Reducing the microcapsule diameter by micro-emulsion to improve the insecticidal activity ofBacillus thuringiensisencapsulated formulations

The emulsion/internal gelation method is highly effective to produce microcapsules of Bacillus thuringiensis (Bt) in a short time; however, it has the limitation to produce microcapsules within a wide range of diameters (1–1000 µm). The aim of this study was to reduce the range of small microcapsule diameters by using a water/corn-oil (W/CO) micro-emulsion as the dispersing medium and the mixture Tween 80–Span 80 as the surfactant. It involved the development of the W/CO micro-emulsion and the determination of the suitable agitation time to disperse the gelling medium (sodium alginate) through the micro-emulsion. A micro-emulsion formulation that allowed reduction of the microcapsule diameter was composed of 82% corn oil, 12% alginate solution and 6% surfactant mixture Tween80–Span80 (31:69). Evaluation of four dispersing times showed that 45 min was suitable to produce 75% of microcapsules of an average diameter of 3.1 ± 1 µm containing the spore–protein complex (SPC) produced by Bt. Bioassays carried out at low concentrations of microencapsulated formulations of cry proteins allowed determination of how its insecticidal effect increased if the range of microcapsule diameters was reduced in the range 1–9 µm. Furthermore, the SPC formulation in alginate microcapsules showed high resistance to extreme irradiation (2.9 ± 0.5 × 108 erg) of a long wavelength (365 nm), which made the microencapsulated formulation profitable and of high yield since repeated applications of the biopesticide during the same harvest period may not be necessary.