Chapter 15
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Soil Fate and Non-Target Impact of Bt Proteins in Microbial Sprays and Transgenic Bt Crops Graham Head Monsanto LLC, 800 North Lindbergh Boulevard, St. Louis, MO 63167
Laboratory and field testing of the soil fate and non-target impact of Bt Cry proteins in the form of microbial sprays and plant tissues from transgenic Bt crops indicate that these proteins break down relatively rapidly in soil, do not bio -accumulate, and have few or no detectable effects on non -target organisms. In contrast, other components of microbial sprays such as Bt spores can persist for several years and may bioaccumulate. These components of sprays, and other formulation ingredients, can have limited non-target impacts. However, both microbial Bt sprays and transgenic crops expressing Bt proteins have minimal impact on non-target species relative to commonly used conventional insecticides. Thus microbial Bt sprays and Bt crops can form the basis for sustainable I P M programs.
The common soil bacterium Bacillus thuringiensis (Bt) has been shown to produce a wide variety of insecticidal proteins. Different insecticidal proteins are produced by different strains of Bt and any given strain may produce different proteins at different points in their life cycle. The group of Bt proteins that have received the most attention are the so-called crystalline (Cry) δendotoxins. Over 30 classes of Bt Cry proteins have been identified and classified based on sequence identity and spectrum of activity. For example, C r y l proteins are active against various Lepidoptera, Cry2 proteins are active against a set of Lepidoptera and Diptera, and Cry3 proteins are active against
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© 2007 American Chemical Society In Crop Protection Products for Organic Agriculture; Felsot, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.
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213 certain Coleoptera (7,2). A number of C r y l , Cry2, and Cry3 proteins have been used as environmentally benign insecticides in agriculture and forestry for over 30 years to control several key pest Lepidoptera and Coleoptera. However, the scale of use has been relatively limited because they generally only provide partial control of pest populations. The Bt Cry proteins in microbial formulations are relatively slow acting and tend to break down rapidly when exposed to ultraviolet light and high temperatures. Within the last 10 years, technologies have been developed that enable Bt proteins to be used more efficaciously in agriculture. The genes coding for the C r y l A b , C r y l A c , C r y l F , Cry2Ab2, Cry3Aa and Cry3Bbl proteins, among others, have been synthesized and successfully inserted into crop plants through recombinant biotechnology, producing transgenic crops that express the relevant proteins (so-called Bt crops). Expression of these proteins in crop plants has been optimized so that the Bt Cry proteins generally are present at high levels in all of the plant parts that are vulnerable to target pest consumption. These high levels of expression, and the stability in expression over time that results from the proteins being protected from ultraviolet degradation, mean that Bt crops can be extremely effective in the level of pest control they achieve, thereby overcoming the difficulties associated with microbial sprays. At the same time, only organisms that feed on plant tissues will be exposed to the Bt proteins in Bt crops, meaning that potential non-target organism effects are minimized. Thus far, the commercial applications of Bt crops include expressing C r y l A b and C r y l F in corn for lepidopteran pest control, Cry3Bbl in corn for control of corn rootworm species (Diabrotica spp.\ C r y l A c and Cry2Ab2 in cotton for lepidopteran pest control, and Cry3Aa in potato for control of the Colorado potato beetle, Leptinotarsa decemlineata. Bt corn varieties are now grown on over 52 million acres in countries that include the U.S., Canada, Spain, Argentina, South Africa and the Philippines. Bt cotton varieties are grown on over 37 million acres in countries that include the U.S., Mexico, Argentina, Colombia, Australia, India and China (3). The widespread use of Bt row crops in agriculture and of microbial Bt sprays in organic agriculture and forestry makes it critically important that the impact of Bt proteins from either of these sources on agroecosystems is thoroughly understood. This paper focuses on potential impacts of Bt proteins from these different sources on soil organisms, reviewing available literature from laboratory tests, field experiments and longer-term monitoring. For there to be ecological impacts on non-target organisms from the use of Bt proteins, both exposure and hazard must be present. Therefore, a simple ecological risk assessment approach is used that separately considers the potential exposure of soil organisms to Bt proteins from various sources and the potential hazards posed by Bt proteins to soil organisms. Finally, the non-target impacts of Bt proteins are contrasted with the demonstrated impacts of alternative pest control practices.
In Crop Protection Products for Organic Agriculture; Felsot, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.
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Potential Exposure of Soil Organisms to Bt Proteins
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Movement of Bt Proteins into the Soil Potential routes of exposure to Bt Cry proteins will be rather different for Bt microbial sprays compared with Bt crops (4). For microbial sprays, exposure of soil organisms will result from some portion of the spray application falling directly upon the soil surface or being washed into the soil by rain or irrigation. Degradation of the Bt proteins in microbial sprays tends to be rapid, though encapsulation of the Cry protein can slow degradation by reducing exposure to ultraviolet radiation and other environmental factors (5). Overall, this means that movement of Bt proteins from microbial sprays into the soil and exposure of soil organisms will occur primarily at or shortly after the time of spray application. In contrast, Cry proteins from Bt crops can enter the soil in three ways. First, tillage of plant material into the soil at the end of the season can introduce Bt proteins into the soil. However, Bt protein levels in the tissues of Bt crops tend to decline as the plant senesces (6). Thus, the amount o f Bt protein being introduced into the soil by tillage of Bt crops at the end of the season will be relatively small. Second, plant material may fall sporadically from live plants during the season, including leaves dropping throughout the year and pollen deposition during anthesis. The amount o f plant material involved will be small compared to end of season tillage. Furthermore, expression of Bt proteins tends to be relatively low in the pollen of Bt crops (7). Third, samples of soil from the rhizosphere of Bt corn seedlings under various conditions suggest that Bt protein may be exuded from the roots into the rhizosphere (*), but sluffage of root tissue containing Bt protein may be a confounding factor for the results obtained. Collectively, the available data indicate that Bt proteins from either microbial sprays or Bt crops may enter the soil in various ways, though in small amounts. For example, conservative calculations indicate that end of season tillage of a Bt cotton field results in
