INFLUENCE OF OIL DROPLET SIZE ON THE TOXICITY OF BIFENTHRIN ON COTTONTO TOBACCO BUDWORM (HELIOTHIS-VIRESCENS)

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