Biopesticides

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Biopesticides

Plants, growing in the wild or in cultivation, face numerous threats from insects, bacteria, viruses, and fungi, as well as from other plants. Biopesticides are inert substances or living organisms that can help protect plants from such threats. Chemical pesticides can offer similar protection but, by contrast, are neither alive nor made by living organisms.

Natural Chemical Defenses

A variety of chemicals produced by plants help ensure that parasites, predators, plant feeders, and herbivores seldom increase in number sufficiently to destroy the plant populations they prey upon. Chemicals found in very low concentrations in certain plants have been found to help keep locusts from feeding on those plants, and some trees produce nearly 1,000 different chemical compounds that help them resist herbivores and parasites.

Living Organisms Serving as Biopesticides

Plant predators are themselves subject to attack by predators, parasites, and microbes, all of which can indirectly help protect a plant and therefore are also considered biopesticides. An oak tree may have about 100 species of insect herbivores feeding on it. In turn, there can be up to 1,000 species of predators, parasites, and microbes feeding on the herbivores. The microbes, parasites, and predators attacking the herbivore populations are considered "biopesticides," as are any protective chemicals produced by the tree.

Such living biopesticides play a vital role in agriculture and nature, helping to control insect pests, plant pathogens, and weeds. Numerous organisms, including viruses, fungi, protozoa, bacteria, and nematodes , as well as insects, such as parasitic wasps, can attack pest insects and weeds. In some cases, biologists search around the world to find natural organisms to help control an insect, a plant pathogen , or weed populations.

The use of natural organisms as biopesticides is sometimes hampered by the presence of chemical pesticides, which can threaten populations of a pest insect's natural enemies. Pest outbreaks that result from chemical pesticides destroying a pest's natural enemies are estimated to cost the United States more than $500 million per year.

Genetically Modified Organisms as Biopesticide Producers

Since the 1980s, many crops have been genetically modified to produce biopesticides that will help protect them from insects and pathogens (including viruses). In 1998, 40 million hectares of engineered crops were planted throughout the world (though 74% of the modified cropland is in the United States). Globally, 20 percent of this area has been planted with herbicide-tolerant crops, 8 percent with insect-resistant crops, and 0.3 percent with insect-and herbicide-resistant crops.

Disease Resistance in Crops

More than 95 percent of all crops have some degree of pathogen resistance bred into them, with resistance to fungi, bacteria, and viruses being most common. Most of this resistance was either added by farmer selection or plant breeder selection, rather than through genetic engineering. It is because of this natural resistance that has been bred into the crops that only 12 percent of the pesticides used in U.S. agriculture are fungicides.

Some viral resistance, however, has been bred into a number of crops through insertion of viral genes into the plant chromosomes. These genes may lead to the plant's producing viral proteins—biopesticides of a sort—that hamper a virus's own actions. This pathogen-derived resistance has been successfully used to protect Hawaii's papaya crop from the devastating papaya ringspot potyvirus. The viral gene was inserted into the papaya genome using a "gene gun," which shoots viral genes into papaya embryo cells.

Insect-Resistant Crops

About 8 percent of land covered by genetically modified crops is planted with insect-resistant crops. Although insect-resistant crops have not been employed as widely as disease-resistant crops, there are some notable examples. These include Hessian fly resistance in wheat and European corn borer resistance in corn. Resistance to corn borers has been provided by using a naturally occurring toxin produced by Bacillus thuringiensis (BT). This bacterium has traditionally been applied to corn and other crops. Genetic engineering has allowed the toxin to be manufactured by the corn plant itself.

The most serious problem with the use of BT toxin genes has been with STARLINK corn. This variety of genetically modified corn was approved for use only as animal feed, not as human food. STARLINK corn nevertheless found its way into the food-processing industry when farmers did not keep their STARLINK corn separate from corn to be used as human food. Some grain elevator operators as well did not keep the STARLINK corn separated. The STARLINK mix-up cost the United States an estimated $5 billion of processed food that had to be destroyed because of the BT toxin restriction.

Plants that have been genetically modified to make BT toxin produce it in every cell. BT corn pollen may harm nontarget moths and butterflies, like Monarch butterflies. Milkweed leaves dusted with drifting BT corn pollen are toxic to Monarch butterfly larvae, but some corn does not naturally pollinate at the same time of the year as butterflies are in their larval stage, so the effects may not be great. The full extent to which natural populations of butterflies are affected by BT corn is not known.

Herbicide-Resistant Crops

Some crops (e.g. corn) are being engineered to contain both herbicide tolerance and the BT toxin. Generally, the use of herbicide-tolerant crops will likely increase the use of herbicides. This has the potential to increase environmental pollution since it might increase the farmers' reliance on chemicals rather than mechanical and other means of weed control.

see also Agricultural Biotechnology; Genetically Modified Foods; Transgenic Plants.

David Pimentel