Expression of Bollgard I & Bollgard II Bt Proteins in Reproductive Cotton Plant Tissues and Associated Feeding Behavior & Survivability of Helicoverpa Zea.

Maintenance of high dose expression for crystal (Cry) proteins continues to be a factor of high importance in effective resistance management programs.  Season long decline of Cry protein expression in Bollgard cotton plants remains a concern for ensuring sustained control of lepidopteran pests (Greenplate 1999). In addition, Cry1Ac expression has been observed to differ among plant structures.  Research has shown that some Lepidopteran larvae are more mobile on Cry1Ac expressing cotton than on conventional cotton (Gore et al 2002).  Helicoverpa zea has also been shown to selectively feed on plant structures with lower expression (Gore et al. 2001). 

This project evaluated levels of Cry1Ac and Cry2Ab expression within specific floral structures and characterized the survival of Helicoverpa zea neonate larvae on Bollgard and Bollgard II flowers.  A two-year evaluation utilizing caged insects indicated a higher level of damage and survivorship in flowers than expected for tissue expressing Cry proteins.ELISA testing of reproductive structures indicated that Cry protein expression levels in the terminal, squares and fully-expanded leaf tissue, which are used in expression equivalency trials before commercialization, were observed to be adequate for larval control, but were consistently lower in expression than floral tissue components, with the exception of pollen.

These data indicate that Bollgard I & Bollgard II reproductive structures, with the exception of pollen, provide adequate expression of Cry proteins for effective control of Helicoverpa zea.  As Cry protein expression in pollen was immeasurable in all but a few cases, it is likely that pollen is a probable avenue for survival of target lepidopteran pests.  Selective feeding behaviors that cause larvae to target pollen as a primary food source may play a key role in Helicoverpa zea survival on Bt cotton; the impact of this selective feeding on potential resistance development to Bt technologies should be explored further.