Reduced foliage herbivory in Bt cotton benefits phloem-feeding insects

Genetically modified cotton plants that express Lepidoptera-active Cry proteins from Bacillus thuringiensis (Bt) are grown on millions of 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 proteins. However, potential indirect effects of Bt cotton have received little attention. We have thus studied the impact of the insecticidal trait on the natural inducible defence mechanisms of cotton. Cotton is defended by a range of terpenoids (e.g., gossypol) that are induced by herbivore damage. We hypothesize that the reduction of damage caused by the elimination of caterpillars in Bt cotton would lead to a lower concentration of cotton terpenoids. Thereby making the plant more vulnerable to attack by other herbivores such as aphids, which do not induce a defence response by the plant. We used the cotton aphid Aphis gossypii and its parasitoid Lysiphlebus testaceipes as model organisms to test if different levels of terpenoid induction in Bt and non-Bt plants have an impact on these non-target organisms.