THIRD WORLD NETWORK BIOSAFETY INFORMATION SERVICE
Dear Friends and Colleagues
Efficacy of Bt Pyramided Crops in Question
During the past decade, the rapid and large-scale adoption of Bacillus thuringiensis (Bt) crops has led a number of targeted insect pests to evolve resistance to Bt toxins. Bt crop pyramids produce two or more active Bt toxins to broaden the spectrum of action and to delay the development of resistance in exposed insect populations.
However, there are increasing reports of lack of pest control with pyramided Bt crops; a case in point is that of Louisiana, USA (Item 1). In 2017 and 2018, notable ear damage and larval survival of the corn earworm, Helicoverpa zea, were observed on pyramided Cry1A.105/Cry2Ab2 maize in some fields in northeast Louisiana. H. zea is considered to be the most costly crop pest in North America with more than 200 host plants, many of which are economically important crops. Researchers found that the observed field control problems of Cry1A.105/Cry2Ab2 maize against H. zea in northeast Louisiana were due to resistance development of the insect to both the Bt toxins in plants. This is the first documentation of field-evolved resistance to pyramided Bt maize in a target insect species in southern USA.
In a mini review of studies on cross-resistance among more than 20 Bt toxins that are most commonly used against 13 insect pests (Item 2), researchers found that insects can develop resistance under the stress of one or more toxins. Cross-resistance occurs when one Bt toxin selected pest population leads to a genetic decrease in susceptibility to other Bt toxins. For example, pyramided Cry1A+Cry2A Bt cotton has replaced the Cry1A crop in Australia, the United States, and India; however, resistance to Cry1Ac could also rapidly accelerate the development of resistance to Cry2Ab, resulting in a high potential risk of resistance to pyramided Cry1A+Cry2A. The researchers stress that more careful evaluation of cross-resistance between Bt toxins must be considered. They conclude that pyramids strategies are not enough for resistance management and pest control; other pest control measures also need to be considered.
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Third World Network
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Item 1
FIELD-EVOLVED RESISTANCE OF HELICOVERPA ZEA (BODDIE) TO TRANSGENIC MAIZE EXPRESSING PYRAMIDED CRY1A.105/CRY2AB2 PROTEINS IN NORTHEAST LOUISIANA, THE UNITED STATES
Gagandeep Kaura et al.
Journal of Invetebrate Pathology 2019 May (163:11-20)
doi: 10.1016/j.jip.2019.02.007.
e-published: 27 Feb 2019
https://www.ncbi.nlm.nih.gov/pubmed/30825480
Abstract
The corn earworm, Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae), is a major target pest of pyramided Bt maize and cotton in the U.S. In 2017 and 2018, notable ear damage and larval survival of H. zea were observed on pyramided Cry1A.105/Cry2Ab2 maize in some fields in northeast Louisiana, U.S. The objective of this study was to determine if the field control problem was due to resistance development to the Bt proteins in plants. A total of 15 H. zea populations were collected from Bt and non-Bt maize plants in 2017 and 2018 in multiple locations in Louisiana, Florida, and Georgia. Diet-overlay bioassays showed that LC50s of Cry1A.105 and Cry2Ab2 for populations collected from the areas with control problems in northeast Louisiana were as much as >1623- and 88-fold greater than that of a susceptible strain, respectively. In addition, two field trials in 2018 validated that Cry1A.105/Cry2Ab2 maize failed in managing natural H. zea populations, while Bt maize containing Vip3A was effective in northeast Louisiana. Results of the study documented that the observed field control problems of Cry1A.105/Cry2Ab2 maize against H. zea in northeast Louisiana were due to resistance development of the insect to the Bt proteins in plants. This is the first documentation of field-evolved resistance to pyramided Bt maize in a target insect species in southern U.S. However, susceptibility levels to Cry1A.105 and Cry2Ab2 varied greatly among populations collected from the three states, suggesting uneven distributions of the resistance in the region.
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Item 2
THE PROGRESS IN INSECT CROSS-RESISTANCE AMONG BACILLUS THURINGIENSIS TOXINS
Jizhen Wei, Yaling Zhang and Shiheng An
Wiley Online Library
https://doi.org/10.1002/arch.21547
First published: 12 March 2019
https://onlinelibrary.wiley.com/doi/10.1002/arch.21547
Abstract
Bt crop pyramids produce two or more Bt proteins active to broaden the spectrum of action and to delay the development of resistance in exposed insect populations. The cross-resistance between Bt toxins is a vital restriction factor for Bt crop pyramids, which may reduce the effect of pyramid strategy. In this review, the status of the cross-resistance among more than 20 Bt toxins that are most commonly used against 13 insect pests was analyzed. The potential mechanisms of cross-resistance are discussed. The corresponding measures, including pyramid RNA interference and Bt toxin, “high dose/refuge,” and so on are advised to be taken for adopting the pyramided strategy to delay the Bt evolution of resistance and control the target pest insect.