Influence of oil droplet size on the toxicity of bifenthrin on cotton to tobacco budworm (Heliothis virescens)

Sprays of bifenthrin and paraffinic oil mixtures were applied to cotton leaves (Gossypium hirsutum L.) to investigate droplet size effects on the mortality of Heliothis virescens (F.). An air‐assisted nozzle applied mixtures at 9.4 litre ha −1 spray rate with volume median diameters (VMD) of 96 and 337 μm, as verified with a Malvern laser diffraction analyzer. An hour after spraying, susceptible 3rd‐instars walked for 2‐min through wet droplets of spray mixtures applied at bifenthrin rates of 22, 45, 67, 90. and 112 g a.i. ha −1. Mortality was assessed at 21, 26, 45, 50, 74, and 117h after exposure. Final mean mortality was affected (P =0.01) by bifenthrin rate and droplet size. Bifenthrin applied in 337 μm VMD droplets caused 75.8% mortality, whereas 96‐μm VMD droplets resulted in a mortality of 67.9%. Also, the 337 μm VMD droplets killed larvae in 13% less time than the 96μm VMD droplets, based on analysis (P =0.01) using the mean elapsed time till death as the dependent factor.