A biochemical analysis of Black Soldier fly (Hermetia illucens) larval frass plant growth promoting activity

by Terrence Green

Black Soldier fly (Hermetia illucens) larval (BSFL) frass was examined for its nutrient nitrogen, phosphate and potassium (N:P₂0₅:K₂O), phytohormone and biogenic amine content, its plant growth promoting activity, and screened to test the hypothesis that bacteria characteristic of the genus Enterococcus (present in the biome of decaying catering waste and the larval gut) are excreted by BSFL in their frass. Frass plant growth promoting activity was measured by comparing the growth of winter wheat berry (Triticum aestivum) in frass treated soil to that of untreated (control) soil. The N:P₂0₅:K₂O percent dry matter average, biogenic amine and phytohormone content of frass was determined by standard soil analysis, HPLC and HPLC/GC-MS methodologies, respectively. All were at too low of concentrations to account for its plant growth promoting activity. Frass added to soil induced a 11% increase in aerial mass and shoot length in treated plants over controls. Numerous colonies of Enterococci growing out on BEA (bile-esculin-agar) plates were detected in frass collected directly from larvae confirming the hypothesis that viable Enteroccoci are passing from the larval gut into their frass. Since a number of rhizobacteria, including Enterococci, have previously been identified as part of the larval gut biome, the passage of Enterococci from the larval gut into frass in the face of only trace N:P₂0₅:K₂O percent dry matter averages, biogenic amine and phytohormone content is consistent with the hypothesis that Enterococci exhibiting rhizobacterial activity have a role in conferring to frass its plant growth promoting activity.