Production of Bt Cry1Ac Resistance in Cotton Bollworm, Helicoverpa zea (Boddie)
Konasale J. Anilkumar and William J. Moar. Auburn University, 301 Funchess Hall, Department of Entomology and Plant Pathology, Auburn University, Auburn, AL 36849
Insecticide resistance is the primary concern with Bt crops in the US. For Bt cotton (Bollgard®), target pests include tobacco budworm, Heliothis virescens, pink bollworm, Pectinophora gossypiella, and cotton bollworm (CBW), Helicoverpa zea. Although results from lab-selected Cry1Ac-resistant TBW and PBW have contributed to insect resistance management (IRM) policy making, this has not occurred with CBW; there is no stable highly Bt-resistant CBW strain. Reasons include high tolerance to Cry1Ac, difficulties in mass rearing, and access to Cry1Ac. Because each species has different Bt resistance characteristics, potentially impacting IRM strategies, characterizing Bt resistance in CBW is critical, especially because Bollgard® does not express a “high dose” against this pest. Additionally, selection experiments cited above have been conducted with MVP II, a commercial formulation containing Cry1Ac. Because Bollgard® expresses Cry1Ac protoxin that is at least partly activated to toxin, we hypothesize that selection using MVP II may not adequately reflect resistance selection occurring in planta. Therefore, we initiated selection experiments with MVP II and Cry1Ac toxin. A susceptible laboratory strain was established from a Monsanto colony, September, 2004. The baseline susceptibility (LC50) of this strain to MVP II and Cry1Ac toxin was 26µg/g and 9µg/g diet, respectively. Subsequently, we selected CBW populations resistant to MVP II or Cry1Ac toxin. Current resistance ratio for AR strain is over 100-fold after 11 generations of selection. The MR strain completely crashed after 9 generations after developing only 17-fold resistance after 7 generations due to higher fitness costs associated with fertility. Results on the rate of resistance development, observed fitness costs and cross resistance to other Cry proteins will also be discussed.