Impact of nitrogen fixing and plant growth-promoting bacteria on a phloem-feeding soybean herbivore

• Aphid presence negatively affected plant growth parameters.
• Identity of rhizobial seed inoculants can impact soybean aphid densities.
• Aphid densities were negatively related to number of soybean root nodules per plant.

In soybean (Glycine max L.) production systems, growers often inoculate seeds with the symbiotic N-fixing species Bradyrhizobium japonicum along with other bacterial species or chemicals intended to enhance plant growth and yield. However, microbes associated with plant roots can also impact the biology of above-ground insect herbivores through influencing various aspects of plant physiology, with effects dependent on the bacterial species. Because rhizobial seed inoculants can potentially affect densities of soybean herbivores, we investigated the performance of phloem-feeding soybean aphids (Aphis glycines Matsumura) on: (1) soybeans receiving one of four commercially available rhizobial seed inoculants, (2) non-inoculated soybeans associated with existing soil bacteria, or (3) non-inoculated soybeans receiving high levels of fertilizer to suppress N-fixation while still providing adequate N as nitrate. We quantified effects of inoculants and aphid presence on parameters associated with plant growth, N-fixation, and foliar N levels, and explored relationships between aphid densities and these plant parameters. Regardless of inoculation treatment, aphid presence negatively affected plant biomass, pod density, and total N concentration in aerial plant tissues, although, effects on the concentration of ureide N (primary products of N-fixation) were not significant. Inoculant identity significantly impacted aphid populations in both short- (1 week) and longer-term experiments (2 months), with pest densities negatively related to the number of root nodules per plant. This research indicates that nodulation status of soybeans can influence above-ground herbivores.