Update: Plant Health News (21 May 14)

Scientists have discovered the mechanism for pink bollworm resistance to Bt cotton (David Nance, USDA)
Scientists have discovered the mechanism for pink bollworm resistance to Bt cotton (David Nance, USDA)

Here’s a taste of some of the latest stories about plant health, including the discovery of how pests develop resistance to biotech cotton, the intensification of the battle against coffee rust, and a biodiversity assessment of over 10,000 Ghanaian cocoa farms.

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New strategy required for delaying insect resistance to Bt crops

Kenyan farmer Mary Ngare in her maize field damaged by stem borers © CIMMYT (CC BY-NC-SA)
Kenyan farmer Mary Ngare in her maize field damaged by stem borers © CIMMYT (CC BY-NC-SA)

Transgenic Bt crops have been grown around the world since the 1990s and have contributed to increased yields by controlling agricultural pests. Due to the importance of this technology, there has been continuous study into the development of resistance to Bt crops and how best to avoid this happening. A recent investigation into the rapid spread of Bt resistance in South Africa has revealed one of the more surprising discoveries to date, that the maize stalk borer (Busseola fusca) has evolved Bt maize resistance inherited as a dominant trait for the first time. This has significant impacts on the management of Bt crops, as current methods for sustaining susceptibility rely on the recessive inheritance of Bt resistance.

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The Evolution of Insect Resistance to Bt Crops

A group of scientists at the University of Arizona have this week published a paper in Nature Biotechnology on the evolution of resistance in insect pests populations to insecticidal proteins from Bacillus thuringiensis (Bt) that are produced by transgenic crops. Resistance is defined as the phenotype of an individual that gives the individual the ability to survive on a transgenic insecticidal plant from egg to adult and provide viable offspring. The team analysed field and laboratory data from seventy-seven studies of thirteen pest species in eighteen countries across five continents. Entomologist Bruce Tabashnik and colleagues found well documented cases of field-evolved resistance to Bt crops in five major pests as of 2010. 60% of these cases occurred in the U.S.A, where approximately half of the world’s Bt crop acreage is planted. In some cases, resistance to Bt evolved within as little as two to three years, whilst in other cases Bt crops have remained effective for more than 15 years. The research team aimed to better understand how quickly insect populations are evolving resistance to Bt crops and how this is occurring.

Workshop participants assess a range of fodder and cereal crops that can be used as “refugia”, fostering stem borers susceptible to the Bt toxin. In a longstanding partnership under the Insect Resistant Maize for Africa (IRMA) project , CIMMYT works with the Kenya Agricultural Research Institute (KARI) to offer farmers maize varieties that resist borers, which otherwise cause heavy losses (approximately 12% of Kenya’s annual maize crop). In addition to conventional breeding, one source of resistance in developing these varieties has been the soil bacterium Bacillus thuringiensis. A gene from this bacterium inserted into “Bt maize” causes it to produce a protein that is selectively toxic to certain borer species. However, mutant resistant borers unaffected by the toxin will flourish and eventually predominate, unless farmers use refugia to maintain a susceptible population. At this workshop in December 2005, sponsored by IRMA at KARI’s Kitale center, 50 participants—including researchers, extension workers, and farmers—learned about progress in the development of insect-resistant maize and the importance of refugia, evaluating numerous crops in the field for their potential as refugia. For more information, see CIMMYT's December 2005 e-news story "Bug Havens Keep Maize Pest-Proof," available online at: http://www.cimmyt.org/newsletter/86-2005/344-bug-havens-keep-maize-pest-proof.  Image © CIMMYT (CC-BY-NC-SA 2.0)
Women assess a range of fodder and cereal crops that can be used as “refugia for stem borers susceptible to the Bt toxin. In a longstanding partnership under the Insect Resistant Maize for Africa (IRMA) project , CIMMYT works with the Kenya Agricultural Research Institute (KARI) to offer farmers maize varieties that resist borers, which otherwise cause heavy losses (approximately 12% of Kenya’s annual maize crop). In addition to conventional breeding, one source of resistance in developing these varieties has been the soil bacterium Bacillus thuringiensis. A gene from this bacterium inserted into “Bt maize” causes it to produce a protein that is selectively toxic to certain borer species. However, resistant borers unaffected by the toxin will reproduce and eventually predominate, unless farmers use refugia to maintain a susceptible population. 
Image © CIMMYT (CC-BY-NC-SA 2.0)

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