Effects of droplet size and volumetric application rate of tractor-applied lambda cyhalothrin (Karate Z 2.08 SC) on larval tobacco budworm, Heliothis virescens (F.), mortality and insecticide deposition on and within the canopy of mature cotton were explored by use of a laboratory bioassay incorporating field-sprayed cotton and a field trial incorporating natural insect infestations. Insecticide was applied in all combinations of three distinct droplet sizes and three volumetric application rates. Mortality of third-instar tobacco budworm occurring in leaf-disk bioassays was highly correlated with insecticide deposits (μg/leaf disk) at upper- and midcanopy levels. At the upper canopy level, mortality was negatively correlated with volumetric application rate and was not significantly correlated with droplet size. Deposit was negatively correlated with volumetric application rate at the lower plant level and larval mortality decreased with increasing droplet size. Results from this study do not support the recommendations of high volumetric application rates; and although droplet size was less influential than volumetric application rate in deposit and insect mortality, the data indicate a significant trend toward increased midcanopy larval mortality with smaller droplets.
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Vol. 94 • No. 3