Casting of kraal manure by larvae of the African rhinoceros beetle, Oryctes boas (Fabricius) and the physico-chemical properties of the casts

Abstract

The influence of manure ingestion by larvae of Oryctes boas on casting and cast properties was studied in an incubation microcosm experiment. Inoculation of manure with larvae of O. boas significantly reduced the proportion of large (>8.0 mm) aggregates from 81 % in non-cast manure to 3.5% in cast manure. In turn, the larvae converted the majority (92.7%) of the aggregate sizes into the 2–8 mm range compared with 14.5% for the un-inoculated manure. Mean weight diameters of dry aggregates were 5.34 mm and 18.45 mm for the treatments with and without O. boas, respectively. The casts also had significantly higher proportions of water stable macro-aggregates than non-cast manure (99.8 vs 90.2%). The higher stability of casts corresponded with their higher tensile strength (769 vs 408 kPa) and bulk density (1.58 vs 1.07 Mg m⁻³) compared with the non-cast manure. Chemically, the production of casts from manure resulted in a pH shift towards neutrality and significant increases in the concentration of plant nutrient elements (N, P, Ca, K and Mg) compared to non-cast manure. The results suggest that the larvae of O. boas can play an important role as soil ecosystem engineers and provides a promising option for converting organic wastes into strongly aggregated granular compost.