- Agronomic: cotton
- Animals: bees
- Crop Production: application rate management
- Education and Training: extension, on-farm/ranch research
- Farm Business Management: risk management, whole farm planning
- Pest Management: biological control, genetic resistance, integrated pest management, prevention
- Production Systems: agroecosystems
Limiting unwanted gene flow between transgenic Bacillus thuringiensis (Bt) crop varieties and non-Bt varieties is of paramount importance for sustainable agriculture. Such gene flow creates liabilities for farmers and seed producers, threatens the use of refuges for delaying insect resistance to Bt crops, and complicates the removal of Bt transgenes from the environment if unexpected problems arise. To identify potential sources of Bt contamination, we monitored 15 non-Bt cotton seed production fields throughout the 2007 growing season. Our results demonstrate that honey bee abundance, proximity to Bt cotton fields, and human factors contribute to seed contamination of non-Bt cotton fields.
Transgenic cotton that produces insecticidal toxins from Bacillus thuringiensis (Bt) comprised 63% of cotton acreage in the United States in 2008 (Economic Research Service 2008). Bt cotton provides substantial control of several key pests and has reduced broad-spectrum insecticide use (Shelton et al. 2002, Cattaneo et al. 2006). While Bt crops offer economic and environmental benefits, their widespread adoption raises concerns that Bt transgenes will contaminate other cotton varieties (Smyth et al. 2002). Such contamination could have negative economic, political and ecological consequences that are detailed below. Moreover, research conducted on today’s Bt crops could provide valuable information for preventing transgene escape from future pharmaceutical and industrial crops.
In 2006, we discovered low rates of Bt transgenes in the seed supply of non-Bt cotton (Heuberger et al., 2008a). We tested 100 seeds from each of 11 previously unopened non-Bt cotton seed bags purchased by Arizona farmers. Seeds were tested for the Bt toxin Cry1Ac using enzyme-linked immunosorbent analysis (ELISA) (ImmunoStripsTM test system, Agdia, Elkhart, IN). Three of the bags were contaminated, each at a rate of 1%. Although rates were low, this raised concern that widespread use of Bt crops and potential fitness advantages of contaminant plants could lead to accumulation of Bt transgenes in the seed supply through time. Risks associated with such contamination include compromising the refuge strategy for insect resistance management, liabilities for farmers, and complication of transgene removal from the environment, as detailed below.
Growers of Bt cotton commonly plant refuges of non-Bt cotton near their Bt fields to delay insect resistance to Bt crops (U.S. EPA 2006, Matten and Reynolds 2003, Carrière et al. 2005). Refuges produce abundant Bt-susceptible insects that can mate with rarely occurring resistant insects, thus delaying the frequency of resistant insects (Gould and Tabashnik 1998, Tabashnik et al. 2004a, Sisterson et al. 2004). Production of Bt toxins in refuges, resulting from introgression of Bt transgenes, decreases the efficacy of refuges by increasing mortality of Bt-susceptible insects (Chilcutt and Tabashnik 2004, Heuberger et al. 2008b). Insect evolution of resistance to Bt toxins is a threat to sustainable agriculture for two reasons: 1) such resistance could drive conventional growers to return to using conventional, more environmentally harmful insecticides, and 2) organic farmers relying on Bacillus thuringiensis sprays could lose this important tool for organic insect control.
Moreover, the presence of Bt seeds in the non-Bt seed supply creates liabilities for farmers, and could be of particular concern if transgenes entered organic crops (Mellon and Rissler 2004, Andow and Zwahlen 2006). Concern about transgene contamination led to collapsing markets for organic canola production in western Canada and North Dakota (Brummond 2001, Smyth et al. 2002). The USDA National Organic Program prohibits products containing transgenes from bearing the “organic” label in the United States (National Organic Program 2006). Transgene contamination also threatens conventional farmers who want to preserve the integrity of their crop varieties. For example, the recent contamination of rice by an experimental variety known as “Liberty Link” triggered a drop in prices of exported rice from the U.S. and resulted in several lawsuits from rice farmers (Vermij 2006).
Unintended introgression of transgenes into crops could greatly complicate their removal from the environment if unanticipated problems occur, such as detrimental impacts on non-target organisms. Transgenic toxins could persist and proliferate in the environment for many years though cross-pollination between transgenic plants and non-transgenic members of the same species. For example, StarLink corn with Bt toxin Cry9C, was approved in the U.S. for livestock feed but not human consumption. However, after being found in human food, its registration was not renewed in 2000 (Lin et al. 2003). Three years after StarLink corn was withdrawn from the market, the Cry9C transgene was still detectable in commercial corn varieties, illustrating the difficulty of expunging transgenes from the environment (Mellon and Rissler 2004).
Contamination of non-Bt cotton by Bt varieties can result from cross-pollination, emergence of volunteer Bt plants in non-Bt fields, and/or seed mixing by human error. Cross-pollination of non-Bt cotton plants by Bt cotton (i.e., “outcrossing”) results in non-Bt plants with some seeds that produce Bt toxin. While cotton is a predominantly self-pollinating crop, outcrossing can occur when insect pollinators, particularly bees, are present (Free 1970, Umbeck et al. 1991). In fact, cotton plants surrounded by cotton of a different variety can outcross at rates up to 10-48% (Free 1970). Studying cross-pollination between Bt cotton fields and non-Bt cotton seed production fields increases our understanding of gene flow in insect pollinated crops and provides a valuable contrast to the more widely-studied wind-pollinated crops.
In addition to outcrossing, gene flow can enter fields via the accidental cultivation of Bt plants in non-Bt fields. Such plants, which are commonly called “adventitious,” can emerge as volunteer plants from previous years crops, or can result from accidental seed mixing (i.e., human error). Adventitious Bt plants are homozygous for the Bt trait unless previous generations outcrossed with non-Bt varieties.
Seed Contamination in the Literature:
Since the widespread adoption of transgenic crops in the mid 1990’s, several reports have documented transgene contamination in the conventional seed supply. A 2004 Union of Concerned Scientists study found genetically engineered seeds in conventional seed bags of canola, corn, and soybeans (Mellon and Rissler 2004). More than half of all varieties tested contained transgenic seeds. Seeds were pooled in the study, but contamination rates were estimated to range from less than 0.05% to greater than 1% of seeds (Mellon and Rissler 2004). In corn, up to 1% contamination by Bt transgenes was detected, although most transgenes detected in the study conferred herbicide resistance (Mellon and Rissler 2004). Of particular interest, soybean varieties were contaminated at levels similar to corn and canola. This was a surprising result because soybean varieties are almost entirely self-pollinating, in contrast to the highly outcrossed corn and canola crops. The similarity in contamination rates among these crops suggests that sources other than outcrossing were responsible for transgene introductions (Mellon and Rissler 2004).
Another study tested 10 cultivars of conventional canola for contamination by herbicide resistance transgenes and reported that all cultivars were contaminated (Friesen et al. 2003). Nine cultivars had less than 1% of seeds with herbicide resistance transgenes whereas one cultivar contained 3-5% herbicide-resistant seeds (Friesen et al. 2003). Similarly, bags of transgenic glyphosate-resistant canola seed contained up to 0.3% seeds with transgenic resistance to both glyphosate and glufosinate herbicides (Beckie et al. 2003).
Conventional crops have also been contaminated by experimental transgenic events. Between 2001 and 2005, hundreds of tons of seed from Bt10, a transgenic variety of corn not approved for sale, were accidentally sold to farmers (Macilwain 2005). In another event, an experimental transgene from rice was found in the foundation seed stock of a conventional rice variety (Vogel 2006). A common feature of these contamination events is that the source of transgene introduction was not identified. Mechanisms of transgene introgression are extremely difficult to trace in hindsight. Thus, we saw a need for a study that monitored fields throughout the growing season to evaluate multiple potential sources of transgene contamination.
Here, we monitored 15 non-Bt cotton seed production fields throughout the growing season, to determine the pathways by which Bt contamination was entering. We hypothesized that bee abundance, proximity to Bt cotton fields, and presence of adventitious Bt plants in fields would enhance cross-pollination of seed by Bt cotton. We also hypothesized that we would encounter 1) Bt plants emerging as volunteers in fields and, 2) pre-existing contamination in the seed bags used to plant fields; and that these factors would result in adventitious plants in seed lots. Our goal was to develop a predictive model to identify fields with particularly high risk of Bt contamination.
Knowledge gained in this project will be useful for seed companies and policy makers in designing methods that limit Bt contamination of non-Bt cotton seed. Insights from this study could also be broadly applicable to understanding crop contamination by other transgenic plants, including those producing pharmaceutical or industrial compounds.
- 1) Determine the importance of seed bags as a source of adventitious Bt plants.
2) Identify factors that influence the level of Bt outcrossing in non-Bt cotton seed production fields. Specifically, examine adventitious Bt plants, landscape attributes, and bee activity as potential explanatory variables. Also examine the potential for human error to contribute to the presence of adventitious Bt plants.
3) Identify changes needed in seed production guidelines to reasonably limit contamination.