Since the introduction of Bt corn, many primary corn pests such as corn borers have buzzed off. Bt's effectiveness on secondary pests, however, depends on the amount of available nitrogen (N), according to new research.

“Managing nitrogen fertility rates to get the full benefits of the Bt is as critical as managing it for high yields,” says Arnold Bruns, Stoneville, MS, USDA-ARS plant physiologist.

Through his two-year study of the effects of N on Bt toxin, Bruns found that since the Bt toxin is a protein, and N is part of protein's structure, more N should equal more Bt toxin. Since Bt toxin protects against insect damage, not using enough N could compromise insect control, especially for secondary insects.

Secondary corn pests like fall armyworms and corn earworms are typically more tolerant of Bt toxin than European corn borer and southwestern corn borer, says Craig Abel, a USDA-ARS entomologist working with Bruns.

“When we reduced the amount of nitrogen we applied in the field, there was less Bt in the leaves,” says Abel. “That means more fall armyworms survived after eating those leaves.”

The experiment measured Bt levels in plants fertilized with zero, 100, 200 and 300 lbs. of applied N/acre.

Abel collected Bt toxin levels from the silks, leaves and leaf sheath tissue. “In the field, we saw about a 5% increase in Bt for every 100 lbs. of nitrogen,” he says.

Abel notes that the more N applied, the fewer armyworms survived — and the ones that did were considerably smaller. Consequently, they ate less and caused less damage.

“For those few armyworms that did survive in the 300-lb./acre N plots, the larvae weighed about 3-4 times less than those in the zero N plots,” Abel says. “And because their development was slowed, there's also a greater possibility they would not develop into adults, therefore reducing the pest population.”

Bruns tested Monsanto's YieldGard Corn Borer gene. Although he didn't test Dow AgroScience's Herculex gene, he expects the results to be similar, since both primarily target the same family of insects. Since rootworm is not really a problem in Mississippi, Bruns also didn't test Monsanto's YieldGard Rootworm gene, which is a protein, too. However, he won't speculate how N affects rootworm control.

While the level of N applied had noticeable effects on secondary pests, it didn't seem to influence the effectiveness of Bt against its primary targets — European and southwestern corn borers.

“Even in the corn tissue with the lowest amounts of toxin, we saw 100% mortality on southwestern corn borer,” says Abel. “And that's probably what you would find with European corn borer, since they're both equally susceptible to Bt.”

Even so, he warns, it is important not to over-apply. Bruns recommends following local university or Extension N application guidelines for achieving realistic yield goals.

“Applying more N than you need is a financial waste,” says Bruns. “You want to have enough to meet your yield goal — and by doing so, it will also maximize the benefits of Bt corn.”