TWN Info Service on Biosafety
19 March 2025
Third World Network
www.twn.my
Dear Friends and Colleagues
Significant Insect Resistance Detected in GM Maize with Two Bt Proteins in South Africa
The African maize stem borer, Busseola fusca, is one of the most important pests of maize in Africa. Cry1Ab maize was planted commercially in South Africa for 6 years since 1998-1999 before field resistance of B. fusca was first reported. Over time, this was followed by reports of Cry1Ab-resistant B. fusca populations throughout the maize production region and the conclusion that it evolved practical resistance to Cry1Ab maize.
To mitigate the problem with Cry1Ab resistance in South Africa, MON 89034 maize, which contains two Bt proteins (Cry1A.105 and Cry2Ab2) was commercialized in 2011. However, growers in localized areas in the KwaZulu-Natal province and Mpumalanga province reported the presence of B. fusca infestations in MON 89034 maize in 2017 and 2023, respectively.
High levels of resistance of B. fusca populations to the Cry1Ab protein in MON 810 and to the Cry2Ab2 protein in MON 89034 were identified during this study in three production regions in South Africa. The observed resistance to Cry2Ab2 likely evolved in part because MON 89034 acts as a single-protein event against B. fusca, with all activity coming from Cry2Ab2. This can also lead to resistance to MON 89034 in other regions of South Africa.
With best wishes,
Third World Network
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INCIPIENT RESISTANCE OF THE AFRICAN MAIZE STEMBORER, BUSSEOLA FUSCA (LEPIDOPTERA: NOCTUIDAE) TO CRY2AB2 MAIZE IN SOUTH AFRICA
Strydom, E., Erasmus, A., Friis, S., Magson, J., Parimi, S., Martinelli, S., … & van den Berg, J. Pest Management Science.
https://scijournals.onlinelibrary.wiley.com/doi/10.1002/ps.8529
14 November 2024
ABSTRACT
BACKGROUND
Resistance of Busseola fusca (Lepidoptera: Noctuidae) to Cry1Ab was documented in 2006, 7 years after the first cultivation of MON 810 in South Africa. This was mitigated by introducing a second-generation Bacillus thuringiensis (Bt) maize (MON 89034), which contains the Cry1A.105 and Cry2Ab2 proteins. The first reports of B. fusca infestations of MON 89034 maize came in the KwaZulu-Natal province (2017–2018 cropping season), followed by reports in the Mpumalanga province (2022–2023 season). Here we report results of artificial diet- and plant-based laboratory assays to assess the susceptibility of B. fusca populations to the Bt proteins in MON 89034.
RESULTS
Larvae were sampled from nine locations which included three where greater than expected injury due to B. fusca had been reported to MON 89034. Larval mortality in assays with diet-incorporated Cry2Ab2 protein was 100% for all except the three problem populations, showing that the Cry2Ab2 protein in MON 89034 is highly efficacious against B. fusca. In contrast, assays with Cry1A.105 did not cause significant mortality in any of the B. fusca populations including larvae from a susceptible reference. Larval survival on leaf tissue of MON 89034 maize after 7 days ranged between 75% and 91% for the three problematic B. fusca populations, compared to 0.4% to 9.6% for the five other populations.
CONCLUSION
Therefore, MON 89034 is effectively a single-mode-of-action technology against B. fusca and carries an inherent high risk for the evolution of resistance. This study shows that the three B. fusca populations collected from locations with greater than expected damage to MON 89034 have resistance to the Cry2Ab2 protein and therefore to MON 89034 maize. This research emphasizes the importance of resistance monitoring and implementation of effective insect resistance management tactics.