ABSTRACT

A selection regime was conducted with 14 insecticides against strains of the beet armyworm from Rio Bravo and Estacion Cuauhtemoc, Tamaulipas, Mexico, and Tiquisate, Guatemala for four to six generations from 1991 to 1992. Four classes of insecticides were tested, pyrethroids, organophosphorus, a carbamate and a pyrrole. The LD50s value for resistance threshold to each insecticide was established at 20 µg/larva. For the Rio Bravo, Tiquisate and Estacion Cuauhtemoc strains resistance was indicated by 82%, 77% and 59% of the LD50s values, respectively. Cross resistance was shown by the strain from Estacion Cuauhtemoc to fenvalerate and cypermethrin. Multiple resistance by the three strains was shown for the pyrethroids and anticholinesterase insecticides in 58% of the generations. The strains were susceptible to the cyclopropane pyrethroid bifenthrin, the organophosphorus insecticide profenofos and the pyrrole, chlorfenapyr.