A comparative field study of commercially-available rhizobial inoculants on soybean aphid density (Aphis glycines)

Commercial soybean production often involves the use of rhizobial inoculants to decrease the cost of nitrogen (N) fertilizer and increase plant health and yield. These commercial inoculants typically contain a mixture of Bradyrhizobium japonicum and other molecules or organisms, such as lipo-chitooligosaccharides, Azospirillum brasilense, and/or Delfita acidovorans. Bradyrhizobium japonicum is a N-fixing bacteria symbiotically associated with soybean roots that fixes atmospheric nitrogen for the plant. The other chemical and biotic components in the inoculant affect plant physiology in various ways, including altering nodulation, N-fixation, and/or plant growth. Thus, specific inoculants may differentially affect host plant quality for herbivorous insects. To investigate the impact of commercial inoculants on soybean aphid densities we conducted a two year field study using four commercial inoculants (N-Dure, Optimize 400, Primo and BioBoost Plus), a non-inoculated control, and a high N control intended to suppress nodulation. Each treatment was replicated six times. Two cages (2 x 2 x 4 ft) were erected in each plot (one with aphids and one without aphids) to assess how treatments impacted plant parameters independent of aphid presence. Aphid cages were infested with five aphids, their densities quantified after 24 hrs, and then aphid populations were monitored weekly for nine weeks. Plant parameters were also assessed, including: height, number of pods, growth stage, above-ground biomass and N-content, nodule weight, and number of nodules. Implications of inoculant use on soybean aphid management will be discussed.