Impact of nitrogen and a soil inoculant on soybean aphid density (Aphis glycines)

Soybean aphids (Aphis glycines) are exotic pests that arrived in the United States from Asia. Since their arrival, these pests have caused major economic damage to soybean (Glycine max) crops. Researchers have determined that bottom-up factors (soil nutrients and bacteria associated with soybean root nodules) can impact soybean aphid densities. However, there is little information on how specific factors impact pest densities, namely the use of nitrogen (N) fertilizer and N-fixing bacteria (Bradyrhizobium japonicum). Inoculation with B. japonicum can increase N-fixation and plant growth and affect the identity and quantity of N compounds within the phloem. However, N fertilizer applications may impact these processes, as high levels of fertilization decrease nodulation. Adding N fertilizer has also been shown to increase aphid body weight and growth rate, although the converse can occur when fertilizer rates are too high. We investigated interactive effects of N fertilizer and a B. japonicum inoculant on soybean aphid densities in greenhouse experiments using a 4x2 factorial design with four levels of nitrogen and two levels of rhizobia (present/absent). Aphids were added at the V1 stage (first trifoliate) and their densities were measured twice a week. At the conclusion of the experiment nitrate, ureide N (product of N-fixation) and total N were analyzed along with root nodule counts, nodule weight, aboveground biomass and plant height. We will discuss how farming practices (fertilization, addition of commercial inoculants) may impact soybean aphid densities in the field.