In order to determine the main resistance mechanisms of the beet armyworm to insecticides, bioassays with insecticides mixed with sinergists inhibitors of the related enzymes in the detoxification of insecticides have been made. It was determined that the maximum sublethal dosis of the sinergists piperonyl butoxide (PBO) and butifos (DEF) on the larvae of S. exigua was 2.0 (µg/µl). The population of beet armyworm in southern Tamaulipas shows a strong tendency to non-metabolic mechanisms, such as insensitivity to cyclodienes, acetilcolinesterase insensitive to organophosphorus and carbamates and knock down resistance (kdr) to pirethroids. Therefore the management of resistance and the control of this pest with conventional insecticides would be very difficult. Then, the use of products with a different mode of action is needed, as long as their effectivity is determined in the field.