THIRD WORLD NETWORK BIOSAFETY INFORMATION SERVICE
Dear Friends and Colleagues
Cases of Pest Resistance to Bt Crops Increased Five-fold From 2005-2016
Transgenic or genetically modified Bt (Bacillus thuringiensis) crops (mainly corn, cotton and soybean) cover more than 830 million hectares worldwide. In addition to the crystalline (Cry) proteins from Bt produced by transgenic crops for the past two decades, some recently introduced types of Bt corn and cotton produce a vegetative insecticidal protein (Vip) from Bt.
The efficacy of Bt crops has been threatened by the evolution of pest resistance. A recent study analyzes relevant literature on this topic from the past two decades to elucidate the current status of pest resistance to transgenic crops. Compared with previous reviews on this topic, the field-monitoring data analyzed in this study represent a more diverse set of Bt toxins (one Vip and nine Cry toxins), crops (corn, cotton, and soy), pests (15 species from two insect orders), and countries (ten countries on six continents).
The study found that the number of cases of pest resistance to Cry proteins produced by transgenic crops increased from 3 in 2005 to 16 in 2016. For the 16 cases of practical resistance, the average time for evolution of resistance was only 5.2 years. In four situations, practical resistance has reduced the number of Bt toxins that are available in commercialized transgenic crops and are still effective against some pest populations to two, one, or none.
Modeling results and empirical evidence show that refuge requirements must be tailored to each pest–transgenic-crop combination. For plants producing a single Bt toxin, when the high-dose standard is met and resistance is rare, refuges accounting for as little as 20% of a pest’s host plants may be sufficient to delay resistance for a decade or more. Conversely, when the high-dose standard is not met and resistance is not rare, larger refuges (e.g., 50%) are needed to substantially delay pest adaptation. Similar principles apply to pyramids. Much larger refuges are needed to substantially delay resistance if each of the toxins or traits in a pyramid is not highly effective, either inherently or because of field-evolved resistance. The primary lesson gleaned from this study is that abundant refuges of non-Bt host plants can delay pest resistance to transgenic crops.
With best wishes,
Third World Network
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Malaysia
Email: twn@twnetwork.org
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SURGE IN INSECT RESISTANCE TO TRANSGENIC CROPS AND PROSPECTS FOR SUSTAINABILITY
Tabashnik, B. E., & Carrière, Y. (2017)
Nature biotechnology, 35(10), 926
https://www.nature.com/nbt/journal/v35/n10/full/nbt.3974.html
11 October 2017
Abstract
Transgenic crops have revolutionized insect pest control, but their effectiveness has been reduced by evolution of resistance in pests. We analyzed global monitoring data reported during the first two decades of transgenic crops, with each case representing the responses of one pest species in one country to one insecticidal protein from Bacillus thuringiensis (Bt). The cases of pest resistance to Bt crystalline (Cry) proteins produced by transgenic crops increased from 3 in 2005 to 16 in 2016. By contrast, in 17 other cases there was no decrease in pest susceptibility to Bt crops, including the recently introduced transgenic corn that produces a Bt vegetative insecticidal protein (Vip). Recessive inheritance of pest resistance has favored sustained susceptibility, but even when inheritance is not recessive, abundant refuges of non-Bt host plants have substantially delayed resistance. These insights may inform resistance management strategies to increase the durability of current and future transgenic crops.