The recent research from India's Central Institute of Cotton Research showing the Bt protein in Gm cotton is not always enough to kill insects has been presented in some quarters as a problem that is particular to Bt cotton in India
But, in fact, the Indian research on Bt cotton isn't the only one to show these Bt expression in cotton boll problems - it is confirmed by a recent Chinese study and a recent US study.
The US study even suggests a mechanism (poor expression of the Bt toxin in tissue with low levels of chlorophyll) which indicates a systemic problem with Bt cotton.
All of which suggests this problem is not just something limited to Indian varieties or to growing conditions in India.
The study abstracts are given below.
J Econ Entomol. 2005 Feb;98(1):195-201.
Seasonal expression profiles of insecticidal protein and control efficacy against Helicoverpa armigera for Bt cotton in the Yangtze River valley of China.
Wan P, Zhang Y, Wu K, Huang M.
Institute of Plant Protection, Chinese Academy of Agricultural Sciences, Beijing 100094, China.
Seasonal levels of Bacillus thuringiensis (Bt) insecticidal protein and its control efficacy against Helicoverpa armigera (Hubner) in Bt transgenic cotton GK19 (carrying a Cry1Ac/Cry1Ab fused gene) and BG1560 (carrying a Cry1Ac gene) were investigated in Tianmen County, Hubei Province, located in the Yangtze River valley of China, in 2001 and 2002. The results showed that the toxin content in Bt cotton changed significantly over time, and that the structure, growth stage, and variety were significant sources of variability. Generally, insecticidal protein levels were high during the early stages of cotton growth; they declined in mid-season, and rebounded in late season. On most dates sampled, the toxin contents in leaf, square, petal, and stamens (including nonovule pistil tissue) were much higher than those in ovule and boll. Compared with BG1560, the expression of Cry1Ac/Cry1Ab protein in GK19 was more variable during the whole growth period of cotton. The field evaluation on larval population dynamics of H. armigera in Bt and conventional cotton showed that the larval densities in BG1560 and GK19 fields decreased, respectively, 92.04 and 81.85% in 2001, and 96.84 and 91.80% in 2002.
J Econ Entomol. 2004 Oct;97(5):1737-44
Relative concentration of Cry1A in maize leaves and cotton bolls with diverse chlorophyll content and corresponding larval development of fall armyworm (Lepidoptera: Noctuidae) and southwestern corn borer (Lepidoptera: Crambidae) on maize whorl leaf profiles.
Abel CA, Adamczyk JJ Jr.
USDA-ARS Southern Insect Management Research Unit, P.O. Box 346, Stoneville, MS 38776, USA.
To manage insect resistance to transgenic crops that express insecticidal proteins from Bacillus thuringiensis (Bt) Berliner, the U.S. Environmental Protection Agency recommends a refuge-based insect resistance management strategy where a percentage of non-Bt (refuge) crop is grown in proximity to a Bt-expressing crop. An important requirement for this strategy is that the toxin exists at a high effective dose for control of the target pest(s), so that heterozygous individuals in the population do not reach adulthood. Factors that cause reduced levels of toxin in the plant are a threat to this strategy. We quantified Cry1Ab from different areas of the maize, Zea mays L., leaf. In general, the distal tip of the V7 maize leaf had a higher concentration of Cry1Ab compared with the middle section of the V7 leaf, and the middle section of the developing V9 leaf had the lowest concentration of Cry1Ab. When these sections of maize tissue were fed to fall armyworm, Spodoptera frugiperda (J.E. Smith), and southwestern corn borer, Diatraea grandiosella Dyar, there was not a reduction in development or an increase in mortality with tissue that had higher concentrations of toxin.
Another study tested the relative concentration of Cry1Ab between the white-yellow, yellow-green, and green portions of the developing ninth leaf within the maize whorl. There were differences in Cry1Ab concentration among these leaf areas. The green tissue had the highest concentration of toxin followed by the yellow-green and white-yellow tissues. Correlations between concentration of Cry1Ab and 5-d fall armyworm larval weights among the three leaf color profiles were all significant and negative, i.e., decreased concentration of Cry1Ab in the leaf tissue resulted in increased 5-d larval weights. There was 100% mortality to the southwestern corn borer larvae fed Cry1Ab maize leaf tissue. Differences in the amount of Cry1Ab in the developing V9 leaf profiles did not alter the absolute susceptibility of the southwestern corn borer to the toxin. In cotton, Gossypium hirsutum L., the amount of Cry1Ac was significantly lower in boll tips where flowers had remained attached compared with normal boll tips. Boll tips where the flowers remained attached are often the site where corn earworms, Helicoverpa zea (Boddie), penetrate Bt cotton bolls. This study demonstrated that, in two diverse plant species, tissue that has low chlorophyll content does not fully express Cry1A. Photosynthesis regulating factors related to mRNA transcription and translation should be studied for their effect on Cry1A production and insect control.
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