Beet army worm (Lepidoptera: Noctuidae): selection for resistance to insecticides by a five strain cross.

The beet armyworm (Spodoptera exigua (Hubner) is an important pest of cotton in southern United States, Mexico and Central American countries. Resistance by crosses to methomyl, fenvalerate and methyl parathion was indicated in many generations but susceptible in others. These are the standard insecticides for this pest. Susceptibility was determined for bifenthrin, profenofos, chlorfenapyr, emamectin benzoate, spinosid and zeta cypermethrin in all generations tested. The objective of this experiment was to determine if we could select for resistance to a five strain cross during 15 generations to 15 insecticides. All larvae were treated each generation. Eighty-seven significant and non-significant regressions indicated resistant and susceptible populations. Percentage insecticides that were susceptible ranged from 0% to 100% in each of the 15 generations. Cross reverted to susceptibility to methomyl and methyl parathion in the last two generations. A resistance threshold of 20.0 micro g/larva for the organophosphorus and carbamate, classes of insecticides and 5.0 micro g/larva for pyrethroids, macrolactone and pyrolle classes of insecticides.