Synergistic interactions have been documented when binary mixtures of cypermethrin with amitraz or piperonyl butoxide were applied to the eye of pyrethroid resistant tobacco budworm (TBW), Heliothis virescens (F.), adults. Amitraz is thought to be a "target site synergist" in the case of formamidine/pyrethroid interactions, whereas piperonyl butoxide is known to block oxidative metabolism of pyrethroids. Therefore, these observations suggest that both kdr type and metabolic resistance are present in adults. Thus, the adult vial bioassay appears to be measuring resistance that is similar mechanistically to that previously demonstrated in TBW larvae.