Effect of three defatted soybean meals on insecticidal activity of Bacillus thuringiensis formulation

The insecticidal activity of Bacillus thuringiensis (BT) formulation for silkworms (Bombyx mori) is affected by the defatted soybean meal manufacturing and refinement. The LC50 of silkworms reared on artificial diets with SP2 and SP600 soybean meals (3.02 and 3.38 mu g/g diet, respectively) was significantly higher than the LC50 of silkworms reared on artificial diets with STN meal (2.41 mu g/g diet). STN soybean meal is an ordinary meal (Katakura Kogyo Co., Tokyo) used by ACIS (Agricultural Chemicals Inspection Station). SP2 and SP600 are products of another manufacturer (Nissin Cosmo Food Co., Tokyo), but the refinement processes are different. SP600 contains more protein (68%) and less breakdown products (NSI>30%) than SP2 (53% protein, NSI<30%). Western blot analysis using a monoclonal antibody against Cry1Aa (a BT delta-toxin) showed Cry1Aa bindable proteins in the soybean meals. The band-intensity of each meal was different (STN<SP2<SP600). It is suggested that these proteins in the soybean meals bind to Cry1Aa competitively and inhibit the activity of the BT formulation used in these experiments.