THIRD WORLD NETWORK BIOSAFETY INFORMATION SERVICE
Dear friends and colleagues,
Re: Pest resistance to Cry1Ab Bt maize in South Africa
A recently published paper in the journal Crop Protection traces the development of resistance of the African maize stem borer to Bt maize containing the toxin Cry1Ab, in South Africa, which was the first country to commercially produce Bt crops in Africa.
The paper discusses the contributing factors to resistance development and highlights that many of the early warning signs that should have alerted regulators to the impending problems were ignored. Likewise, while such early warnings should have prompted intensive monitoring of resistance levels as well as strict refuge compliance, this did not happen. In fact, between the first plantings of Bt maize in the 1998/99 cropping season and the first report of resistance in 2007, no systematic evaluation or monitoring for resistance was done.
Currently, resistant populations of the African maize stem borer are being reported at new localities in South Africa on a regular basis. Cry1Ab-toxin therefore has lost its efficacy against the pest in many areas throughout the maize-producing region where single-gene Bt maize events are planted.
This case highlights that for countries who may chose to plant Bt crops, insect resistance management should be a high priority, starting with appropriate monitoring subsequent to release of Bt crops, followed by accurate reporting and development of strategies to counter the problem as soon as it appears. If one waits until pest-induced yield losses start to occur, the level of resistance of the target pest to the Bt crop is most likely such that it cannot be corrected for. Furthermore, the South African experience shows that the predicted rate of evolution of resistance in many instances was underestimated.
According to the scientists, where resistance is prevalent, the only viable options to reduce selection pressure are withdrawal of the product and/or enforcement of high-dose/refuge requirements. The latter may however be of no value under conditions where resistance is prevalent. Remedial actions taken in South Africa included the propagation and enforcement of refuge compliance followed by the release of pyramided maize hybrids in 2011. However, it remains uncertain if cross-resistance occurs between the pyramided toxins (Cry1A.105/Cry2Ab2) and the closely related Cry1Ab toxin, and for how long this pyramided event will endure.
With best wishes,
Third World Network
131 Jalan Macalister
10400 Penang
Malaysia
Email: twnet@po.jaring.my
Website: www.biosafety-info.net and www.twn.my
To subscribe to other TWN information services: www.twnnews.net
Crop Protection 54 (2013) 154-160
http://dx.doi.org/10.1016/j.cropro.2013.08.010
Pest resistance to Cry1Ab Bt maize: Field resistance, contributing factors and lessons from South Africa
Johnnie Van den Berg a, *, Angelika Hilbeck b, Thomas Bøhn c
a Unit for Environmental Sciences and Management, North-West University, Private Bag X6001, Potchefstroom 2520, South Africa
b Swiss Federal Institute of Technology, Institute of Integrative Biology, Universitätsrasse 16, Zürich 8092, Switzerland
c GenØk – Centre for Biosafety, Post Box 6418, 9294 Tromsø, Norway
Abstract
This paper documents the historical development of resistance of the African maize stem borer, Busseola fusca (Fuller) (Lepidoptera: Noctuidae) to Bt maize (Zea mays L.). This pest was one of the first to evolve resistance to Bt maize expressing Cry1Ab protein. A time-line of events and contributing factors are presented, from the commencement of efficacy testing through to the present situation, where the Cry1Ab toxin has lost its efficacy against B. fusca at many localities throughout the maize producing region, and single-gene Bt maize events often require insecticide treatments for which farmers are compensated. Significant levels of pest survival on Bt maize was observed in the first season after commercial release in 1998 and confirmed seven years later. Reduced selection pressure on the target pest is the objective of insect resistance management (IRM), and strategies to accomplish this should receive highest priority. Where resistance is prevalent, the only viable options to reduce selection pressure are withdrawal of the product and/or enforcement of high-dose/refuge requirements. The latter action may however be of no value under conditions where resistance is prevalent, since the value of refugia to an IRM strategy may be compromised. Remedial actions taken in South Africa included the propagation and enforcement of refuge compliance followed by the release of pyramided maize hybrids in 2011. These pyramids combine Cry1A.105 and Cry2Ab2 toxin-producing transgenes, replacing the ineffective single-transgene. However, it remains uncertain if cross-resistance occurs between Cry1A.105/Cry2Ab2 and the closely related Cry1Ab toxin, and for how long this pyramided event will endure. Cultivation of Cry1Ab-expressing hybrids continues in areas where resistance levels have been confirmed to be high. In retrospect, this case provides lessons regarding IRM, not only in South Africa, but wherever Bt crops are being introduced.