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Beet armyworm, Spodoptera exigua (Hubner), larvae, collected from flowering plants in Alva, FL, were selected for resistance for 13 generations. Selection was conducted with 10 insecticides which represented three classes of insecticides. First generation of larval treatment was filial one. A resistance threshold of 20 µg/larva for each insecticide was used to indicate resistance or susceptibility in each generation. None of the insecticides showed selection for resistance. Strain was susceptible to bifenthrin and profenofos in all generations. Cross resistance was shown in generation nine by chlorpyrifos and methyl parathion, but not by profenofos. Multiple resistance to non-cyclopropane pyrethroids, i.e. fenvalerate and esfenvalerate, and organophosphorus insecticides was evident in generations three, six and 10 through 12. Susceptibility (LD_< 20 µg/larva) to the insecticides tested was present in 25%, 67%, 67%, 100%, 86%, 25%, 89%, 50%, 50%, 0%, 100% and 25% of this strain in generations 1 through 12, respectfully. A laboratory reference susceptible strain had lower LD_s for each insecticide than shown by Alva strain in any generation. Only non-significant regressions were determined in generation 13. With this variation in response resistance factors could be polygenic. |
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Document last modified XXXXXX, XXX XX 2001
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