The role of ozone-reactive compounds, terpenes, and green leaf volatiles (GLVs), in the orientation of Cotesia plutellae

The emission of inducible volatile organic compounds (VOCs), i.e., inducible terpenes, and green leaf volatiles (GLVs), is a common response of plants to herbivore attack. These VOCs are involved in the orientation of natural enemies, i.e., predators and parasitoids, toward their herbivore prey or hosts (indirect defense of plants). Terpenes and some GLVs are readily oxidized by ozone (O-3), an important oxidant of the low atmosphere and predicted to increase as a result of anthropogenic activity. It has been recently reported that O-3 degradation of terpenes and GLVs does not affect signaling in two selected tritrophic systems. Natural enemies may have learned to use oxidation products that are more stable in nature to locate their prey. To understand the role of these compounds on the tritrophic system Brassica oleracea-Plutella xylostella-Cotesia plutellae, we assessed the preference of wasps to different combinations of cabbage VOCs (intact vs. herbivore-induced and herbivore-induced vs. herbivore-induced VOCs) in the presence or absence of O-3. We found that C. plutellae preferred P. xylostella-damaged plants at 0 and 120 nl l(-1) O-3 to intact plants at 0 nl l(-1) O-3. However, wasps preferred P. xylostella-damaged plants at 0 nl l(-1) to P. xylostella-damaged plants at 120 nl l(-1) O-3. The results suggest that compounds other than terpenes and GLVs are crucial for the orientation of the wasps, but terpenes and GLVs contribute to the behaviorally active VOC blend of herbivore-damaged cabbages by increasing their attraction to them. The products resulting from oxidation of terpenes and GLVs do not seem to play a role in the host location process as speculated previously.