PANNA: EPA Analysis of Bt Corn Flawed


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EPA Analysis of Bt Corn Flawed
March 1, 2001

A recent U.S. Environmental Protection Agency (EPA) study gives high marks to Bt corn — attributing decreased insecticide use, increased yields, and substantial economic benefits to the new technology. By contrast, however, a Union of Concerned Scientists (UCS) critique faults the agency’s overly rosy assessment. Bt corn has been genetically engineered to produce an insecticidal toxin originally derived from a soil microorganism, Bacillus thuringiensis or Bt.

The UCS analysis, prepared by Dr. Charles Benbrook, found that since the introduction of Bt corn there has actually been an increase in the percentage of U.S. acres sprayed with insecticides to control the European corn borer (ECB), the pest targeted by the new corn. In addition, while Bt corn may appear to increase yields under certain circumstances, the UCS report concluded that economic benefits are, at best, modest. Moreover, alternative techniques can deliver comparable control at the same or less cost. The analysis concludes that “the added costs of compliance with refugia requirements and resistance management plans, field monitoring and regulatory reviews, market segregation and impacts on export demand exceed the onfarm benefits associated with the technology.”

The EPA is currently engaged in determining whether Bt corn and cotton should continue to be grown in the United States. Because the permits under which the agency first approved Bt corn and cotton expire in 2001, EPA must decide by next summer whether or not — or under what conditions — to allow farmers to plant the crops in 2002 and beyond. (A future PANUPS will ask readers to submit comments to EPA regarding the Bt-crop review.)

In September, to launch the renewal process, EPA published a “Biopesticides Registration Action Document: Bt Plant-Pesticides” detailing the agency’s preliminary assessment of the risks and benefits of Bt crops. The UCS analysis prepared in response criticizes EPA’s optimistic view of the benefits of Bt corn, citing the agency’s use of flawed assumptions and incorrect information. For example, UCS found that the percentage of acres treated for ECB rose from 6.75% in 1995 (prior to introduction of Bt corn) to 8.5% in 1999 — a 26% increase. By contrast, EPA concluded that ECB-treated acres fell 37%. The increase of ECB insecticide use may be due at least in part to shifts in the composition of non-target and/or beneficial species in Bt cornfields as a result of exposure to the Bt-toxin in plant tissues. Since several important predators depend on ECBs as a primary food source during parts of the season, absence of these pests may be reducing predator populations.

The European corn borer becomes a pest only periodically. Damaging population levels are triggered by a combination of weather, cropping history, timing of planting and harvest, tillage and pest management systems, past control tactics and other factors. In most corn-growing regions, the ECB reaches damaging levels in just a few years out of 10, an indication that most farmers are effectively managing it under most conditions. Many experts agree that more can be done to suppress pest populations further and limit areas infested at levels that trigger the need for control actions such as insecticides or Bt corn.

Many farmers using sustainable farming systems report little or no trouble with ECBs in any year. The National Academy of Sciences’ 1989 report, Alternative Agriculture, outlines the basic components of such systems and their success in suppressing the ECB and rootworms. Research has shown that nutrient management and general plant health appear to be key factors in managing ECB losses.

There are several additional management strategies that have been shown to suppress the ECB. Planting of shorter season varieties as early as possible can often lead to harvest before damaging levels of second or third generation ECBs emerge. Steps can also be taken post-harvest to reduce over-wintering populations. These include chopping corn stalks, certain tillage practices, and planting cover crops to encourage diversification of insects feeding in corn stubble and trash. In addition, crop rotation is a proven control measure. Seed companies are also continuing to make progress in breeding conventional hybrids with higher levels of resistance to the ECB.

In terms of yields, entomologists in several states reported that a lack of pest pressure in 1998 and 1999 resulted in virtually no difference in fields planted to Bt hybrids in contrast to otherwise similar hybrids. For example, University of Illinois extension entomologists concluded that due to low infestation levels in most growing areas in 1998 and 1999, few growers planting Bt hybrids in either year had received an economic return on their investment. U.S. farmers planted about 25% less Bt corn in 2000 compared to crop year 1999.

To find out more about the UCS Bt crop renewal campaign and for a complete copy of their analysis of Bt crop benefits, visit

Source/contact:, Jane Rissler, Ph.D., Senior Staff Scientist, Union of Concerned Scientists, 1707 H Street, NW, Suite 600, Washington, DC 20006-3919.

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