Common Corn Pest Presents Major Challenge to Bt Crops
By Andrew Porterfield
The fall armyworm (Spodoptera frugiperda) is a major corn pest in South America, known to devour corn crops from Argentina to the Southeastern United States. Farmers have typically battled the worm’s infestation with chemical insecticides, but they have been met (unsurprisingly) with resistant insects.
Transgenic corn raised to express Bacillus thuringiensis (Bt) proteins were introduced in the last decade and were quickly adopted by farmers in Brazil. However, a paper by entomologists at Ohio State University, the University of Sao Paulo, and the agricultural company Syngenta showed that armyworm resistance to Bt corn started appearing in Brazil just three years after the transgenic product’s introduction. Worse, this resistance is increasing, calling for a major strategy shift in managing this pest, the researchers write in a study published this week in the open-access Journal of Integrated Pest Management.
Ohio State researcher Julio Fatoretto and his colleagues demonstrate that Bt’s ability to fend off the armyworm was thwarted by a number of issues in Brazil: the worm’s genetics and ability to quickly evolve resistance traits to all but one of the Bt proteins, the high reproductive efficiency of the armyworm in tropical areas, and the intense agricultural practices carried out in tropical areas like Brazil.
The armyworm can cause devastating losses to corn crops—before Bt, yield reductions reached 34 percent to 38 percent of a total crop. More frequent plantings, including a second corn season, have helped raise this number significantly, because of the creation of “green bridges,” or extended growing seasons that allow the armyworm larvae (and their winged adult phases) to easily migrate from crop to crop throughout the year. The larvae easily adapt to pesticides, too: Currently, the 2016 fall armyworm is resistant to 24 active insecticide ingredients. Introducing Bt corn provided $98.2 billion in economic benefits globally between 1996 and 2011 and boosted corn productivity—in 2013, more than 12 million hectares (29.7 million acres) were planted with genetically modified Bt corn.
In Brazil, multiple types of Bt proteins were introduced to reduce armyworm infestations, including Cry1Ab in 2008 and Cry1f in 2009, and a “pyramid” Cry1A.105+Cry2Ab in 2010, but none could prevent resistance and crop damage a few years following their introduction. Only a non-Cry endotoxin, called Vip3Aa20, which is produced during the vegetative stage of Bt, has successfully held S. frugiperda at bay without signs of resistance.
To the researchers, the experience in Brazil provides some important lessons for managing pests like the fall armyworm:
- Look at genetics. Higher frequencies of resistance alleles in pests usually results in faster evolution of resistance. However, most crop experts and farmers do not measure these frequencies. Reproductive rates of fall armyworms are also very high (one female can produce more than 1,600 offspring), which quickly incorporate resistance genes in a population, neutralizing the effects of Bt. To make matters worse, the resistance alleles to Cry proteins appeared to be dominant, and having these resistance alleles did not result in any apparent weakening of the pest population.
- The environment matters. The armyworm can produce 10 generations a year in the tropics, and adults can migrate hundreds of miles to lay their eggs. Meanwhile, the armyworm doesn’t limit its diet to corn, also feeding on cotton and soybean, which also contain protein-expressing Bt that the pest can begin to resist. This varied diet has helped the armyworms develop the ability to easily metabolize toxic compounds, including Cry proteins.
- Manage land use. Tropical climates have usually resulted in more intensive land use, and Brazil is no exception. Rapid crop succession creates what the researchers called a “year-round mosaic” for armyworms to feed on, including corn, wheat, cotton, soybean, and millet.
To combat these issues, the researchers recommended several strategies, including refuges, which involve planting non-Bt crops near Bt crops, so a number of “naïve” pests are born and can be affected by Bt. Compliance with this rule is one of Brazil’s biggest agricultural problems, however. Using Vip3Aa20 continues to be successful, too. Partial insecticide spraying of leaves in refuge crop areas also can help keep populations lower.
“Development of a perfect IRM [insecticide resistance management] system is challenging due to the interaction of biology, ecology, and even society,” the researchers write. But, “as only one Bt product remains effective (Vip3Aa20), all agronomic groups have a vested interest in extending its durability.”
Journal of Integrated Pest Management
Andrew Porterfield is a writer, editor, and communications consultant for academic institutions, companies, and nonprofits in the life sciences. He writes frequently about agriculture issues for the Genetic Literacy Project. He is based in Camarillo, California. Follow him on Twitter at @AMPorterfield or visit his Facebook page.