Degree Type


Date of Award


Degree Name

Master of Science



First Advisor

Walter R. Fehr


The soybean aphid (Aphis glycines Matsumura) can reduce the yield of soybean [Glycine max (L.) Merr.] cultivars that are susceptible to the insect. Two genes that confer resistance to some biotypes are Rag1 (R1) from PI548663 and Rag2 (R2) from PI200538. These genes individually can limit the development of soybean aphids and prevent yield loss. The impact of these genes combined is not known. One objective of my research was to determine the impact of the Rag1 and Rag2 genes on the performance of lines for agronomic and seed traits. A second objective of my research was to compare the growth of aphid populations on soybean lines with both resistance genes (R1/R2), Rag1 alone (R1/S2), Rag2 alone (S1/R2), and neither resistance gene (S1/S2). The four types of lines were selected from the same backcross population to achieve a similar genetic background for traits other than the aphid resistance genes. In Experiment 1, the lines were grown at three locations in 2010 to evaluate their agronomic and seed traits. In this experiment, the mean seed yields of the S1/S2 lines yielded 17.4% more than the R1/R2 lines, 10.8% more than the R1/S2 lines, and 8.8% more than the S1/R2 lines. Even though the mean yields of the resistant types were significantly less than the susceptible type, high yielding lines of R1/R2, R1/S2, and S1/R2 lines were identified.

For Experiment 2, a bulk of lines was made for each of the four genotypes. The lines were grown at a single location in 2010 to evaluate their agronomic and seed traits. The mean seed yield of the S1/S2 line was 2.5% more than the R1/R2 line, 19% less than the R1/S2 line, and 11.8% more than the S1/R2 line.

In Experiment 3, a bulk of lines for each genotype was grown in netted cages and artificially infested with soybean aphids to achieve five treatment levels: aphid-free, 675 aphids per plant (AP), 25,000 cumulative aphid days (CAD), 50,000 CAD, and 75,000 CAD. The intrinsic rate of growth of soybean aphids from after the initial infestation until the population reached 675 AP or reached a plateau was significantly different for the S1/S2 line than the R1/R2, R1/S2, or S1/R2 lines. The R1/R2, R1/S2, and S1/R2 lines were not significantly different during this same time. From after the second infestation until the end of the season, the intrinsic rate of growth of soybean aphids on R1/R2, R1/S2, and S1/R2 lines were still not significantly different. The yield reduction of the S1/S2 lines was 2.1% at 675 AP, 18.8% at 25K CAD, and 26.9% at 50K CAD. Aphid development on the R1/S2 and S1/R2 lines after two infestations reached a maximum of 25K CAD at the same time. The yield of the R1/S2 line increased by 5.8% at 675 AP and decreased by 2.3% at 25K CAD compared to a decrease of 9.9% at 675 AP and a decrease of 11.8% at 25K CAD for the S1/R2 line. The maximum treatment level reached for the R1/R2 line was only 675 AP, at which there was no significant yield reduction. The results indicated that for the biotype used in the study, cultivars with R1/R2 would be expected to have less aphid development and less yield reduction than R1/S2 or S1/R2 lines.

Copyright Owner

Shaylyn Lena Wiarda



Date Available


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88 pages