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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Vol. 97 • No. 5