Rag1 soybeans [Glycines max (L.) Merr] were recently introduced into soybean production in the United States to help manage the invasive soybean aphid, Aphis glycines Matsumura. It is not yet fully understood how Rag1 varieties will perform across different elite lines, or the influence the expression of Rag1 will have on the soybean defoliator community. This project studied these issues in two parts. One part studied the performance of several varieties expressing Rag1 in the field across two years, measuring the average aphid exposure throughout the season as well as harvest yield. The second part of this project evaluated how the expression of the Rag1 gene influenced the preference and performance of several species of Coleoptera and Lepidoptera through choice and no-choice testing, as well as evaluating several nutritional indices of immature Lepidoptera when consuming Rag1 leaf tissue.

The field portion of this study evaluated eight unique soybean varieties that included four expressing the Rag1 gene, and four that were considered aphid-susceptible. Treatments included each of these varieties with either an application of insecticide, following current recommendations for management of the soybean aphid, or each of these varieties grown without an application. Evaluations of each treatment were done in terms of cumulative aphid days and harvest yield adjusted to 13% moisture. After two years of this study, we found that incorporating insecticide applications with Rag1 varieties reduced aphid exposure greater than the reduction seen in Rag1 plants left untreated. We also observed that the expression of Rag1 was effective in managing soybean aphid regardless of the genetic background of the variety in which it was expressed. Differences in harvest yield between each variety appeared to be based on difference in the genetic background of each variety, and not due to a source of resistance.

The second part of this project evaluated the preference of eight different insects known to be soybean defoliators: the northern corn rootworm, Diabrotica barberi Smith & Lawrence; the southern corn rootworm, Diabrotica undecimpuncata howardi Barber; bean leaf beetle, Ceratoma trifurcata (Förster); the Japanese beetle, Popillia japonica Newman; fall armyworm, Spodoptera frugiperda (J. E. Smith); corn earworm,Helicoverpa zea (Boddie); soybean looper, Chrysodeix includens (Walker); and the velvetbean caterpillar, Anticarsia gemmatalis Hübner. Members of Coleoptera (D. barberi, D. u. howardi, C. trifurcata, P. japonica) were tested as adults, whereas members of Lepidoptera (S. frugiperda, H. zea, C. includens,A. gemmatalis) were used in immature stages. In the laboratory, each of these insects was offered leaf tissue in choice and no-choice testing Rag1 or aphid-susceptible tissue, and measurements of leaf tissue area consumed were used to determine the preference of each insect. Analysis of choice and no-choice tests showed that preference appear to be different between different species and different varieties of soybeans. We found, most notably, that D. barberi and H. zea showed a preference for resistant tissue of several varieties.

Also, S. frugiperda,H. zea,C. includens and A. gemmatalis were also included in an analysis of nutritional indices when allowed to develop on Rag1 leaf tissue. Third instar larvae of each of these species were allowed to consumed Rag1 or susceptible leaf tissue over the course of seven days. During this time, measurements were taken to determine the indices of relative growth rate, approximate digestibility, and conversion efficiency of ingested and digested materials. S. frugiperda, H. zea, and A. gemmatalis had reduced relative growth rate on Rag1 tissue. Also, each of these insects had reduced conversion of efficiency of ingested materials on Rag1 tissue, possibly signifying an effect of antixenosis. Finally, A. gemmatalis had reduced conversion efficiency of digested materials on Rag1 tissue, indicating an effect of antibiosis suffered by this insect.

The data obtained in this study offers a possible recommendation for the use of Rag1 soybean varieties in an integrated pest management program by incorporating the economic threshold of the soybean aphid with insecticidal application. This study also points out the differences in feeding preferences of soybean defoliators when incorporation Rag1 varieties, and the possibility that the preference appear to be based on variety and species more than resistance. We also show how the expression of Rag1 may have negative effects on developing lepidopteran larvae, leading to a reduction in the number of pest insects through antixenosis and antibiosis.