AMES, Iowa – Not since the introduction of synthetic insecticides in the 1950s, has agriculture seen such a potentially powerful technology as genetically altered plants to fight pests.
But that potential must be weighed against the risks of using it, according to researchers at three Midwestern universities.
Genetically altered Bacillus thuringiensis, or Bt corn, is one such technology. While it does have a place in agriculture today, it should be used with restraint as scientists learn more about its risks as well as its rewards, says John Obrycki, Iowa State entomologist. This is especially important in light of evidence that the use of Bt corn has not lessened the use of insecticides nor has it significantly increased yields.
Obrycki reviewed recent scientific studies that detail Bt corn’s effects on insects and other microorganisms. He also looked at planting rates of Bt corn, use rates of insecticides and yield studies of resultant crops from U.S. Midwest states that make up the Corn Belt.
His report was coauthored by Cornell University entomologist John Losey; Orley Taylor, University of Kansas, Lawrence; and graduate student Laura Jesse at Iowa State.
Bt corn was developed in the 1990s and promoted as a way for farmers to battle the European corn borer without having to apply insecticides.
The altered corn produces insecticidal proteins derived from genes of the Bt bacterium, making the corn resistant to corn borers, bollworms and other pests. Most toxicity tests show that Bt corn has been very effective in combating the corn borer.
But Bt corn has been controversial, too. Many European countries shy away from products made of Bt corn and studies over the past two years – one by Losey in 1999 and one by Obrycki in 2000 – have shown the pollen from Bt corn raises the mortality rates of Monarch butterfly larvae in controlled experiments.
Yet, Bt corn has been very popular with U.S. farmers. Up to 30 percent of the acreage in the Midwest has had some variety of Bt corn planted in it.
But Obrycki and his co-authors state that several studies show Bt corn use has not reduced insecticide use. Bt corn appears to be widely used as “insurance” against the possibility of corn being attacked by the corn borer rather than to fight an actual problem.
“Unlike the use of transgenic potatoes and cotton, the use of transgenic corn will not significantly reduce insecticide use in most of the corn growing areas of the Midwest,” the scientists state. “Bt plantings are not being used as a replacement for insecticides, but in addition to them.”
“We feel there is a limited role for Bt corn in relation to its use for controlling the European corn borer-that is, use it if corn borer numbers have been consistently high,” Obrycki added.
In terms of corn yields, use of the Bt product has not significantly increased yields, the scientists state.
Iowa State University Extension studies that compared transgenic and genetically similar non-transgenic corn hybrids grown in replicated plots in Iowa showed “only 34 percent of the transgenic lines produced significantly higher yields in 1997.”
Where corn borer damage was highest in nontransgenic lines, 50 to 58 percent of the transgenic hybrids produced significantly higher yields. When corn borer densities were generally lower than usual in Iowa, 12 percent of the transgenic lines produced significantly higher yields.
“The economic benefits of this technology are highly dependent on the population densities of the corn borer and the market value of the corn,” the scientists state.
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