Evaluation of different media formulations on spore production and toxicity of Bacillus thuringiensis subsp. aizawai
Sehaole, Thibello Perseverance.
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One of the major constrains in beekeeping is damage caused by the greater wax moth (Galleria mellonella). Due to problems and concerns regarding the use of chemical insecticides, biopesticides have been introduced to manage problematic insects. For the biological control of greater wax moth, Bacillus thuringiensis subsp. aizawai (B.t.a) has been widely researched and used. Large scale production of B.t.a as a biopesticide is expensive because of the high cost of commercial media formulations. In this study, three raw materials, horse manure, soy meal and maize chops, were selected for evaluation as raw materials for media production based on their economical feasibility and availability and compared to two commercial media (Tryptone soy broth and bactopeptone). Sixty three raw material formulations and two commercial media were evaluated in terms of spore yield and toxicity towards G. mellonella. For each formulation the spore and vegetative cell yield was determined using direct microscopy. From the results obtained from the 63 formulations, the 12 highest spore yielding formulations were selected and re–evaluated for confirmation of the results. Spores obtained from the 12 highest spore yielding formulations were used in a bioassay to determine their toxicity against G. mellonella larvae and the protein concentration were also determined. Spore morphology was investigated for the 12 highest spore yielding formulations. Formulations containing 3.75 g horse manure, 5 g maize chops and with soy meal ranging between 1.25 g and 3.75 g yielded the highest spore yields. In particular the combination of 3.75 g horse manure, 5 g maize chops and 1.25 g soy meal had the highest spore yields but also resulted in 100 % mortality against G. mellonella larvae within 3 days. The yields obtained were also significantly higher than those for the commercial media against which they were evaluated. As such it was concluded that the selected formulation represents a feasible alternative to commercial media for the production of B.t.a.