FEATURE ARTICLE
Ecology of Transgenic Crops
Genetically engineered plants might generate weed problems and affect nontarget organisms, but measuring the risk is difficult
Michelle Marvier
Troubles for Nontargets
The second major ecological concern surrounding insect-resistant transgenic crops is that they might harm nontarget animals. For example, a plant that is toxic to Colorado potato beetles could conceivably also be toxic to nonpest beetles or to beetles that actually benefit farmers, including ladybird beetles. Two recent studies on this topic attracted considerable media attention.
The beginning of this article mentioned one of those studies, which involved monarch butterflies. Losey and his colleagues studied the effects of pollen produced by transgenic corn that resists lepidopteran pests, including the European cornborer. Plant breeders can transfer a gene from a bacterium called Bacillus thuringiensis into corn, which causes the corn plant to produce an insecticidal compound, commonly called Bt toxin. In fact, there are several distinct Bt toxins, each capable of binding to receptors in the midgut of particular groups of insects, but humans and other vertebrates do not express these receptors. Wind pollinates corn, and its copious pollen can move up to 60 meters, coating the surfaces of neighboring noncrop plants. Thus, nontarget insects, including monarch larvae that feed on milkweed plants, consume some of the windblown corn pollen.
Losey and his colleagues coated milkweed leaves with transgenic corn pollen in quantities similar to those observed in nature. Only 56 percent of the monarch larvae survived when fed milkweed plants coated with transgenic corn pollen, whereas 100 percent of them survived when the plants were coated with nontransgenic corn pollen. Given that roughly half of the monarchs in the United States spend their summer months feeding on milkweeds in corn-producing regions, the effects on monarch populations could be substantial. Nevertheless, critics of this study point out that the toxins in transgenic corn's pollen might become inactive more quickly in the field than in a laboratory setting. In addition, C. Lydia Wraight of the University of Illinois and her colleagues reported that some varieties of so-called Bt corn express low quantities of toxin in pollen and do not significantly affect the survival of nontarget butterflies.
A second recent study, by Deepak Saxena and his colleagues at New York University, revealed that transgenic corn releases an insecticidal compound through its roots into the soil. Although Bt toxin generally becomes inactive quite quickly, the toxin can bind with soil particles and retain its insecticidal properties for 230 days or more. As a result, Bt toxin might accumulate over time and build up to much higher concentrations than previously anticipated. High levels of Bt toxin persisting in the soil could harm a variety of earth-bound organisms, which could affect rates of decomposition and nutrient cycling. Given that about 15 million acres of Bt corn were planted in the United States in 1998, scientists must quickly determine the consequences for soil ecosystems.
The findings from these studies highlight two key concerns regarding the safety of transgenic plants. First, the effects of transgenic crops will not be as localized or as transient as was initially anticipated. Rather, these studies suggest that the effects of transgenic crops might spread on the wind and persist in the soil. Second, the effects of these plants might move through food webs to species seemingly distantly associated with the transgenic crops. For both of these reasons, the ecological effects of transgenic crops will be quite difficult to predict.
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