Release and Degradation of Microencapsulated Spinosad and Emamectin Benzoate

Abstract

The dynamics of release and degradation of the microencapsulation formulation containing spinosad (SP) and emamectin benzoate (EM) were evaluated in the present study. SP and EM were microencapsulated using biodegradable poly-lactic acid (PLA) as the wall material. Their release from and degradation within the prepared SP and EM microspheres (SP-EM-microspheres) were studied. It was found that the encapsulation significantly prolonged the insecticide release. The release could be further extended if the external aqueous phase was pre-saturated with the insecticides and the microspheres were additionally coated with gelatin. On the other hand, increasing the water content of the emulsion or the hydrophilic polycaprolactone (PCL) content in the PLA/PCL mixture accelerated the release. Due to the photolysis and hydrolysis of SP and EM by sunlight, the toxicity of the non-encapsulated insecticides in water declined continuously from 0 through the 9^th day (d), and dissipated in 13 d. In contrast, an aqueous suspension containing 5% SP-EM-microspheres maintained a mostly constant toxicity to Plutella xylostella for 17 d. The biodegradable SP-EM-microspheres showed significantly higher long-term toxicity to P. xylostella due to lower release, reduced photolysis and hydrolysis of the encapsulated insecticides, which were affected by the varied preparation conditions.

Figure 1

Release rates of SP + EM in SP-EM-microspheres prepared under different conditions. SP + EM contents were determined by spectrophotometer method. In typical solvent evaporation method, the external aqueous phase was not saturated with SP and EM, the emulsion was not added with additional water, PLA was the lone wall material, and no gelatin was applied for coating the microspheres. (A) External aqueous phase with or without insecticides saturation. (B) With or without added distilled water in emulsion. (C) PLA and PCL in different ratios for microsphere wall. (D) Concentrations of gelatin for microsphere coating. In (A), (B) and (D), only PLA was used. In (A), (C) and (D), no water was added to emulsion.

Figure 2

Light degradation of insecticides in SP-EM-microspheres or as naked insecticides.

Figure 3

Graphical representation of research work. Biodegradable microspheres containing spinosad and emamectin benzoate with poly-lactic acid as wall material showed significantly lower release rate, photolysis and hydrolysis of the two insecticides, and higher long-term toxicity.