DEVELOPMENT OF HYDRATEABLE, COMMERCIALLY-RELEVANT ARTIFICIAL COTTON LEAVES AND ASSAY ARCHITECTURE FOR MONITORING INSECT RESISTANCE TO BT.

Transgenic cotton varieties that express Bacillus thuringiensis (Bt) toxins have been adapted readily because they reduce the need of insecticide application, while their environmental impacts are minimal. Recently, pyramided cotton varieties that express two different Bt toxins have been introduced; these varieties may reduce the risk of resistance development in lepidopteran pests populations. However, resistance monitoring in Bt cotton fields are necessary and mandated. Based on a previously described feeding disruption test (FDT) from our laboratory, the objective of this project was to develop a bioassay that uses Bt toxin obtained directly from Bollgard II plant material. The technology under development should be applicable to other pyramided and single gene commercial plants. The assay hardware is a novel 16 well plastic plate with each well of the plate containing a hydrateable, artificial diet containing leaf extract from Bollgard II. Dose-response studies were conducted using a laboratory susceptible strain of the tobacco budworm, Heliothis virescens, where different concentrations of leaf extract were used to hydrate a standard meal pad in each well. From these studies, a diagnostic dose was determined of 1.3 µg cotton / meal pad, which essentially stopped the production of fecal pellets by neonates of the budworm after a 24 h incubation. This diagnostic dose was further evaluated using three different populations of H. virescens assayed directly from the field from central North Carolina, USA. The diagnostic dose was also evaluated with the Bt resistant, laboratory reared H. virescens strains, YHD2 and CxC. The bioassay developed is a practical, reliable monitoring tool that can be used to monitor Bt susceptibility as part of an integrated resistance program in pyramided cotton fields. [ABSTRACT FROM AUTHOR]