This dissertation is about how adoption of a genetically engineered plant works as a prevention method for aflatoxin, a carcinogen that commonly contaminates food crops such as corn and peanuts interacting with climate condition. First produced in the United States in 1996, Bt corn has become one of the most commonly grown transgenic crops worldwide. Aside from improving corn growers' yields, it has also resulted in an 11% decrease in insecticide use on US corn. Multiple experimental studies found that the level of aflatoxin, a fungal toxin, can be mitigated by reducing the presence of insects because fungi colonize corn through kernel wounds from insect feeding. However, the relationship between Bt corn and aflatoxin has yet to be examined in field settings, wherein many environmental factors are at play. Given the higher environmental risks of temperature and drought, revealing the mechanism by which Bt corn reduces aflatoxin can inform farmers' decision-making on crop choices and seed choices.In the first essay, I developed a regression model that estimates causal relationships between aflatoxin-related insurance claims and Bt corn adoption rates, drought index, and climatic variables. From 2001-2016, a significant inverse correlation existed between Bt corn planting and aflatoxin-related insurance claims in the United States when controlling for temperature, drought, state, and year. Estimated benefits of Bt corn's aflatoxin reduction were about $120 million to $229 million per year in over 16 states on average. These results suggest that Bt corn is an important strategy with corresponding economic benefits for reducing aflatoxin risk in the United States. Climate change-typically increased temperature-may expand prevalence zones for aflatoxin because warm temperatures and dry conditions are associated with aflatoxin accumulation. The second essay's objectives were to predict both areas with high aflatoxin risks in 2031-2040 based on 16 climate models, as well as the extent of aflatoxin-related economic loss due climate change. To do so, growing season impacts on aflatoxin risk were modeled by allowing for the adjustment of planting season under different climate scenarios. It was found that more than 89% of corn planting areas are likely to experience increased aflatoxin risks in 2031-2040 when compared to aflatoxin risks from 2007-2016 in the United States. Ignoring health-related costs, aflatoxin-related economic loss was expected to amount to $36 million - $70 million per year.In the third essay, I examined an additional potential benefit of Bt crops (corn and cotton): a decrease in the incidence of aflatoxin in peanuts (non-Bt crops). Bt crops can help reduce aflatoxin levels by reducing insect damage, a key path of fungal infection. However, the effect of Bt crops should not be limited to the adopted crops, because insects controlled by Bt have a relationship with other crops and insects in the broader ecosystem. Furthermore, the sales of aflatoxin-contaminated peanuts are currently under regulation. As such, the negative relationship between Bt and aflatoxin in peanuts means that Bt crops can also provide a meaningful, unintended economic benefit to peanut farmers by reducing aflatoxin. In addition to the Bt crops, temperature and drought at key stages of peanut growth were included as potential confounding factors in aflatoxin-related insurance claims. The results indicate that a county with a higher Bt crops adoption rate was less likely to have aflatoxin-related insurance claims in peanuts. This means that, by reducing the incidence of aflatoxin, Bt crops adoption in the United States has saved losses of $0.45 million per year. Overall, my dissertation study increases current understandings of the unintended effects of Bt in protecting crops from aflatoxin damage in the broader ecosystem. It aims to shed light on the benefits of Bt crops in countries that suffer from aflatoxin-related damage and transgenic seed traits that are not planted. Additionally, this study contributes to improved knowledge about climate conditions that affect either aflatoxin levels or host plants (corn and peanut). As climate change is expected to increase temperature and dryness, it is likely to increase the risk of aflatoxin in the US. Bt crops and new biotechnology are thus expected to play an important role in protecting crops from aflatoxin damage.
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