Functional transgenes in Mexican maize: Benefits and risks for insect pest management in Mexico and the United States

Corn earworm (Helicoverpa zea) and fall armyworm (Spodoptera frugiperda) are major migratory pests of maize (Zea mays) in the United States and Mexico. They are primarily controlled in the United States with genetically engineered (GE) maize, while the 25-yr moratorium on cultivating GE maize in Mexico has forced growers to control these pests with insecticides, where maize productivity remains 35% below the world's average. The United States annually exports 5% of its maize grain to Mexico, where it provides human food and animal feed. This seed is often sown by smallholder growers, leading to plantings of GE transgene-expressing maize and potential hybridization with local landraces. As a result, transgenes are now present in Mexican maize products and landraces. In this study, we examined the F1 offspring of GE maize to better understand the frequency of different transgenes expressed in maize seeds exported to Mexico. We show that exported seed contains numerous transgenes, including an estimated ~68% epsps expressing resistance to the herbicide glyphosate; ~80% pat and bar expressing resistance to the herbicide glufosinate; and ~82% Bacillus thuringiensis (Bt) genes that effectively protect maize plants from several insect pests. We tested 134 samples, including landraces from 10 Mexican states, and found that 35% expressed resistance to glyphosate and 33% to glufosinate. Many samples containing herbicide resistance also expressed 11%-100% functional Bt transgenes, which can effectively reduce the refuge area provided by Mexican maize and increase the Bt-resistant allele frequency. We discuss ways that the introgression of transgenes could provide pest management benefits to Mexican growers but, at the same time, accelerate the development of Bt-resistance in corn earworm and fall armyworm. Our cost-effective screening methods can be used to determine the introgression of functional herbicide resistance and Bt transgenes in maize.