Beet armyworm (Spodoptera exigua) resistance to spinosad

Susceptibility to spinosad (Success(R)/Tracer(R)) of beet armyworm (Spodoptera exigua) from the southern USA and Southeast Asia was determined through exposure of second- and third-instar larvae to dipped cotton leaves. LC,,, estimates of susceptibility of second- and third-instar larvae of field populations ranged fi om 0.279 to 6.14 and 0.589 to 14.0mg spinosad litre(-1), respectively. A Thailand population was 22- and 24-fold less susceptible than the six other US field populations evaluated, and 85- and 58-fold less susceptible than a reference laboratory population, respectively. From these results, we initiated experiments to test the hypothesis that the Thailand population was resistant to spinosad. Ft crosses between the resistant Thailand population and a susceptible reference strain yielded individuals that were 22-fold less sensitive to spinosad than the susceptible parent. This same resistant strain exhibited significantly greater survivorship on plants treated with spinosad in the field. Lastly, selection of an Arizona population resulted in a significant reduction in susceptibility to spinosad, further substantiating the hypothesis of a genetic basis for resistance to spinosad. These findings indicate a vulnerability of this new insecticide to resistance development in beet armyworm and should serve as a warning against excessive use of it. (C) 2000 Society of Chemical Industry.