RESUMO
Insecticide susceptibility in tobacco budworm, Heliothis virescens (F.) (Lepidoptera: Noctuidae), was determined for 8 yr (1991-2001) with larvae sampled from cotton in southern Tamaulipas, Mexico. Before 1996, when Bollgard cotton expressing the Cry1A(c) delta-endotoxin was introduced into the region, two important patterns were documented. The first was economically significant increases in resistance to certain insecticide groups. The second was occurrence of virtually complete control failures in the field during 1994 and 1995. The largest resistance changes were recorded for the type II pyrethroids cypermethrin and deltamethrin. These products are the most widely used products in the region. Resistance ratios for these products increased up to > 100-fold from 1991 to 1995. After 1996, the resistance levels declined. These findings did not occur with other products of scant use (e.g., permethrin, profenofos, and endosulfan) or low tobacco budworm efficacy coupled to a high use pattern (e.g., methyl parathion). This clear trend toward reversal of resistance to type II pyrethroids can be understood, in part, with respect to two factors: 1) the high adoption rate of transgenic cotton in the region, from 31.2% in the beginning (1996) to approximately 90% in 1998; this has considerably curbed the use of synthetic insecticides, with the attending loss of selection pressure on this pest; and 2) the potential immigration to the region of susceptible tobacco budworms from cultivated and wild suitable hosts as well as from transgenic cotton might have influenced the pest population as a whole. The influence of transgenic cotton on southern Tamaulipas can be more clearly seen by the drastic reduction of insecticide use to control this important pest. Now tobacco budworms in this region are susceptible to type II pyrethroids. Two effective and fundamentally different pest management tools are now available to cotton growers in southern Tamaulipas: transgenic cotton, coupled with careful use of pyrethroids, offers the possibility of sustainable and profitable cotton production.