The effect of polymers on the stability of microencapsulated formulations of Bacillus thuringiensis subsp kurstaki ( Bt-KD2) after exposure to Ultra Violet Radiation

We compared the protective effect of three polymers; starch, gelatin and sodium alginate (2, 3, 5%) as coating materials, on the stability of microencapsulated formulation of Bacillus thuringiensis after exposure to Ultra Violet (UV) R. Microencapsulated Spore Crystal Aggregate (SCA) formulations were prepared by the emulsion gelling method. The protective effect of polymers was evaluated by measuring spore viability. Bioassay and release tests were done on the microencapsulated formulations. Sodium alginate (5% w/w) showed the highest viabilities of 90 and 86% after exposure to Ultra Violet in long term (UVB 385 nm) and Ultra Violet in short term (UVC 254 nm) radiation, respectively, while viabilities of non-microencapsulated spores under these conditions were 40 and 50%, respectively. The crystal activity (mortality) of irradiated and non-irradiated free spore formulations on second-instar larvae of Ephestia kuehniella were 15 and 93%. However, the mortalities caused by irradiated and non-irradiated microencapsulated formulations were 70 and 80% on the 10th day of the experiment. The size range of the microcapsules was 7-20 mu m while the microcapsulation efficiency was 86%. The release behaviour of microspheres conformed best to Korsmeyer-Peppas semi-empirical model with the correlation of R-2 = 0.98.