Rhizobia–Bean Symbiosis Increases Root Herbivore Attraction and Growth via Volatile Signals and Enhanced Nutrition

The symbiosis between nitrogen-fixing rhizobia and plants is considered mutually beneficial, yet its indirect effects on other organisms remain understudied. We examined how rhizobia symbiosis in Phaseolus vulgaris influences the behaviour and performance of Diabrotica balteata larvae. Specifically, we tested larval preference for nodulated (R⁺) vs. non-nodulated (R^-) roots and assessed the impact on larval growth. We also analysed root nutrient content and volatile organic compounds (VOCs) to identify potential chemical cues driving feeding preferences. Larvae strongly preferred R⁺ roots, where they exhibited enhanced growth and higher survival post-metamorphosis. Nutritional analysis revealed that R⁺ roots had greater nutrient content, supporting improved larval performance. VOC profiles differed significantly between treatments, and olfactometer assays confirmed that larval attraction was mediated by VOCs, likely signalling enhanced nutritional benefits from rhizobia symbiosis. Our results demonstrate that rhizobia-induced metabolic changes in bean roots make them more attractive and nutritious to herbivorous larvae. This highlights a complex belowground interaction between nitrogen-fixing bacteria, host plants and herbivores, with potential implications for ecological theory and sustainable agriculture. Understanding these interactions could inform pest management strategies and improve legume cultivation by balancing plant-microbe mutualisms with herbivore dynamics.