Concern for the environment has increased interest in reducing the amount of pesticides applied to agricultural land. This can be accomplished by immobilization of the pesticides in polymer supports, which prevents volatilization degradation and leaching losses, and provides controlled release of the pesticides. In the present study, acetamiprid, a novel pesticide, was enclosed in polylactide (PLA)-based microspheres using solvent evaporation method via oil-in-oil (O/O) emulsion. Amount of acetamiprid released from PLA microspheres was less than 5% in phosphate buffered saline. On the other hand, incorporation of water-soluble polymer, such as poly(ethylene glycol) (PEG) and poly(oxyethylene) diglycolic acid, into the PLA microspheres resulted in increased amount of released acetamiprid (similar to 70%). Planting hole application by greenhouse pot test demonstrated that the efficacy of the PLA/PEG microspheres enclosing acetamiprid against cotton aphids was superior to that of PLA microspheres enclosing acetamiprid. (C) 2008 Wiley Periodicals, Inc.