Effects of Nitrogen Fertility on Bt Endotoxin Levels in Maize

Abstract

Three Bt maize hybrids (two with Bt event MON-810, one with Bt event DBT-418) and one nonBt maize hybrid were grown at Stoneville, MS, USA in 2002 and 2003 with N-fertility rates of 0, 112, 224, and 336 kg N/ha, respectively. Tissue samples of the 7th leaf at growth stage V5 (5 fully extended leaves) and primary ear leaf sheaths and outer ear husks at growth stage R3 (milk) were collected and analyzed for endotoxin concentrations. At V5 CrylAb Bt concentrations of the MON-810 hybrids increased whereas there were no N-fertility effects on Cry1Ac in the DBT-418 hybrid. Bioassays from tissue samples at growth stage V5 of all N-fertility treatments for both Bt events were nearly 100% fatal to southwestern corn borer larvae (Diatraea grandiosella_Dyar). Endotoxin concentrations at growth stage R3 increased in both the primary ear leaf sheaths {[Bt] = 0.0006(kg N/ha) + 0.167 r2 = 0.8817 for both MON-810 and DBT-418} and outer ear husks {[Bt] = 0.0006(kg N/ha) + 0.148) r2 = 0.9985 and [Bt] = 0.0007 (kg N/ha) + 0.16) r2 = 0.8702 for MON-810 and DBT-418, respectively} as N fertilization increased in 2003. Bioassays determined that southwestern corn borers were susceptible to even the lowest [Bt] in tissues of MON-810 at R3 but not DBT-418. Diets made from the nonBt hybrid at all stages and N-fertility rates had a nearly 80.0% larval survival rate. Because other Noctuidae pests [i.e., corn earworm (Heliothis zea (Boddie)) and fall armyworm (Spodoptera frugiperda (J. E. Smith))] of corn are more tolerant to Bt toxins, growing Bt maize under adequate levels of N-fertility may be even more important to insure maximum protection from these pests.