Bt maize and cotton fall victim to armyworms and bollworms

Pest resistance to the Bt toxins in GM Bt crops has ballooned in Brazil (item 1 below) and India (item 2), according to two studies from those countries. Many reviews hyping the supposed on-farm benefits of GM Bt crops use data that pre-date these resistance problems.

In a study that GMWatch first reported on last year, the Brazilian researchers voice the desperation induced by the mass failure of GM Bt technology, concluding, “There is an urgent need to revise and implement more effective insect resistance management measures to extend the durability” of Bt crops for controlling the fall armyworm, a major maize pest in Brazil.

The researchers suggest that multiple Bt toxin traits should be stacked or “pyramided” into crops in an attempt to delay resistance. But research from the University of Arizona has already shown that this approach doesn’t work well, for two reasons:

i) Survival of both Bt toxin-susceptible pests and pests that are resistant to one of the toxins in the pyramid is higher than expected

ii) Exposure to one Bt toxin causes insect pests to become resistant to other kinds of Bt toxin also. This phenomenon is called cross-resistance.

The study carried out in India examines the problem of pink bollworm resistance to Bt cotton. The researchers found Bt-resistant bollworms in both Bt and non-Bt cotton fields. That implies that planting refuges of non-Bt cotton may not be an effective strategy against resistance, albeit the researchers recommend it.

It’s interesting how researchers go to great lengths to come up with novel solutions to the problem of pest resistance to Bt crops to try to extend the life of this GM technology – which solutions, however, have already failed elsewhere. Sooner or later they will have to admit that exposing pests to pesticides 24/7 is simply a recipe for evolved resistance.

1. Geographical and temporal variability in susceptibility to Cry1F toxin from Bacillus thuringiensis in Spodoptera frugiperda (Lepidoptera: Noctuidae) populations in Brazil
2. Analysis of resistance to Cry1Ac in field-collected pink bollworm, Pectinophora gossypiella (Lepidoptera:Gelechiidae), populations
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1. Geographical and temporal variability in susceptibility to Cry1F toxin from Bacillus thuringiensis in Spodoptera frugiperda (Lepidoptera: Noctuidae) populations in Brazil

Juliano R. Farias, Renato J. Horikoshi, Antonio C. Santos, and Celso Omoto
Journal of Economic Entomology Dec 2014 : Vol. 107, Issue 6, pg(s) 2182-2189 doi: 10.1603/EC14190
http://jee.oxfordjournals.org/content/jee/107/6/2182.full.pdf (open access)

Abstract

The genetically modifed maize TC1507 event with the cry1F gene (Cry1F maize) has been used to control Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae) in Brazil since the 2009-2010 cropping season. As part of the insect resistance management program, we conducted studies to determine the baseline susceptibility to Cry1F before the widespread planting of Cry1F maize. Subsequently, we evaluated the geographical and temporal variability of susceptibility to this toxin in populations of S.frugiperda collected from major maize-growing regions in Brazil. The baseline susceptibility to Cry1F was determined using a diet-overlay bioassay for a susceptible reference population and four field populations of S. frugiperda. We then monitored the susceptibility to Cry1F in 43 populations of S. frugiperda sampled in nine States of Brazil between 2011 and 2013. In the baseline study, the MIC50 (the concentration that inhibits molting to second instars in 50% of individuals) ranged from 3.59 to 72.47 ng Cry1F toxin per centimeter square. Based on the upper limit of the MIC99 value of the joint analysis from the baseline susceptibility data, the concentrations of 200 and 2,000 ng of Cry1F toxin per centimeter square were defined as diagnostic concentrations for potentially resistant individuals, and these were used to monitor the susceptibility of S. frugiperda to Cry1F. Survival at 2,000 ng Cry1F toxin per centimeter square increased significantly throughout the cropping seasons in S. frugiperda populations from Sao Paulo, Santa Catarina, Rio Grande do Sul, Bahia, Mato Grosso, Goias, Mato Grosso do Sul, and Parana. The highest survival (50%) was reached in populations collected from Bahia, Mato Grosso, Goias, Mato Grosso do Sul, and Parana during the 2012-2013 cropping season. Therefore, a significant decrease in susceptibility to Cry1F was detected in S. frugiperda throughout cropping seasons, especially in regions with intensive maize production in Brazil, according to field-evolved resistance that has been demonstrated in Brazilian populations.
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2. Analysis of resistance to Cry1Ac in field-collected pink bollworm, Pectinophora gossypiella (Lepidoptera:Gelechiidae), populations

Ojha A, Sree KS, Sachdev B, Rashmi M, Ravi K, Suresh P, Mohan KS, Bhatnagar RK.
GM Crops Food. 2014 Oct 2;5(4):280-6. doi: 10.4161/21645698.2014.947800.
http://www.ncbi.nlm.nih.gov/pubmed/25523173

Abstract

High survivorship of pink bollworm, Pectinophora gossypiella in bolls of Bollgard® cotton hybrids and resistance to Cry1Ac protein, expressed in Bollgard cotton were reported in field-populations collected from the state of Gujarat (western India) in 2010. We have found Cry1Ac-resistance in pink bollworm populations sourced from Bollgard and non-Bt cotton fields in the adjoining states of Maharashtra and Madhya Pradesh in Central India. Further, we observed reduced binding of labeled Cry1Ac protein to receptors localized on the brush-border membrane of pink bollworm larval strains with high tolerance to Cry1Ac. These strains were sourced from Bollgard and conventional cotton fields. A pooled Cry1Ac-resistant strain, further selected on Cry1Ac diet also showed significantly reduced binding to Cry1Ac protein. The reduced binding of Cry1Ac to receptors could be an underlying mechanism for the observed resistance in pink bollworm populations feeding on Bollgard hybrids.