Too much of a good thing can be bad.
Insect-resistant Bt corn hybrids are a case in point.
As Bt corn hybrids make their way into Western Canada, farmers must use proper management strategies associated with growing the genetically modified crop.
Insects such as the European corn borer are poisoned by a bacterium that is grafted into the genetic structure of the hybrid corn plants. The problem develops when not all the insects die.
A few insects might develop tolerance of the Bt and will survive. If they mate with other Bt-resistant insects, some of the offspring will also be resistant and the problem will magnify.
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The Canadian and United States governments require farmers using the hybrid Bt gene to sign a compliance agreement when buying the seed. Growers must agree to plant five to 20 percent of their cropped acres to a non Bt gene corn.
“Farmers must provide a refuge that allows some of the non-affected insects to survive and mate with survivors of the Bt hybrid,” said Adele Pelland, of Monsanto Canada.
This crossbreeding of resistant and non-resistant insects provides a dilution of the resistant genetics in the corn borer and keeps the number of resistant mutants to a minimum.
The Bt genetically modified corn, first released in 1996, has recently been introduced for widespread distribution in Manitoba and Alberta where growers are facing insect pressure from the European corn borer.
Annually, the insect is responsible for more than $1 billion in corn crop damage in Canada and the U.S., say researchers at the University of Minnesota. In that state a corn borer outbreak in 1995 caused a reported $285 million (U.S.) in damage.
“There are no worries about the safety of Bt. It has been used for decades as an insecticide. You can take this stuff and throw it onto your cabbage plants in your garden today and eat the cabbages tomorrow,” said Tom Turpin, a professor of entomology at Purdue University, in Indiana.
“The problem is that there is already some resistance to the Bt toxin in some insect populations.”
Pelland said Monsanto is focusing on grower education, rather than enforcement of the signed grower agreements.
“It is in the growers’ best interests to provide the refuge for the corn borers. They will lose the effect of the gene if they don’t manage its use well,” she said.
Art Stirling, of Pioneer Hi-Bred in Canada, which has Bt corn varieties said, “There is a lot of debate in the U.S. about the size of refuges. We feel that the 20 and five percent balance will be enough to keep resistance to a minimum. It is important for growers that do set aside the refuges.”
An American report done last year by more than 20 universities, eight seed companies, the Environmental Protection Agency and the U.S. department of agriculture, urges producers using the Bt gene corn to plant 20 to 30 percent of their acreage to non-Bt varieties. If they spray for corn borer using a Bt insecticide and grow Bt corn, then 40 percent of total acreage should be set aside.
Stirling said most growers have shown no opposition to planting some non-Bt varieties.
A few farmers planting larger corn acreages are “grumbling” but they understand the need and are complying, said Stirling.
Bt corn is created using a few of the genes from the bacterium Bacillus thuringiensis. The genes are genetically married into corn plants and can be passed to insects feeding on stalks and leaves.
Once in the gut of the corn borer, the crystal-like protein mixes with enzymes already there and kills the bug. It does not affect people or other organisms, with the exception of other plant stalk-boring insects in the Lepidoptera family.
“We haven’t seen the need to go to the Bt corn on our farm yet,” said Barry Bragg, a Bownmanville, Ont., corn grower.
“It harvests two to five percent wetter than non-Bt varieties and has slightly better yields, but is tough to thresh. For those who grow corn without rotations it works out pretty well, but for many of us it isn’t a necessity yet. … It is good to know it’s there, though.”
Bragg said there is enough non-Bt corn grown that resistance is not a big issue in Ontario yet.
Alberta is monitoring the situation, said provincial entomologist Mike Dolinski.
“There isn’t much (Bt corn), if any, being planted in Alberta this year. We don’t know how growers are going to take to interplanting and planting of refuges either. We don’t know how processors are going to take to the whole Bt thing either. I guess we’ll wait and see how things pan out in Manitoba and learn from their experiences.”
