GM Crops and the Environment

Aside from Golden Rice, most crops that have been genetically modified have been modified to increase their yield. For centuries, farmers have tried to increase yields by killing the pests that damage crops and by controlling the growth of weeds that compete for nutrients, rain, and sunlight. In the United States, farmers typically spray chemical pesticides and herbicides directly on

Figure 7.16 Crop dusting. Pesticides and herbicides are sprayed on crops to improve yields and are found on the surface of many crop plants. The chemicals also leach into the soil, where they can contaminate the groundwater.

to their fields (Figure 7.16). This practice concerns people worried about the health effects of eating foods that have been chemically treated. In addition, both pesticides and herbicides leach through the soil and contaminate the groundwater.

To help decrease farmer's reliance on pesticides, agribusiness companies have engineered plants that are genetically resistant to pests. For example, corn plants have been engineered to kill the European corn borer (Figure 7.17a). To do this, scientists transferred a gene from the soil bacterium Bacillus thuringien-sis (Bt) into corn. The Bt gene encodes proteins that are lethal to corn borers but not to humans (Figure 7.17b). The idea of using this bacterium for pest control

Figure 7.17 The European corn borer. (a) The European corn borer damages corn and decreases yields. (b) A gene present in the bacterium Bacillus thuringiensis produces a protein that is toxic to the corn borer. When this gene is inserted into corn DNA, the plant produces the protein that kills the corn borer, thereby providing resistance to the pest. (c) Some researchers have shown that Bt corn may be harmful to other organisms.

Figure 7.17 The European corn borer. (a) The European corn borer damages corn and decreases yields. (b) A gene present in the bacterium Bacillus thuringiensis produces a protein that is toxic to the corn borer. When this gene is inserted into corn DNA, the plant produces the protein that kills the corn borer, thereby providing resistance to the pest. (c) Some researchers have shown that Bt corn may be harmful to other organisms.

actually came from organic farmers, who have sprayed unengineered B. thuringiensis on crop plants for many years. Genetically modified Bt corn has proven to be so successful at resisting the corn borer that close to one-half of all corn currently grown in the United States is engineered with this gene.

Shortly after the arrival of Bt corn, concern arose about its impact on organisms in the surrounding areas. One laboratory study showed that milkweed, a plant commonly found on the edges of cornfields that had been dusted with pollen from Bt corn, was lethal to Monarch butterfly caterpillars, for which milkweed is the only source of food (Figure 7.17c). This research was performed in a laboratory and has been difficult to duplicate on farmers' fields, but there may still be cause for concern about how GM crops will affect other organisms.

Modified corn also caused controversy in 1996 when Bt corn was found in Kraft Taco Bell™ taco shells. Since corn with high levels of Bt had not yet been approved for human consumption, there was a massive recall of the product.

Critics of Bt corn point out that it is only a matter of time before corn borers develop resistance to Bt corn, which will require the development of new varieties of genetically engineered corn. This is true of pesticides applied to crops as well—pests develop resistance because application of a pesticide does not always kill all of the targeted organisms. The few that have preexisting resistance genes and are not susceptible survive and produce resistant offspring. Eventually, widespread resistance develops and a new pesticide must be developed and applied.

This problem is particularly vexing for the organic farmers who were the first to use B. thuringiensis for controlling the corn borer, but who did so in a targeted way. When a farmer's chemical overspray drifts to the farms of nearby organic farmers, the organic farmer has lost a powerful tool when the bacterium is killed, and must find another method of controlling this pest.

The continued need for the development of new pesticides in farming is paralleled by farmers' reliance on herbicides. Herbicide-resistant crop plants, such as Round-Up Ready™ soybeans, have been engineered to be resistant to Round-Up™ herbicide, used to control weeds in soybean fields. Farmers can now spray their fields of genetically engineered soybeans with herbicides that will kill everything but the crop plant. Some people worry that this resistance gene will allow farmers to spray more herbicide on their crops, since there is no chance of killing the GM plant, thereby exposing consumers to even more herbicide.

There is also concern that GM crop plants may transfer engineered genes from modified crop plants to their wild or weedy relatives. Wind, rain, birds, and bees carry genetically modified pollen to related plants near fields containing GM crops (or even to farms where no GM crops are being grown). Many cultivated crops have retained the ability to interbreed with their wild relatives; in these cases, genes from farm crops can mix with genes from the wild crops. While this is unlikely to happen with corn or soybeans, which do not have weedy relatives, it has already been seen with canola and is likely to happen with squash and rice. Thus, the herbicide is rendered ineffective since both the crop plant and its weedy relative share the same resistance gene. It may become impossible to determine whether weed plants surrounding fields of engineered crops have been pollinated with pollen containing the modified gene, and there could be unintended consequences for the ecology of the surrounding environment. Also, if pollen from GM crop plants drifts to farms that are not growing modified crops, it becomes impossible to determine whether a crop plant has engineered genes or not. This would be disastrous in the event of a recall.

Genetic manipulation could lead to decreasing variation within a species, and this too can have evolutionary consequences. Most GM corn, in addition to carrying the Bt resistance gene, has also been selectively bred to mature all at once, produce uniform ears, and have a particular nutrient profile. If an unforeseen disease or pest were to sweep through the area containing this corn variety, the disease would probably devastate a large portion of the crop.

Most, but not all, of the genetic engineering that occurs to produce crop plants resistant to pesticides and herbicides is performed by private companies and is designed to maximize profits. For example, Round-Up Ready soybeans are purchased by farmers who then apply Round-Up herbicide; both the GM soybean and herbicide are sold by Monsanto. Some day the techniques pioneered by agribusiness firms may be used to help solve the problem of world hunger, but this has not been the case to date.

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