Pro-active Management of Beet Armyworm (Spodoptera exigua) Resistance to the IGRs, Tebufenozide and Methoxyfenozide

Susceptibility to tebufenozide and methoxyfenozide of beet armyworm (Spodoptera exigua) from the southern United States and Thailand was determined through exposure of first and third instar larvae to dipped cotton leaves. LC50 estimates of first instar larvae ranged from 0.377 to 32.7 micrograms of tebufenozide per milliliter and 0.034 to 11.5 micrograms of methoxyfenozide per milliliter. LC50 estimates of third instar larvae ranged from 4.37 to 715 micrograms of tebufenozide per milliliter and 0.393 to 47.4 micrograms of methoxyfenozide per milliliter. These estimates translated into 87-fold and 164-fold decreases in susceptibility to tebufenozide and 338-fold and 121-fold decreases in susceptibility to methoxyfenozide of first and third instar larvae from a Thailand strain when compared to the most susceptible of eight United States populations evaluated. Among the United States field populations evaluated, a collection from Belle Glade, Florida was the most susceptible and one taken near Parker, Arizona was the least susceptible. Selection of the Thailand population with tebufenozide or methoxyfenozide resulted in significant reductions in susceptibility to both analogs, indicating a common mechanism of resistance. Isolation and characterization of resistance will provide information that will be helpful for pro-active management of resistance for this valuable group of insecticides in the United States.