Texas Resistance Monitoring Program Reveals Increase In Cypermethrin Susceptibility In Helicoverpa Zea (Boddie) Populations In 2010

The monitoring program evaluated the susceptibility of males of bollworm, Helicoverpa zea (Boddie), populations to cypermethrin in Texas. Moths were captured near cotton fields using traps with synthetic pheromone.  Bioassays were conducted with the adult vial test. Seven counties were surveyed from April to October in 2010. Data were collected from collaborators for estimation of lethal concentrations (LC₅₀ and LC₉₀), calculation of resistance ratios, and for likelihood ratio tests of equality and parallelism. Nueces populations exhibited the highest LC₅₀ (1.31 µg/vial) followed by Burleson (1.13 µg/vial) while maximum susceptibility to cypermethrin was exhibited by Hockley populations (LC₅₀ 0.53µg/vial).  The populations from Nueces and the Burleson Counties exhibited the highest LC₅₀ resistance ratios of 3.9 and 3.4, respectively. Uvalde populations had a resistance ratio of 2.3. The resistance ratios for Uvalde and Nueces showed the most drastic decline in comparison to 2009.  Populations in the High Plains of Texas remained susceptible except in Parmer County which for the first time showed a resistant population in July with a resistance ratio of 3. In summary, the Gaines, Hockley, Swisher, Nueces and Uvalde populations exhibited increased susceptibility with respect to 2009; the Burleson populations maintained the same level of resistance compared as in previous years. The overall decrease in resistance in 2010 may be attributed to several factors: first, the extensive area subjected to drought in 2009 which caused a decrease in planting of summer crops in South Texas, especially sorghum and cotton, reducing source populations and second, likely a larger area was planted with Bt corn in 2010, also reducing populations.  Finally, the impact of the area wide monitoring programs may not be neglected in creating awareness among growers regarding the resistance status of H. zea (http://insecticideresistance.tamu.edu).  Overall decreased population densities observed across the state may indicate areawide suppression by Bt crops.