TWN Info Service on Biosafety
18 October 2023
Third World Network
www.twn.my
Dear Friends and Colleagues
GM Bt crops losing effectiveness as insect resistance to Bt toxins increases
GM Bt crops, which are genetically engineered to produce insecticidal proteins, are losing effectiveness as pests develop resistance to the Bt toxin insecticides.
The area planted globally to transgenic crops that produce Bt proteins increased to 109 million ha in 2019 in 27 nations. Evolution of resistance by pests, however, continues to reduce the effectiveness of Bt crops, despite widespread adoption of Bt crops called pyramids that produce two or more distinct Bt proteins that are toxic to each target pest.
There are three categories of resistance to Bt crops: (1) practical resistance, (2) early warning of resistance, and (3) no decrease in susceptibility. The first two categories are subsets of field-evolved resistance. Practical resistance means that more than half of the individuals in a population are resistant and the field efficacy of the Bt crop has decreased. No decrease in susceptibility indicates the monitoring data show no statistically significant decrease in susceptibility in any population tested after field populations have been exposed to a Bt crop.
A recent study analysed global resistance monitoring data for 24 pest species based on the first 25 years of cultivation of Bt crops, including corn, cotton, soybean, and sugarcane. Each of the 73 cases examined represented the response of one pest species in one country to one Bt toxin produced by one or more Bt crops. The rapid evolution of practical resistance to Bt crops has reduced their efficacy in at least 26 cases in 2020 compared to 3 in 2005. The U.S. accounted for half of the world’s 26 cases of practical resistance, which is consistent with its high adoption of Bt crops. Practical resistance was documented in some populations of 11 pest species, collectively affecting nine widely used Bt toxins in seven countries.
Conversely, 30 cases reflected no decrease in susceptibility to Bt crops in populations of 16 pest species in 10 countries. The remaining 17 cases provide early warnings of resistance, which entail genetically based decreases in susceptibility without evidence of reduced field efficacy.
With best wishes
Third World Network
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Global Patterns of Insect Resistance to Transgenic Bt Crops: The First 25 Years
By Bruce E Tabashnik, Jeffrey A Fabrick, and Yves Carrière
Journal of Economic Entomology, Volume 116, Issue 2,
April 2023, Pages 297–309,
https://doi.org/10.1093/jee/toac183
https://academic.oup.com/jee/article/116/2/297/6968925?login=false
*A correction has been published: Journal of Economic Entomology, Volume 116, Issue 2, April 2023, Page 648, https://doi.org/10.1093/jee/toad013
ABSTRACT
Crops genetically engineered to produce insecticidal proteins from the bacterium Bacillus thuringiensis (Bt) have improved pest management and reduced reliance on insecticide sprays. However, evolution of practical resistance by some pests has reduced the efficacy of Bt crops. We analyzed global resistance monitoring data for 24 pest species based on the first 25 yr of cultivation of Bt crops including corn, cotton, soybean, and sugarcane. Each of the 73 cases examined represents the response of one pest species in one country to one Bt toxin produced by one or more Bt crops. The cases of practical resistance rose from 3 in 2005 to 26 in 2020. Practical resistance has been documented in some populations of 11 pest species (nine lepidopterans and two coleopterans), collectively affecting nine widely used crystalline (Cry) Bt toxins in seven countries. Conversely, 30 cases reflect no decrease in susceptibility to Bt crops in populations of 16 pest species in 10 countries. The remaining 17 cases provide early warnings of resistance, which entail genetically based decreases in susceptibility without evidence of reduced field efficacy. The early warnings involve four Cry toxins and the Bt vegetative insecticidal protein Vip3Aa. Factors expected to favor sustained susceptibility include abundant refuges of non-Bt host plants, recessive inheritance of resistance, low resistance allele frequency, fitness costs, incomplete resistance, and redundant killing by multi-toxin Bt crops. Also, sufficiently abundant refuges can overcome some unfavorable conditions for other factors. These insights may help to increase the sustainability of current and future transgenic insecticidal crops.