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United States Department of Agriculture

Agricultural Research Service

Research Project: AGRICULTURAL PRACTICES, ECOLOGICAL AND VARIETAL EFFECTS ON AFLATOXINS AND OTHER MYCOTOXINS IN CORN

Location: Biological Control of Pests Research Unit

Title: Effects of Nitrogen Fertility on Bt Endotoxin Levels in Corn

Authors
item Bruns, Herbert
item Abel, Craig

Submitted to: Journal of Entomological Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 26, 2006
Publication Date: February 1, 2007
Citation: Bruns, H.A., Abel, C.A. 2007. Effects of Nitrogen Fertility on Bt Endotoxin Levels in Corn. Journal of Entomological Science. 42(1):35-34.

Interpretive Summary: Some corn hybrids have been modified through biotechnology and have a gene inserted into them from bacteria called Bacillus thuringiensis (Bt). This causes the plant to produce a natural toxin that kills the caterpillars of southwestern corn borer and other similar insect pests. The U.S. Environmental Protection Agency recommends that corn with a Bt gene be grown with sufficient nitrogen fertilizer to produce high levels of Bt-toxin and prevent the insects from surviving and developing resistance to it. When corn hybrids with a MON-810 Bt gene had five fully extended leaves, Bt-toxin levels in the plants increased as the nitrogen fertilizer rates increased from 0 to 100, 200, and 300 pounds per acre. A corn hybrid with a DBT-418 Bt gene had no such increases in Bt-toxin at this early growth stage, but its plant tissue was as fatal as tissue from MON-810 Bt hybrids when fed to southwestern corn borer caterpillars. Later when the developing corn kernels were at the milk stage of growth, MON 810 hybrids still had consistent increases in the Bt-toxin in ear husks and ear-leaf sheaths as nitrogen fertilizer rates increased. Southwestern corn borer caterpillars all died when fed plant tissue collected at all nitrogen fertility rates. The DBT 418 Bt hybrid had similar increases in Bt-toxin in the same plant tissues as nitrogen fertilizer rates increased. However, southwestern corn borer caterpillars were able to survive on these plant tissues. Despite the differences in sensitivity of southwestern corn borer caterpillars to these two Bt-toxins during kernel development, this experiment demonstrates that, during this growth period, corn containing a Bt gene will have higher levels of Bt-toxin as nitrogen fertilizer rates are increased.

Technical Abstract: Three Bt hybrids (two with Bt event MON-810, one with Bt event DBT-418) and one non-Bt 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 seventh 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 Cry1Ab Bt concentrations of the MON-810 hybrids increased while 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 non-Bt hybrid at all stages and N-fertility rates had a nearly 80.0% larvae survival rate. Because other Noctuidea 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.

Last Modified: 8/22/2014
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