Floral odors can interfere with the foraging behavior of parasitoids searching for hosts

Authors: DESURMONT, GAYLORD - EUROPEAN BIOLOGICAL CONTROL LABORATORY (EBCL); VON ARX, MARTIN - UNIVERSITY OF ZURICH; TURLINGS, TED - UNIVERSITY OF NEUCHATEL; SHIESTL, FLORIAN - UNIVERSITY OF ZURICH

Submitted to: Frontiers in Ecology and Evolution
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/30/2020
Publication Date: 5/27/2020
Citation: Desurmont, G.A., Von Arx, M., Turlings, T.C., Shiestl, F.P. 2020. Floral odors can interfere with the foraging behavior of parasitoids searching for hosts. Frontiers in Ecology and Evolution. 8:148. https://doi.org/10.3389/fevo.2020.00148.
DOI: https://doi.org/10.3389/fevo.2020.00148

Interpretive Summary: Plants produce odors that can be attractive to different types of beneficial insects. On the one hand, odors produced by flowers attract pollinators that help plants to achieve their reproductive cycle. On the other hand, odors produced by leaves that have been damaged by caterpillars attract natural enemies of the caterpillars (predators or parasitoids) that effectively defend the plant. This study investigates the consequences of the mixture of flower odors and leaf odors of the plant Brassica rapa on natural enemies of caterpillars and aphids. Results showed that flower odors decrease plant attractiveness to natural enemies in general, and that this effect is quantitative: the more flower odors are present, the least attractive the plant becomes to natural enemies. These results have important implications for common pest management techniques, for instance the use of floral strips on the edges of crop field to enhance biological control.

Technical Abstract: Plants produce distinct blends of volatile compounds to attract pollinators (floral odors) or natural enemies of insect herbivores (herbivore-induced plant volatiles). These two types of volatile blends may interact with each other, which may alter the attraction of either type of mutualist and ultimately affect plant fitness. Here, using synthetic blends of Brassica rapa floral volatiles and real B. rapa flowers, we investigated how floral odors impact the foraging behavior of five species of parasitoids. In a 4-arm olfactometer setting, the addition of floral odors to plants infested by herbivores reduced plant attractiveness to parasitoids by 43.5%, a decrease that was significant for four out of five parasitoid species. Additionally, experiments focusing on the parasitoid Cotesia glomerata revealed that the effects of floral odors are dose-dependent and start affecting the behavior of parasitoids past a certain quantitative threshold. This threshold depended on the experimental setup: parasitoids were less sensitive to floral odors under wind tunnel conditions than under olfactometer conditions. Electro antennogram bioassays showed that C. glomerata perceive and respond to floral compounds, but that floral compounds do not inhibit antennal responses to herbivore-induced leaf volatiles. In conclusion, our study shows that floral odors can have a detrimental effect on the attractiveness of chemical blends used by natural enemies to locate plants attacked by herbivores. Under natural conditions, such interferences could affect the outcome of tritrophic interactions between plants, herbivores, and natural enemies, and may play an important role in the evolution of plant volatile signals.