The objective of this study was to evaluate the effects of natural phytosanitary products (NPs) on spores and crystals of Bacillus thuringiensis subsp. kurstaki S-1905 (Btk S-1905). For the spore assay, NPs and bacteria were applied in combination and individually. For the combined application, Btk S-1905 + NP mixtures were inoculated on nutrient agar (NA), and for the separate applications, the NPs were spread on NA plates, which were later inoculated with the pathogen. The number of colony-forming units (CFU) per milliliter was quantified after 18 h of incubation. For the crystal protein degradation assay, the Btk S-1905 + NP mixtures were added to the diet of Anticarsia gemmatalis (Lepidoptera: Erebidae), and mortality was evaluated at the following time points: 12, 24, 48, and 72 h. Scanning electron microscopy and agarose gel electrophoresis were carried out. Biogermex and Ecolife® reduced the CFU ml−1 in both combined and separate applications. Biogermex, Ecolife®, and Planta Clean were antagonistic to the action of bacterial toxins, and no product affected the morphology or resulted in the degradation of the crystal proteins. The remaining products evaluated did not reduce the CFU ml−1 and had additive effect when combined with the crystal toxin.

Most field experiments in the microbial control of Chilo spp. have been performed using strains of Bacillus thuringiensis (B. t.). Generally, low levels of control were obtained, which is probably due to the relatively narrow host range of the B. t. strains used. B. t. formulations and different B. t. strains obtained from overseas culture collections or isolated from material collected in Kenya were screened for activity against C. partellus.

C. partellus was found to be susceptible to commercial preparations based on B. t. kurstaki only. The screening and bioassay work on identified B. t. strains showed differences in the toxicities of the various subspecies groups for C. partellus. There were also differences in the toxicity of isolates within the same subspecies. The most toxic exotic strain obtained was B. t. entomocidus. Excellent control of C. partellus larvae, on artificially infested plants, was obtained. Plant damage was significantly reduced and yield levels increased by 5 to 7 times over the untreated controls. Three strains, M-44–2, A-3, and A-C-2, gave the best levels of plant protection.

Different methods of applying the pathogen to the crop have also been investigated. Aqueous suspensions and a granule/powder formulation have given very good levels of borer control in screenhouse experiments.