From 1990-1993 proportion and timing of tobacco budworm, Heliothis virescens (F.), populations in northern Tamaulipas, Mexico, cotton fields were determined. Percentage tobacco budworm was greater than bollworm, Helicoverpa zea (Boddie), on cotton during the first five weeks of fruiting. Larval parasitism was variable from 1990 to 1993 with 1991 showing greatest parasitism.

In 1991, resistance was not shown by bollworm and tobacco budworm to either methyl parathion or permethrin in northern Tamaulipas. In 1992 LD(50)'s of methyl parathion and permethrin were greater in northern Tamaulipas than southern Tamaulipas. In 1993 LD(50) values increased 11.3 times to methyl parathion and 23.3 times to profenofos in cotton near Matamoros compared to LD(50) values from Rio Bravo in 1992; both locations were in northern Tamaulipas.