Submitted to: International Symposium on the Biosafety of Genetically Modified Organisms
Publication Type: Abstract only
Publication Acceptance Date: 8/1/2010
Publication Date: 11/20/2010
Citation: Hagenbucher, S., Olson, D.M., Ruberson, J., Wackers, F., Romeis, J. 2010. Reduced foliage herbivory in Bt cotton benefits phloem-feeding insects. International Symposium on the Biosafety of Genetically Modified Organisms. Buenas Aires, Argentina on November 20, 2010. Interpretive Summary:
Technical Abstract: Genetically engineered cotton plants that express Lepidoptera-active Cry toxins from Bacillus thuringiensis (Bt) are grown on 15 millions hectares worldwide. Numerous studies have established that these plants pose a negligible risk to non-target arthropods due to the narrow spectrum of activity of the expressed Cry toxins. A potential indirect effect of Bt cotton that has received little attention is the interactions between the introduced insecticidal trait and the natural insect defence system of the cotton plant. An integral part of the cotton defence system is a group of closely related terpenoids: gossypol, hemigossypolone and four heliocides. Those terpenoids are efficient toxins against a broad range of pest organisms and cotton varieties with low terpenoid levels are highly susceptible to insect attack. Terpenoid production in cotton is induced by insect damage. The defence induction, however, is not equally distributed over the entire plant. Cotton follows the optimal defence hypotheses by protecting valuable plants parts, like young leaves, better then less valuable parts such as old mature leaves. Interestingly, not all insects can cause the terpenoid induction in cotton. While most caterpillars with chewing mouthparts induce the cotton defence system, phloem-feeding insects like aphids and whiteflies do not. In the field, cotton plants are usually under the attack by a broad range of insect pests and there is evidence that phloem-feeding herbivores are to some extent controlled by the terpenoids produced in response to caterpillar attack. We have thus hypothesized that the reduced damage caused by caterpillars to Bt cotton would lead to a lower concentrations of cotton terpenoids and subsequently would benefit other herbivores. We tested this hypothesis by monitoring the population dynamics of cotton aphids (Aphis gossypii Glover) on Lepidoptera-damaged, mechanically damaged and undamaged Bt- and non-Bt cotton plants under greenhouse conditions and in a field experiment conducted at the USDA/ARS research station in Tifton (Georgia, USA). As study plants, we used a Bollgard II© variety expressing Cry1Ac, Cry2Ab and an herbicide-resistance trait. As control we used the nearest non-Bt expressing isoline containing only the herbicide-resistance trait. One week after the respective treatment of the study plants, mixed stages of A. gossypii were released on the youngest fully-developed leaf of the plants. Aphid abundance was then recorded after two weeks. As hypothesized, aphids performed better on Bt cotton plants that were less damaged by caterpillars compared to the non-Bt control plants. Similar differences in the aphid populations were observed in the field experiment. Samples of plant material will be analysed for their terpenoid content using HPLC to confirm that the observed differences in aphid populations were caused by differences in the defence induction.