A Culex quinquefasciatus colony was established from surviving late instars and pupae from a semifield microcosm test with Natular® XRG (a granular formulation containing 2.5% spinosad) applied at 14.2 kg/ha. The initial lethal concentrations for 50% and 90% of the population (LC50 and LC90) against Natular XRG were determined in the laboratory for the 1st generation progeny (F1). Selection was applied at LC70–90 levels to 10,000–15,000 of late 3rd and early 4th instars each generation. Susceptibility changes were determined every other generation and referenced to a susceptible laboratory colony as well as freshly collected wild populations of the same species concurrently. Tolerance to spinosad (resistance ratio <5.0 fold) was observed up to F8 generation in the selected population. Thereafter, resistance ratios increased significantly from F10 to F16 generations, being 7.26- to 20.52-fold at the LC50 and 7.48- to 20.08-fold at the LC90 level.
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