Solid lipid microparticle formulations of the pyrethroid gamma-cyhalothrin-incompatibility of the lipid and the pyrethroid and biological properties of the formulations.

Pyrethroids such as gamma-cyhalothrin (GCH) are valuable insecticides that possess a high and unwanted toxicity towards aquatic organisms. The aim of the present study was to test the ability of the solid lipid nanoparticle technology to reduce the aquatic toxicity and concurrently retain the insecticidal activity of GCH. Applying the lipid Compritol) 888 ATO and homogenising the crude o/w emulsions of melted GCH and compritol at different pressures, 150-1500 bar, solid lipid microparticle formulations were produced having average particle diameters between 0.3 and 100 microm. GCH had limited solubility in the solid lipid phase and probably the lipid showed transition from the alpha to the beta' crystal form upon storage. This resulted in expulsion of GCH from the lipid and appearance of GCH crystals in the water phase of the formulations. By using as surfactant polyvinyl alcohol instead of a mixture of Synperonic PE/F68 and sodium lauryl sulphate the appearance of GCH crystals was delayed. This delay was probably due to the fact that the GCH solubility was significantly lower in aqueous polyvinyl alcohol solutions than in solutions of the above-mentioned surfactant mixture. Compared with a traditional emulsifiable concentrate formulation of GCH the solid lipid microparticle formulations reduced the toxicity towards fish (Brachydanio rerio) and daphnia (Daphnia magna) by a factor 10 and 63, respectively. The solid lipid microparticle formulations and the emulsifiable concentrate formulation had about the same insecticidal activity on both Dysdercus cingulatus nymphs and Spodoptera littoralis larvae. Surprisingly the particle size of the solid lipid microparticle formulations only affected the biological activity slightly.