What Risks Do Genetically Engineered Crops Pose to the Environment?
by Dr. Norman C. Ellstrand, Professor of Genetics, Department of Botany & Plant Sciences, University of California, Riverside

Despite their successful development and proliferation over the last decade, genetically engineered (GE), or transgenic, crops carry potential risks.  Environmental biosafety concerns center around problems created directly by growing such crops, and problems created by unintended descendants of them.

In assessing the direct environmental impact of transgenic crops, scientists have focused almost exclusively on the evolution of pests that are resistant to control, and unwanted effects on species in affiliated ecosystems, particularly from the use of transgenic herbicide-resistant plants.  The impact of this herbicide-resistance on diversity, however, depends largely on the type of herbicide, and where and how it is used.

Emergence of resistance in pests targeted for control by or associated with transgenic crops is simply a matter of time.  This inevitability derives from the fact that transgenic products currently use a single, uniform control method over huge areas. Most of the millions of acres of GE corn and cotton now grown in the United States, for example, are engineered with one or two bacterial genes from the species Bacillus thuringiensis (Bt), enabling the plants to make their own pesticides to target certain insect pests.  Because these Bt products do not kill all insect species, they are deemed relatively environmentally benign. But the National Research Council (NRC) considers the evolution of resistance to Bt crops unavoidable, and the evolution of resistant pests is a potential environmental hazard of transgenic crops because more environmentally damaging alternative treatments would then be needed for control.

Meanwhile, the U.S. Environmental Protection Agency has issued guidelines mandating that farmers plant “refuges” of non-Bt varieties in plantations of Bt varieties to prevent or delay the evolution of resistance.  This strategy may be working, given that no pests have yet evolved resistance to Bt crops in the field, despite their commercialization for more than a decade.

Genetically modified corn, commonly called Bt corn, is engineered to kill pests such as the European corn borer. However, an October 2007 study shows that Bt corn may also harm the caddisfly, which serves as food for fish and amphibians. The study also shows that parts of Bt corn, such as leaves, cobs and pollen, can travel as far as 2,000 meters away from source areas – a phenomenon that was not considered when Bt corn was licensed (Credit: Zina Deretsky, National Science Foundation)

A crop engineered to interfere with the reproduction or viability of one or more pests, however, might also interfere with nonpest species. For example, while Bt corn was developed to control certain moth varieties, reports of potentially toxic effects on monarch butterfly larvae surfaced in the late 1990s.  A flurry of subsequent research demonstrated the effects to be highly variable, and current commercial Bt corn varieties are not considered hazardous to the larvae – but one variety that is no longer grown would have been. This illustrates that transgenic crops could impact nontarget organisms and that risk assessment research can help clarify whether a supposed risk is, in fact, a problem.

Bt in corn, for example, reduces environmental effects relative to spraying broad-spectrum insecticides, yet, prior to its advent, many American corn farmers did not spray any insecticides to curb the pests controlled with Bt, the NRC reports.  These farmers simply took their chances, possibly because the damage from the lepidopteran pests of corn varies so much from year to year. 

Meanwhile, the possibility of unintended reproduction by GE crops has raised questions about whether direct undomesticated, or feral, descendants of these crops may prove to be new weeds or invasives, and whether unintended hybrids between transgenic crops and other plants could lead to problems.

Progeny of GE crops could pose risks if their transgenic traits alter their ecological performance such that they evolve increased aggressiveness. Some crop plants – especially those with a long history of domestication like corn and soybeans – pose little hazard because traits that make them useful to humans also reduce their ability to establish feral populations in either agroecosystems or nonagricultural habitats, according to the NRC.  But other cultivated plants, including certain forage and turf grasses, ornamentals, rice, rye and alfalfa, are often found growing on their own as feral populations that create problems, as documented by J. Gressel in the 2005 book Crop Ferality and Volunteerism. This tendency to establish troublesome feral populations could increase as the result of acquiring new traits – but whether new weeds, or any of the other hazards identified on the basis of good research, come to pass still remains to be seen.

** Based on an article by the author published in the July-September 2006 issue of California Agriculture.

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Also Noted: What Risks Do Genetically Engineered Crops Pose to the Environment?