Improving the biological efficacy of small droplets of permethrin by the addition of silicon-based surfactants

Oil-soluble and oil-dispersible silicon-based, non-ionic surfactants were incorporated in two oil-based formulations of permethrin. At the optimal concentration of surfactant, 40 μm droplets of the insecticide were twice as effective against whitefly (Trialeurodes vaporariorum) larvae as droplets without surfactant. The addition of a surfactant to a formulation containing 10 g/litre permethrin resulted in a spray mixture which was at least as effective as the same formulation containing 100 g/litre permethrin without surfactant. These improvements in efficacy were not attributable to droplet spread or perimeter on the leaf surface, or to differences in the surface tension of the mixtures. The implications of these results for ultra-low volume spraying are discussed. A statistical model based on the zero-cell of the Poisson distribution is described and used to analyse the bioassay results.