A STRUCTURAL MODEL OF INSECT CONTROL DECISIONS: ROOTWORM RESISTANCE IN US CORN FIELDS
Wechsler, Seth James
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Recent evidence from field tests and laboratory studies suggest that rootworms are adapting to the toxins produced by genetically modified, insect resistant (Bt) seeds. Given that rootworms cause over 1 billion dollars in yield losses and treatment costs per year, finding larger scale evidence of resistance could have important policy ramifications. This dissertation analyses corn farmers' insect control decisions in an effort to determine whether rootworms have adapted to Bt seeds. Chapter 1 provides a broad overview of the literature on genetically engineered (GE) seeds. It strives to correct many commonly held misperceptions about genetically modified seeds. Chapter 2 provides a detailed description of the economic literature on pesticide productivity and GE seed use. It compares structural to reduced form models, and discusses how previous studies have accounted for endogeniety. Chapter 3 develops a two stage, theoretical model of corn farmers' insect control decisions. This model is used to derive a non-linear, soil insecticide demand function. Chapter 4 presents the dissertation's empirical approach. First, it describes the data used in the analysis. Next, it discusses how to estimate the soil insecticide demand function derived in Chapter 3. Finally, it discusses how the parameters of the structural model can be used to determine how: a) Bt adoption affects yields, b) Bt adoption affects insecticide use, and c) whether the effectiveness of Bt seeds has changed over time. Chapter 5 provides the study's results. These results indicate that using rootworm resistant seeds would have decreased soil insecticide use by 70% in 2005 and 84% in 2010. Bt adoption would have increased yields by .6 percentage points (1.02 bushels/acre) in 2005 and .1 percentage points (.2 bushels per acre) in 2010. Alarmingly, the evidence suggests that rootworm resistant seeds were less effective in 2010 than in 2005, and less effective on farms where selective pressure was high than on farms where selective pressure was low. In other words, the results of this study support the hypothesis that rootworms are adapting to Bt seed use. Chapter 6 concludes.