Preparation And Physicochemical Characteristics Of Biodegradable Microspheres Containing Spinosad And Emamectin Benzoate

Spinosad and ememactin benzoate were both new and highly promising pesticides with efficiency against a wide range of insects. Recently, spinosad resistance to a variety of target pests had developed quickly all over the world. At the same time though emamectin benzoate resistance level was still lower, its risk resistance of emamectin benzoate was great. Compound use of spinosad and emamectin benzoate could delay the development of resistance on target pests effectively. In adding, control efficiency against target pests enhanced and the pressure of pesiticides on environment could be reduced by using mixed pesticides at the same time. But the degeradation rate of spinosad and ememactin benzoate under light was too fast that efficacy durations of pesticides were short obviously. The problem could be solved if spinosad and emamectin benzoate could be prepared into pesticides microspheres.Non-biodegradable material such as polyurea polyurethane was still mostly used in processing procedure of pesticides controlled-release formulations. Environmental problems would be caused seriously if non-biodegradable materials were widely applied for a long time. Biodegradable microspheres containing spinosad and emamectin benzoate were prepared using polylactic acid as wall material by solvent evaporation method and its physicochemical characteristics were studied.1. Preliminary optimized preparation condition of microspheresInfluence of emulsifier type, content of emulsifier, content of PLA, mass ratio of core material to wall material, ratio of oil phase volume to water phase volume and stirring speed on diameter, span, pesticides loading, and entrapment rate of the microspheres was investigated respectively and the preliminary optimized preparation condition of microspheres was obtained. The preliminary optimized preparation condition was that emulsifier type was PVA1788, content of emulsifier was 20 mg/mL, content of PLA was 90 mg/mL, mass ratio of core material to wall material was 1:1, ratio of oil phase volume wasl:10 and stirring speed was 700 r/min respectively. The pesticides loading, entrapment rate, diameter, and span of microspheres prepared as the preliminary optimized preparation condition were that 39.47±1.06%,91.79±2.46%,9.84±0.27μm and 1.468±0.132 respectively.2. Optimized preparation condition of microspheresInfluence of content of PLA in oil phase(X1), theoretical pesticide loading(X2) and stirring speed(X3) in the preparation process was investigated using central composite design. Integrated effect of dependent variables including pesticide loading, entrapment rate, mean diameter, span and release percentage in 5 days was transformed into overall desirability by mathematical method. Data of overall desirabilities were fitted to a second-order polynomial equation, through which three dimensional response surface graphs were described. Optimum experimental conditions were calculated by design-expert v8.06. The results indicated that date of overall desirability conformed to multivariate quadratic equations while coefficient of determination was 0.8325. The highest overall desirability (0.6811) could be obtained when content of polylactic acid, theoretical pesticide loading and stirring speed were 77.79mg/mL,51.06% and 560.91r/min respectively. Microspheres prepared according to the optimum experimental conditions resulted in an overall desirability of 0.6305 while entrapment rate, mean diameter, span, release percentage in 5 days were 84.49%,7.19μm,1.134,52.95% respectively. The bias between observed and predicted values of the overall desirability of the optimum formulation was-5.03%.3. Study of release property of microspheresMethods of pesticides saturated external aqueous phase, modified solvent evaporation method and external coating technology of microspheres were applied in order to improve the release property of microspheres. The results indicated that pesticides release speed of microspheres decreased and the T50 extended from 5.45d to 8.86d when external aqueous phase was saturated by spinosad and emamectin benzoate. The pesticides release speed of microspheres prepared from modified solvent evaporation method increased and the T50 reduced from 5.45d to 3.70d. In adding, when the volume of extraction solution increased, the pesticides release speed increased also. The pesticides release speed decreased greatly and initial burst release effect almost disappear after microspheres were coating with a gelatin firm. The T25 and T50 could be 9.33d and 53.69d evenly. When content of the used gelatin solution increased, the pesticides release speed decreased sequentially.4. Preparation of suspension concentrate of microspheres and its indoor toxicityFormulation of 5% suspension concentrate of microspheres of containing spinosad and emamectin benzoate was prepared. It was proved that its physical and chemical characteristics had been no apparent changes after the formulation was stored under normal,cold and heating conditions respectively. The toxicity of 5% suspension concentrate of microspheres of containing spinosad and emamectin benzoate after different time under nature light against the larva of Plutella xylostella was evaluated using mixture of 2.5% spinosad SC and 1.5% emamectin benzoate EC in certain proportion as CK. The results indicated that the toxicity of suspension concentrate of microspheres was poor than CK group, but its long-last toxicity was excellent.