CFA) in different ratios. The aim was to evaluate the impacts of insoluble inorganic matter on BSFL and larval frass. The growth performance and nutrient composition of the BSFL were measured under different treatments. The intestinal microbiota structure, morphological characteristics, and total proteolytic activity of the gut were analyzed. The larval frass was tested for nutrients and analyzed using energy-dispersive spectroscopy and scanning electron micrographs. In particular, the surface areas of microparticles from the larval frass (diameter < 0.0074 mm) were measured using Brunauer-Emmett-Teller method. It was found that CFA addition prolonged larval development and reduced the maximum larval weights. The mean larval length, crude protein, and highest larval weight showed negative regression with an increase in the CFA ratio (P < 0.05). Morphological images indicated that physical clogging might be the main influencing factor on larval growth. Moreover, the microbial diversity and complexity in the larval gut increased with CFA addition, but CFA addition had little effect on the composition of dominant phyla or genera (P > 0.05). Finally, the nutrient composition revealed that the frass met the organic fertilizer standard when the CFA addition ratio was less than 7.5%. The optimal addition ratio was 5%, at which the larvae had a more stable and healthier gut environment, but there was less of an effect on larval growth and nutrient composition. Moreover, particles from 5% CFA mixture had the highest surface area.