Genetically engineered crops and pesticide use in U.S. maize and soybeans

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Science Advances  31 Aug 2016:
Vol. 2, no. 8, e1600850
DOI: 10.1126/sciadv.1600850

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  • RE: Response to Plewis
    • Edward D. Perry, Assistant Professor, Department of Agricultural Economics, Kansas State University
    • Other Contributors:
      • Federico Ciliberto, Associate Professor, Department of Economics, University of Virginia
      • David A. Hennessy, Professor, Department of Agricultural, Food, and Resource Economics, Michigan State University
      • GianCarlo Moschini, Professor, Department of Economics, Iowa State University

    The target population for the survey was commercial corn and soybean growers in the United States. The samples were designed to be representative of corn and soybean pesticide use at the crop reporting district (CRD) level). The surveys were conducted via computer assisted telephone interviews. We were not provided with information on the annual response rates. We note that the Pesticide National Synthesis Project of the U.S. Geological Survey has also relied on GfK proprietary pesticide data. Thelin and Stone (2013, p. 3) provide some additional information on the nature of these survey data.

    Concerning the longitudinal component of our analysis, we do not know why certain farmers appear more than others. As part of our robustness checks we did estimate versions of the model that excluded farmers who were sampled more than 5 years, obtaining results virtually identical to those with the full sample.

    As for the replicability of the results, unfortunately we do not have permission to share the primary data. But it is important to note that we do not have exclusive access to the data used.

    Thelin, G. P.; Stone, W. W. Estimation of annual agricultural pesticide use for counties of the conterminous United States, 1992−2009. USGS Scientific Investigations Report, 2013; Vol. 5009, pp 1−54.

    EDITOR'S NOTE: please see full published paper for data availability statement at...

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    Competing Interests: None declared.
  • RE: Pest resistance to genetically engineered Bt crops and insecticide use
    • Bruce E. Tabashnik, Regents' Professor and Department Head, Department of Entomology, University of Arizona

    Pest resistance to genetically engineered Bt crops and insecticide use

    Bruce E. Tabashnik, Department of Entomology,
    University of Arizona, Tucson AZ, 85721

    Perry et al. (1) provide a valuable analysis of the association between pesticide use and adoption of genetically engineered corn and soybeans in the United States from 1998 to 2011. They conclude that adopters of glyphosate tolerant (GT) crops increased herbicide use over this period, which they attribute in part to emergence of weed resistance to glyphosate. By contrast, they find no such increase for insecticide use on transgenic corn producing insecticidal proteins from Bacillus thuringiensis (Bt), which they state is “consistent with the evidence that non-Bt maize refugia have been broadly effective as a means to prevent the onset of pest resistance,” citing my 2008 paper (2). That paper explains how refuges of non-Bt host plants yield susceptible adults that can mate with resistant adults surviving on Bt crops and thereby delay evolution of resistance. It does note sustained susceptibility to Bt crops in five insect pests including the European corn borer (Ostrinia nubilalis), but also mentions two pests that had evolved resistance to Bt corn (one in the United States territory of Puerto Rico and the other in South Africa) and a third pest that had evolved resistance to Bt cotton. I proposed that in those three cases, the scarcity of refuges might have hastened resistance.
    Since 2008...

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    Competing Interests: Bruce Tabashnik is coauthor of a patent on modified Bt toxins, "Suppression of Resistance in Insects to Bacillus thuringiensis Cry Toxins, Using Toxins that do not Require the Cadherin Receptor" (patent numbers: CA2690188A1, CN101730712A, EP2184293A2,EP2184293A4, EP2184293B1, WO2008150150A2, WO2008150150A3). Bayer CropScience, Dow AgroSciences, DuPont Pioneer, Monsanto and Syngenta did not provide funding to support this work, but may be affected financially by publication of this letter and have funded other work by the author.
  • RE: Genetically engineered crops and pesticide use in US maize and soybeans
    • Ian Frank Plewis, Emeritus Professor of Social Statistics, University of Manchester, UK

    This article is an important contribution to the debate about the effects of genetically engineered crops. Its strength comes from the authors’ analyses of data at the farm level; previous analyses have had to rely on aggregate data from USDA surveys. But this strength is also a weakness in that the data come from a commercial organisation – GfK Kynetec - and are not publicly available. Moreover, it is not possible to find out just how GfK Kynetec carry out this survey over time and the article is silent on a number of salient issues. In particular:

    1. What is the target population for the survey, what is the sample design and how is the sampling actually done each year?
    2. How are the data collected – face-to-face interviews, mail, phone or web?
    3. What are the annual response rates to the survey?
    4. The survey has a longitudinal component which the authors exploit to identify and estimate their GE effect but we do not know how and why farmers appear in the survey more than once. Table S1 indicates that nearly half the farmers appeared in the survey just once over the 14 year period but some were surveyed more than 10 times.
    These criticisms do not necessarily invalidate the authors’ conclusions although point 4 above is particularly relevant if farmers’ continued participation in the survey is linked to their pesticide use and to changes in their practices. More generally, it is a matter of concern when replication of analyses is not possi...

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    Competing Interests: None declared.