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The Promise—and Limits—of Bt Maize to Manage Fall Armyworm in Africa

fall armyworm (Spodoptera frugiperda) on maize

As fall armyworm continues to threaten food security in Africa, the use of Bt maize offers significant potential to manage the pest. But it will be critical to manage resistance via multi-toxin crop varieties and appropriate use of refuge planting—all in forms accessible to the smallholder farmers that grow nearly all of Africa’s maize. (Photo by Clemson University – USDA Cooperative Extension Slide Series ,

By Andrew Porterfield

Andrew Porterfield

Andrew Porterfield

The fall armyworm (Spodoptera frugiperda), which devastated staple crops like maize in South America, has become a similarly worrisome pest to African farmers since its discovery on the continent in 2015. Maize is the most important staple crop in sub-Saharan Africa, accounting for the highest source of calories per capital of all staple plants. In addition, nearly all (98 percent) of maize is grown by “smallholders,” farmers who grow on plots of less than two hectares (4.9 acres).

The fall armyworm has invaded every country in sub-Saharan Africa except Lesotho. Its larvae develop on many species of plants but prefer grasses, maize in particular. The larvae attack maize in all stages of its development, eating leaves, tassels, and ears. In Cameroon, a 2017 study found fall armyworm infestation in 53 percent of maize, 11 percent of sorghum, 3 percent of potato, and 2 percent of cotton and sweet potato.

food security risk in Africa due to fall armyworm

As fall armyworm continues to threaten food security in Africa, the use of Bt maize offers significant potential to manage the pest. But it will be critical to manage resistance via multi-toxin crop varieties and appropriate use of refuge planting—all in forms accessible to the smallholder farmers that grow nearly all of Africa’s maize. In sub-Saharan Africa, risk of food insecurity due to fall armyworm is highest in places where households have limited coping capacity and governments have low capacity to support farmers in pest management and to absorb production shortfalls. (Image originally published in Van den Berg et al 2021, Journal of Economic Entomology)

Maize that is genetically engineered to contain specific toxic (to armyworms) proteins created by the bacteria Bacillus thuringiensis (Bt) has proven successful in thwarting the crop destruction by the fall armyworm in South and Central America and the United States. However, fall armyworm populations have developed significant resistance to certain proteins expressed by Bt maize, underscoring the need to implement agricultural strategies that can boost maize yields while circumventing the effects of resistance. Moreover, these strategies must be accessible to smallholder farmers, who often lack the resources of larger, commercial farms.

A research team of noted Bt and agricultural genetics researchers—led by Bruce Tabashnik, Ph.D., of the University of Arizona and including Johnnie Van den Berg, Ph.D., and Charles Midega, Ph.D., of North-West University in Potchefstroom, South Africa; Boddupalli Prasanna, Ph.D., of the International Maize and Wheat Improvement Center (CIMMYT) in Nairobi, Kenya; Pamela Ronald, Ph.D., of the University of California, Davis; and Yves Carriere, Ph.D., of the University of Arizona—have conducted an extensive research review and numerous interviews with other scientists on fall armyworm and the use of Bt maize in Africa. This month in the Journal of Economic Entomology the team makes several recommendations for creating a plan for Africa to make Bt maize accessible and at least partly overcome fall armyworm resistance.

So far, only South Africa has approved the use of Bt maize on the continent—but Kenya may be the next nation to make a similar approval. As Bt maize may see more approvals and use in Africa, Tabashnik’s team made several recommendations, especially for smallholder farmers:

  • Use Bt maize with other pest management methods, including native host plant resistance (non-genetically modified), as part of an integrated pest management (IPM) plan.
  • Use Bt maize “pyramids” that produce two or more toxins (ideally, four toxins) that are individually effective against fall armyworm but together significantly reduce the likelihood of the pest developing resistance.
  • Avoid single-toxin Bt crops that enable resistance and reduce effectiveness of multi-toxin strains.
  • Provide Bt pyramids that smallholder farms can afford, in open pollinated varieties as well as in more expensive hybrid plants and seeds.
  • Include refuges of non-Bt maize to more than 50 percent of maize hectares for single-toxin Bt plants and 20 percent of maize hectares for multi-toxin Bt plants.
  • Work closely with smallholder farmers at every step from development to harvest of Bt maize.

Resistance to Bt maize is a significant problem for farmers and can evolve quickly. The fall armyworm developed resistance to single-toxin Bt crops in Brazil in just two years. “When multi-toxin Bt crops are deployed optimally with resistance management programs that include adequate refuges of non-Bt host plants and integration with other control tactics, efficacy can be sustained for decades,” Tabashnik says. “IPM methods are available for smallholder farmers and can be the foundation for sustainable pest management in concert with Bt crops.”

Resistance to single-toxin Bt plants (with either of the toxins known as Cry1Ab or Cry1Fa) has been particularly strong. The researchers found that current requirements for refuge (non-Bt crops that can attract non-resistant fall armyworm) in South Africa at five percent is not adequate to address resistance. Kenya is proposing the same percentage. “We expect these (20 to 50 percent) refuge percentages will be readily attainable for smallholders in Sub Saharan Africa … because they typically plant more than one maize cultivar and are likely to continue to plant many non-Bt maize cultivars after Bt maize is introduced,” the researchers write.

Multi-toxin Bt maize, used as part of an IPM plan, should be an improvement over previous strategies that involved heavy, government-funded use of synthetic insecticides. These treatments were hazardous to health and the environment and were not very effective. Bt maize is far safer and offers the potential for working around resistance.

Bayer Crop Science, a provider of Bt maize, is providing the seeds royalty-free. In addition, the researchers were surprised to discover that one of the multi-toxins in Bt maize, Vip3Aa, is more accessible because its patent has expired. “Vip3Aa could be especially useful because it’s effective against some lepidopteran pests that evolved resistance to the more extensively adopted Bt crystalline (Cry) proteins,” Tabashnik says.

The use of Bt maize in Africa may face social and government resistance as well as insect ones. “Anti-GM activism is impactful and influencing governments,” Tabashnik says. “South Africa is the only African nation where Bt maize has been approved. We want to be honest brokers about the benefits and limitations of Bt maize, so all stakeholders can make informed decisions.”

Journal of Economic EntomologyRead More

Managing Fall Armyworm in Africa: Can Bt Maize Sustainably Improve Control?

Journal of Economic Entomology


Andrew Porterfield is a writer, editor, and communications consultant for academic institutions, companies, and nonprofits in the life sciences. He is based in Camarillo, California. Follow him on Twitter at @AMPorterfield or visit his Facebook page.

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