ABSTRACT

Larvae from four crosses of three field collected strains of beet armyworm (Spodoptera exigua (Hubner)) (BAW) from FL and TX, United States of America (USA) and northern Tamaulipas, Mexico were treated with 14 insecticides for one, two and 10 generations. An LD50s of 20 µg/larva was used as a resistance threshold to separate resistance from susceptibility. Cypermethrin showed selection for resistance in the cross of the three strains, while the other crosses and the three strain cross showed cross and multiple resistance and reversion to susceptibility. Susceptibility was shown to bifenthrin, chlorpyrifos, deltamethrin, zeta cypermethrin, fenpropathrin, lambda cyhalothrin and chlorphenapyr by the Alva x Donna and the Alva x Donna x Rio Bravo crosses. Crosses of strains of the BAW can be resistant to one insecticide one generation yet be susceptible to the same insecticide the next. Factors for resistance and susceptibility of these crosses have to be polygenic.