Submitted to: Journal of Economic Entomology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/14/2004
Publication Date: 10/5/2004
Citation: Abel, C.A., Adamczyk Jr, J.J. 2004. Expression of cry1ac in maize leaves and cotton bolls with diverse chlorophyll content and corresponding development of fall armyowrm (lepidoptera: noctuidae) & southwestern corn borer (lepidoptera: crambidae) on maize whorl. Journal of Economic Entomology. Interpretive Summary: In the southeastern United States, the fall armyworm and southwestern corn borer cause economic damage to corn and the corn earworm/ bollworm causes economic damage to cotton. Bt varieties are available to growers for control of these and other caterpillar pests. In order to preserve the usefulness of the Bt varieties for insect control, a high level of Bt toxin must be expressed throughout the plant. Factors that cause lower expression of toxin may allow insect pests an opportunity to become resistant to the toxin, thus, forcing farmers to rely on insecticide sprays to control these pests. We discovered the Bt toxin is expressed at low levels in the yellow-green leaf portion of the developing corn whorl. We determined that decreased expression of the toxin in the leaf tissue resulted in improved development by the fall armyworm. Differences in toxin expression did not alter the level of control achieved for the southwestern corn borer. In cotton, Gossypium hirsutum L., the amount of Bt toxin was significantly lower in boll tips where flowers had remained attached compared to normal boll tips. Boll tips where the flowers remained attached are often the site where corn earworms/bollworms begin feeding on Bt cotton bolls. This study demonstrated that, in two diverse plant species, tissue that had low chlorophyll content, i.e. is not fully green in color, did not fully express Bt toxin.
Technical Abstract: In order to manage insect resistance to transgenic crops that express insecticidal proteins from Bacillus thuringiensis Berliner, the U. S. Environmental Protection Agency recommends a strategy whereby plants must express toxin at high levels so that insect resistance is functionally recessive. Factors that cause lower expression of toxin in the plant are a threat to this strategy. We analyzed the relative Cry1Ab expression between white-yellow, yellow-green, and green portions of the developing ninth leaf within the maize, Zea maize L., whorl. The green tissue had the highest expression followed by the yellow-green and white-yellow tissues. Correlations between expression of Cry1Ab and 5 d fall armyworm, Spodoptera frugiperda (J. E. Smith), larval weights among the three leaf color profiles were all significant and negative, i.e. decreased expression of Cry1Ab in the leaf tissue resulted in increased 5 d larval weights. There was 100% mortality to the southwestern corn borer, Diatraea grandiosella (Dyar), larvae fed Cry1Ab expressing maize leaf tissue. Differences in Cry1Ab expression 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 to 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.