Reduced foliage herbivory in Bt cotton benefits phloem-feeding insects

Genetically modified 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. However, potential indirect effects of Bt cotton have received little attention. We have thus studied the natural inducible defence mechanisms of cotton, specifically the induction of plant terpenoids, and whether they are affected by the introduced insecticidal trait. We hypothesize that the reduced damage caused by caterpillars in Bt cotton would lead to a lower concentration of cotton terpenoids. This could leave the plant vulnerable to attack by other herbivores such as aphids, which do not induce a defence response by the plant. We tested this hypothesis by monitoring the population dynamics of cotton aphids (Aphis gossypii Glover) on Lepidoptera-damaged and undamaged Bt - and non-Bt cotton plants in the greenhouse and in a field experiment. As hypothesized, aphids performed better on Bt cotton that were less damaged by caterpillars compared to the non-transgenic control plants.