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GES Center Lectures, NC State University
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S14E6 - Ed Perry on GE crops and pesticides: the critical nexus

Oct. 13, 2026 GES Colloquium | This talk examines the evolving relationship between pesticide use and the adoption of genetically engineered (GE) crops.

GES Colloquium:

GE Crops and Pesticides: The Critical Nexus

Edward Perry, PhD

Associate Professor of Economics at Iowa State University, Profile  

Full details at https://ges.research.ncsu.edu/event/colloquium-2026-10-13/ | Watch the video

Related links:

  • Genetically Engineered Crops and Pesticides: The Critical Nexus, Moschini and Perry, 2026 [Pre-print]. Available at SSRN: https://ssrn.com/abstract=6901678 
  • Perry, E. D., & Moschini, G. (2020). Neonicotinoids in US maize: insecticide substitution effects and environmental risk. Journal of Environmental Economics and Management, 102, 102320. https://doi.org/10.1016/j.jeem.2020.102320 
  • Perry, E. D., Ciliberto, F., Hennessy, D. A., & Moschini, G. (2016). Genetically engineered crops and pesticide use in US maize and soybeans. Science advances, 2(8), e1600850. https://www.science.org/doi/10.1126/sciadv.1600850 
  • Lee, S., Moschini, G., & Perry, E. D. (2023). Genetically engineered varieties and applied pesticide toxicity in US maize and soybeans: Heterogeneous and evolving impacts. Ecological Economics, 211, 107873. https://doi.org/10.1016/j.ecolecon.2023.107873 
  • Carrière Y., Brown Z., Aglasan S., et al. (2020) Crop rotation mitigates impacts of corn rootworm resistance to transgenic Bt corn, Proc. Natl. Acad. Sci. U.S.A. 117 (31) 18385-18392, https://doi.org/10.1073/pnas.2003604117 
  • Gould F., Brown Z.S., and Kuzma J. (2018) Wicked evolution: Can we address the sociobiological dilemma of pesticide resistance? Science, Vol. 360, Issue 6390, pp. 728-732. DOI: 10.1126/science.aar3780. Download

Overview

This colloquium, hosted by the Genetic Engineering and Society (GES) Center, featured a presentation by Ed Perry, newly appointed economics faculty at Iowa State University. The talk was based on a working handbook chapter co-authored with Giancarlo Moschini, titled Genetically Engineered Crops and Pesticides: A Critical Nexus. The presentation provided a comprehensive survey of the economic and environmental literature examining the relationship between genetically engineered (GE) crop adoption and pesticide use over the past 30 years, covering farm-level impacts, resistance development, spillover effects, and human health concerns.

Key Concepts or Theories

  • Economic framework of GE crop adoption: Farmers as profit-maximizing agents whose pesticide decisions are shaped by the marginal value of inputs, altered by GE traits
  • Two-phase model of GE crop impact: An initial phase of simplified, reduced-cost pesticide use followed by a resistance-driven rebound in chemical inputs
  • Tragedy of the commons in resistance management: Individual rational behavior leading to collectively suboptimal outcomes in pest and weed resistance
  • Toxicity-weighted pesticide measurement (Total Applied Toxicity - TAT): Weighting pesticide quantities by species-specific LD50 scores to better capture environmental and health hazards
  • Spillover effects: Both positive (pest suppression benefiting neighboring farms) and negative (herbicide drift, resistance spread) externalities extending beyond adopting farms

Important Questions Raised

  • Has GE crop adoption contributed to larger farm sizes, and what confounding factors does that introduce?
  • Can negative birth outcomes observed in studies be specifically attributed to glyphosate, or are they driven by broader land use changes?
  • How should extension programs and policy incorporate research findings on pesticide use and resistance management?
  • What are the unintended consequences of restricting specific pesticides, and could substitutes be more harmful?

Key Takeaways and Summary of Learning Objectives

  • GE crop adoption has had fundamentally different effects on insecticide use versus herbicide use, requiring separate analysis of each trait type.
  • Bt insect-resistant crops have more unambiguously reduced insecticide use and improved yields, though resistance issues persist, particularly with western corn rootworm.
  • Herbicide-tolerant (Roundup Ready) crops initially simplified and reduced herbicide costs but created conditions for widespread glyphosate resistance, now confirmed in 38 weed species globally.
  • Measuring pesticide use by mass alone is misleading; toxicity-weighted measures reveal qualitatively different trends and are essential for accurate environmental and health assessments.
  • Spillover effects of GE crop adoption are bidirectional: positive effects include regional pest suppression and increased no-till adoption; negative effects include herbicide drift complaints and resistance spread.
  • The link between glyphosate use and negative birth outcomes remains an open and contested area of research, with methodological challenges in isolating glyphosate from other co-occurring land use changes.
  • Proactive resistance management policies in the early 2000s could have prevented billions of dollars in current herbicide costs.
  • Policy design must account for unintended consequences, as restricting one pesticide may lead farmers to adopt alternatives that are equally or more harmful.

Topic 1: Farm-Level Impacts of GE Crop Adoption

The presentation began by framing GE crop adoption through an economic lens: farmers seek to maximize profit, and GE traits alter the relative value of different pesticide inputs. Bt (insect-resistant) traits substitute for insecticides by embedding proteins that kill target insects, while herbicide-tolerant traits — most notably Roundup Ready crops — allow broad-spectrum glyphosate application without crop damage, simplifying weed management and reducing costs.

By approximately 2013, over 90% of corn, soybean, and cotton acres in the U.S. were planted with herbicide-tolerant varieties, and similar adoption rates were reached for Bt traits by 2020. This near-universal adoption has had significant implications for pesticide use patterns at the farm level.

Bt Insect-Resistant Crops: A global meta-analysis cited by Ed Perry found that Bt crop adoption was associated with a 25% average yield increase, a 42% reduction in pesticide quantity, and higher profits. U.S.-specific data showed approximately an 11% reduction in insecticide use, a smaller figure reflecting higher baseline insecticide use in the U.S. These outcomes were described as more unambiguously positive compared to herbicide-tolerant crops.

Herbicide-Tolerant Crops: The picture is more complex. While GT soybean adopters used 28% more herbicide by mass than non-adopters, toxicity-weighted measures showed no significant difference in environmental hazard between the two groups. The primary initial benefit was cost reduction: glyphosate, especially after its patent expiration in 2000, was cheap and effective, enabling farmers to dramatically simplify their herbicide programs.

Relevant Q\&A

Question: Has the adoption of GE crops led to larger farm sizes, and what confounding factors might that introduce?

Answer: Ed Perry noted that he was unaware of studies directly quantifying this relationship and suggested intuitively that GE crops may not strongly favor larger farms, since they do not require large fixed-cost investments the way precision agriculture technologies do. He suggested GE crops may have been relatively more beneficial to smaller operations. Zack agreed, characterizing GE crops as a scale-independent technology. Gaylon offered a grower-level perspective, noting that the convenience of GE systems allowed farmers to cover more acres with the same effort, potentially enabling economies of scale. Ed Perry acknowledged this could shift crop mix — for example, increasing soybean acreage — but noted that disentangling GE adoption effects from simultaneous commodity price booms and ethanol mandates is methodologically challenging.

Topic 2: Resistance Development and the Two-Phase Model

Ed Perry described a clear two-phase trajectory in how GE crop adoption has affected pesticide use, using soybean herbicide trends as the primary illustration.

Phase 1 (approximately 1996–2007): Following the rapid adoption of Roundup Ready soybeans, farmers dramatically increased glyphosate use while eliminating nearly all other herbicides. By 2006, over 70% of soybean farms surveyed were using exclusively glyphosate with no supplemental herbicides. This simplified system was highly cost-effective and initially very productive.

Phase 2 (approximately 2007–present): Beginning around 2004–2005, reports emerged of glyphosate-resistant weed populations — an outcome that had not been widely anticipated by agronomists and weed scientists at the time of Roundup Ready's commercialization. Resistance spread rapidly, and by the time of the presentation, 38 weed species globally had confirmed glyphosate resistance, including highly problematic species such as Palmer amaranth and common waterhemp. In response, farmers began reintroducing older herbicides alongside continued glyphosate use, such that by 2018, total herbicide mass applied in soybeans was approaching or exceeding 1992 levels. Herbicide program costs per acre in Iowa rose from approximately 10–15 in 2006–2007 to 60–70 currently, representing potentially billions of dollars in aggregate additional costs.

Bt Resistance: Resistance to Bt traits has been less severe overall. European corn borer resistance has remained relatively controlled due to the high toxicity of the relevant proteins and the biological characteristics of the pest. However, western corn rootworm has developed resistance to multiple Bt toxins, and adoption of rootworm-targeting Bt traits has declined as farmer confidence in their effectiveness has eroded.

Tragedy of the Commons: Ed Perry framed resistance as a collective action problem. Individual farmers optimize for their own operations without internalizing the resistance externalities imposed on neighbors. Additionally, seed company marketing practices — bundling all available traits into a limited set of commercial varieties — may inadvertently accelerate resistance by ensuring widespread deployment of traits even where they are not agronomically necessary.

Topic 3: Spillover Effects Beyond the Farm

GE crop adoption generates a range of effects that extend beyond the adopting farm, both positive and negative.

Positive Spillovers — Regional Pest Suppression: Studies published in PNAS documented that widespread Bt corn adoption suppressed European corn borer populations at a regional level, reducing pest pressure and insecticide costs for neighboring farms — including farms that did not adopt Bt crops — and even for producers of other crops such as vegetables that share the same pest.

Negative Spillovers — Herbicide Drift: The introduction of dicamba-tolerant and 2,4-D-tolerant soybean varieties approximately 8–10 years ago created a new category of spillover harm. Both dicamba and 2,4-D have poor drift properties, meaning applications can migrate to neighboring fields and damage non-tolerant crops. U.S. herbicide drift complaints spiked from approximately 1,000 annually (2013–2016) to 3,000 in 2017 following the commercial release of dicamba-tolerant soybeans. Legal liability has been established in some cases, and regulatory agencies have had to impose restrictions on specific dicamba formulations. Ed Perry noted that some observers have speculated — though not verified — that seed companies may have had strategic incentives to promote drift-prone herbicides as a means of compelling neighboring farmers to adopt tolerant varieties.

Management Practice Spillovers: GE crop adoption also influenced broader farming practices. Glyphosate-tolerant soybeans enabled post-emergence weed control without tillage, contributing to a significant increase in no-till and conservation tillage adoption, with associated reductions in soil erosion and CO2 emissions. However, as resistance has driven a return to mechanical tillage in some regions, these environmental benefits have partially eroded. Conversely, the effectiveness of glyphosate-only systems reduced the perceived need for crop rotation, which may have contributed to resistance development.

Topic 4: Toxicity-Weighted Measurement and Environmental Impact

A central methodological contribution highlighted in the chapter is the shift from measuring pesticide use by mass (pounds or kilograms per acre) to toxicity-weighted measures that account for the heterogeneous hazard profiles of individual chemicals.

Ed Perry introduced the Total Applied Toxicity (TAT) framework, in which the quantity of each chemical applied is weighted by its species-specific lethal dose 50 (LD50) value. This approach can yield qualitatively different conclusions from mass-based measures. For example, in corn, herbicide mass applied in 2018 was approximately equal to 1992 levels (~3 kg/hectare), yet mammalian toxicity (measured by rat LD50) declined dramatically — from approximately 7,000 to 1,000 LD50 doses per hectare — reflecting the displacement of highly toxic older herbicides by glyphosate, which has a relatively low mammalian toxicity score.

In soybeans, the toxicity trajectory was initially positive as glyphosate replaced more toxic herbicides, but has since reversed as older herbicides have been reintroduced alongside glyphosate. Ed Perry suggested that current soybean herbicide toxicity levels may now exceed those of 1992.

The choice of target species also matters significantly. While mammalian toxicity from insecticides has declined substantially — partly due to EPA regulations restricting organophosphates and partly due to GE crop adoption — honeybee toxicity has increased with the rise of neonicotinoid seed treatments, which are now applied to virtually all corn acres. This divergence underscores the importance of evaluating toxicity impacts across multiple non-target species rather than relying on a single metric.

Topic 5: Human Health Implications and Policy Considerations

Glyphosate and Birth Outcomes: Ed Perry identified the relationship between glyphosate exposure and human health outcomes — particularly birth outcomes — as one of the most active and contested areas of current research. Despite glyphosate's relatively low acute mammalian toxicity scores, concerns have emerged around longer-term effects including cancer risk and adverse birth outcomes. Two recent studies were discussed: one by Renner and Rubin published in PNAS, which found correlations between GE crop rollout, increased glyphosate use, and lower birth weight and shorter gestation; and a more recent study examining downstream county-level birth outcomes relative to upstream glyphosate application, which found negative effects concentrated in lower-income areas.

Ed Perry cautioned that a key methodological challenge in this literature is distinguishing glyphosate-specific effects from the broader effects of land use change. The instruments used in these studies — such as land suitability for corn and soybean production — capture the overall intensification of row crop agriculture, not glyphosate use specifically. He noted that parallel studies using the same methods but attributing effects to atrazine rather than glyphosate produce similar results, suggesting the identified effects may reflect land use change more broadly. Ongoing collaborative work with Zack and Greg Ferraro aims to more carefully isolate glyphosate's contribution using product concentration-based instrumental variables.

Policy Implications: Ed Perry outlined several overarching policy lessons from the literature. First, pesticide regulation involves a fundamental trade-off between agricultural productivity and environmental or health externalities, and policy must carefully quantify both sides. Second, unintended consequences of regulation are common: restricting one pesticide may lead farmers to substitute alternatives that are equally or more harmful. Third, resistance management represents a missed opportunity — proactive policies in the early 2000s, such as mandatory crop rotation requirements or extension guidance on diversifying herbicide modes of action, could have substantially slowed or prevented the current glyphosate resistance crisis. Fourth, as new GE traits and CRISPR-based crop modifications are developed, the lessons from the past 30 years should inform more anticipatory regulatory frameworks.

Relevant Q\&A

Question: Given your extension appointment, have you thought about how to convey these research findings to farmers?

Answer: Ed Perry noted that his extension role will primarily involve agricultural cooperatives, providing indirect rather than direct farmer engagement. He acknowledged that much of his research is more directly relevant to policy than to on-farm decision-making. He observed that even well-intentioned extension advice on resistance management faces the collective action problem: individual farmers have limited incentive to adopt resistance-prevention practices if they cannot be assured their neighbors will do the same. He also noted that confusion around glyphosate product concentration levels in the past led many farmers to over-apply, highlighting the practical challenges of translating research into behavioral change.

Actionable Next Steps / Assignments

  • The next GES Colloquium will be held on October 27th (in person), featuring Greg Ferraro, currently a postdoc at the University of Maryland, presenting research on sea level rise and farmland values. Note: there is no Colloquium the week prior due to fall break.

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