Abstract
A series of tests was designed to expose third instar Plutella xylostella L. to discrete deposits of uniformly sized spray droplets containing permethrin. Larvae from five U.S. populations were evaluated for resistance to this pyre throed. The LC50 for dip and spray droplet (100-120 J!m diameter droplets) tests differed by 10- to 1,000-fold for the same population. Interpopulation differences were also 1,000-fold in both as says. The spray assay LC50 of larvae from three of the populations exceeded 40 g/liter (equivalent to approximately 100 times the concentration recommended for conventional spray application). Application of different droplet densities to the upper surface of leaf disks had no significant effect on subsequent mortality in four of the populations, emphasizing the importance of obtaining spray deposition on undersides of leaves.
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