Cotton is an important comodity in Southern Tamaulipas, Mexico. It was in 1994 and 1995 when this crop reached its peak planted area; however, in 1995 the highest Tobacco Budworm (Heliothis virescens) LD₅₀ values were found: Profenofos 5.34x, Permethrin 4.46x, Cypermethrin 6.12x, Deltamethrin 18.25x, and 14.0x for Lambda-cyhalothrin. In 1997 an important decrement of this pest's LD₅₀ values were observed for the above mentioned insecticides. This can be attributed to two factors: a) the use of Bollgard^®; cotton and, b) a reduction in the number of insecticide applications to control this pest, since it is not necessary when using this cotton. Some of these pesticides quickly regained LD₅₀ values similar to those of the reference colony.