Pyramiding CryIA(b) Insecticidal Protein and Terpenoids in Cotton to Resist Tobacco Budworm (Lepidoptera: Noctuidae)

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Abstract
The development of insect resistance to CryIA insecticidal proteins expressed in transgenic cotton, Gossypium hirsutum L., is likely to occur quickly following commercialization, unless strategies are implemented to delay pest adaptation. This study was undertaken to determine the effects of pyramiding (i.e., combining 2 or more resistance traits in the same plant) the transgenic Cry1A(b) insecticidal protein trait with the conventional high-terpenoid plant trait on resistance to tobacco budworm, Heliothis virescens (F.), growth, survival, and injury to the plant. The transformed cotton line MON 81 expressing the cryIA(b) gene was crossed to glandless (terpenoid-free), wild-type (normal terpenoid level), and high-glanded (high-terpenoid) plant isolines derived from the ‘TAMCOT CAMD-E’ and ‘Stoneville 213’ variety backgrounds. F2:4 cotton lines were evaluated in field and laboratory no-choice survival and plant injury experiments. In the field no-choice experiment, the CryIA(b) trait alone reduced larval growth, larval survival, and injury to flower beds, capsels, terminals, and leaves of CryIA(b)-susceptible and CryIA(b)-adapted tobacco budworm larvae compared with wildtype cotton. Larval injury to cotton was also reduced by the high-terpenoid trait compared with wild-type cotton, but larval growth and survival were not significantly different. In a laboratory no-choice study, survival of CryIA(b)-susceptible larvae was reduced more by pyramiding the CryIA(b) protein trait with the high-terpenoid trait in a single plant background than by either trait alone when compared with the terpenoid-free control plants. These results suggest that the relative levels of resistance among the traits were (from highest to lowest): pyramided CryIA(b) + high-terpenoid > pyramided CryIA(b) + normal-terpenoid > CryIA(b) alone> high-terpenoid alone > normal-terpenoid alone > no terpenoid or CryIA(b). These data suggest that pyramiding CryIA(b) insecticidal protein with high-terpenoid should increase plant resistance to tobacco budworm and improve the durability of the CryIA(b) trait in commercial cotton.