Rhizobia-bean symbiosis enhances root herbivore attraction and growth via volatile signals and nutrition.

The symbiosis between nitrogen-fixing rhizobia and plants is regarded as mutually beneficial, but its indirect effects on other interacting organisms remain underexplored. This study investigates how rhizobia associated with Phaseolus vulgaris influence the behaviour and performance of root-feeding larvae of the beetle Diabrotica balteata . We examined whether larvae prefer nodulated (R + ) over non-nodulated (R - ) bean roots and assessed the impact of rhizobia symbiosis on larval growth. Additionally, we analysed the nutrient content of R + and R - roots and characterized their volatiles organic compounds (VOC) profiles to identify potential chemical cues driving larval feeding preferences. Our findings reveal strong larval preference for R + roots on which larvae also exhibited enhanced growth and better survival compared to R - roots. Nutritional analyses showed that R + roots provided higher nutritional value. Furthermore, VOC profiles differed significantly between rhizobia treatments, and olfactometer assays confirmed that larval attraction is mediated by VOCs, likely signalling the nutritional advantages conferred by rhizobia symbiosis. These findings highlight that metabolic changes in bean roots caused by rhizobia not only increase their nutritional value but also enhance their attractiveness to root herbivores. This study sheds light on the complex belowground interactions among nitrogen-fixing bacteria, host plants, and herbivores, offering new insights with implications for ecological theory and sustainable agricultural practices.