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Genetically modified crops limit corn borer, U study reports

While environmental scientists continue to assess the full impact of genetically-modified crops, a new University of Minnesota study concludes they’ve seriously set back one insect: the European corn borer.

No Midwestern farmer will mourn this development. These borers had cost corn growers up to $1 billion a year in losses. During a 1995 outbreak of the borer, Minnesota farmers alone lost an estimated $285 million dollars worth of corn to the destructive insect.

Anyone who spent time in a corn field a few years ago – or even bought fresh corn from a farmers market has seen this insect. In the moth stage it is yellowish brown with intricate wavy bands across its wings.

The destructive stage comes with the black-headed caterpillar. These voracious worms not only mar the juicy corn kernels but also bore tunnels in the stalks, sometimes weakening the plants to the point where they topple over in the fields.     

Then, in 1996, so-called Bt corn was introduced. It featured a gene that enabled plants to produce a toxin from the soil bacterium Bacillus thuringiensis. The toxin kills insect pests such as the corn borer but is not harmful to humans. In fact, Bt-based pesticides are used by organic growers as a natural weapon against destructive insects.

There had been no question that Bt corn helped beat back the borer in fields where farmers chose to use the genetically-modified crops. That success is one reason farmers throughout the American Midwest rushed to embrace the controversial crops.

Now, the U of M study says the widespread adoption of GM field corn has benefitted even non-GM corn patches. U of M entomology professor William Hutchison is the chief author of the study, reported in this week’s edition of the journal Science.

It is the first study to show a direct association between Bt corn use and an area-wide reduction in corn borer abundance, documenting a decline in non-GM corn fields too, said Paul Mitchell, an agricultural economist at the University of Wisconsin-Madison, and a co-author of the study.

Corn borer moths cannot distinguish between Bt and non-Bt corn, so females lay eggs in both kinds of fields, Hutchison said a statement about the findings. Once eggs hatch in Bt corn, young borer larvae feed and die within 24 to 48 hours.

Because Bt corn is effective at controlling corn borers and other pests, it has been adopted on about 63 percent of all U.S. corn acres.

As a result, corn borer numbers have also declined in neighboring non-Bt fields by 28 percent to 73 percent in Minnesota, Illinois and Wisconsin, depending on historical pest abundance and level of Bt-corn adoption. The authors were able to document the borer suppression entomologists in those three states have monitored pest populations for more than 45 years.

The study also documents similar declines of the pest in Iowa and Nebraska. And the authors said that roughly the same declines likely are occurring elsewhere, but they can’t be documented without the historic monitoring data that is available in the Midwestern states.

In the five-state region, the study’s authors estimate the economic benefits of this area-wide pest suppression have totaled $6.9 billion over the past 14 years. Surprisingly, non-Bt corn acres accounted for $4.3 billion, some 62 percent of this total benefit, they said.

The primary benefit of Bt corn is reduced yield losses, and Bt acres received this benefit after the growers paid Bt corn technology fees. But as a result of areawide pest suppression, non-Bt acres also experienced yield savings without the cost of Bt technology fees, and thus received more than half of the benefits from growing Bt corn in the region, the authors concluded.

The analysis does not consider benefits for other important Midwestern crops affected by European corn borer, such as sweet corn, potatoes and green beans.

Hutchison said however, “that additional environmental benefits from corn borer suppression are likely occurring, such as less insecticide use, but these benefits have yet to be documented.”

It is important to note that these generally positive findings for Bt corn do not erase a serious concern about the GM crop: the worry that the widespread use of the Bt toxin will lead to resistant borers. The strategy for minimizing that risk has been to require farmers to plant non-Bt refuges near their Bt-corn fields, giving insects places where they can co-mingle genes as they breed.

The study’s authors emphasize that farmers, industry and regulators need to remain committed to the refuge strategy.

Sustaining the economic and environmental benefits of Bt corn and other transgenic crops for adopters and non-adopters alike depends on the continued stewardship of these technologies, they said.

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