Advances in soybean breeding have produced varieties with single, double and triple gene combinations of soybean aphid host plant resistance. These naturally-occurring genes dramatically suppress aphid populations and will reduce the reliance of insecticides in soybean. Farmers using aphid-resistance genes will save on input costs and minimize negative effects to pollinators and beneficial insects associated with insecticides.
To address both the short-term and long-term goals of soybean aphid management, our proposed project combines growth chambers, field cages and on-farm research in a collaboration that includes both soybean breeders, farmers, a non-profit organization and commodity organization. We propose a progressive and complementary process for screening new aphid-resistance genes against soybean aphid biotypes that have been discovered in the United States.
Although soybean aphid can be patchy within fields and between growing seasons, it is considered the most economically important Iowa soybean insect pest since 2000. Soybean aphid can significantly reduce yield, and therefore regular scouting and timely foliar insecticides have been the primary way to protect yield. Advances in soybean breeding have produced varieties with single, double and triple gene combinations of host plant resistance for soybean aphid. These naturally-occurring genes dramatically suppress aphid populations and will reduce the reliance of insecticides in soybean. Farmers using aphid-resistance genes will save on input costs and minimize negative effects to pollinators and beneficial insects associated with insecticides. Documented pyrethroid resistance in some Midwestern states also stresses the importance of host plant resistance as a management tool.
1) We completed Objective 1 in 2016 (Year 1). Our results confirm that the Rag1+2+3 pyramid effectively manages all known soybean aphid biotypes. Our results indicate that Rag1+2+4 would be an effective management option for biotype-1, biotype-2, and biotype-3 soybean aphid, but had a negligible impact on biotype-4.
2) We are continuing to work on Objective 2 since 2016 (Years 1-3). We have been using seed from increases developed by Brian Diers (University of Illinois), including access to several gene pyramids.
3) We also completed Objective 3a in 2016 (Year 1). Soybean aphid responded to resistant gene combinations in more realistic growing conditions similar to the laboratory testing.
4) We completed the first year of Objective 3b in 2017 (Years 2-3). We worked with three commercial soybean farmers in northern Iowa and incorporated strip trials of host plant resistance. Aphid pressure was low, but overall, host plant resistance was an effective tool compared to susceptible soybean. We will continue a second year of on-farm strip trials in 2018.
Advances in soybean breeding have produced varieties with single, double and triple gene combinations of soybean aphid host plant resistance. These naturally-occurring genes dramatically suppress aphid populations and will reduce the reliance of insecticides in soybean. Our hypothesis is fields with aphid-resistance genes will have fewer aphids than aphid-susceptible genes. Ultimately, farmers will save on input costs and minimize negative effects to pollinators and beneficial insects associated with insecticides.
Objective 1. Screen new aphid-resistant gene combinations. Biotype survival on new Rag gene combinations requires laboratory testing. We maintain four soybean aphid biotypes at Iowa State University and have sufficient growth chambers to test these biotypes on aphid-resistant varieties. Replicates of each variety will be artificially infested in the vegetative stage (V2-3) with 5 aphids and their population growth measured over 14 days. We predict that the Rag 1+2+3 will provide protection from all the currently identified biotypes.
Objective 2. Seed increase for the strongest aphid-resistant gene combinations. Field-scale evaluation outlined in Objective 3 will require a seed increase. In Years 1 and 2, soybean breeders Brian Diers (University of Illinois) and Asheesh Singh (Iowa State University) will generate seed increases of the best-performing Rag varieties. This could include double and triple pyramided gene combinations, within a non-GMO background. Varieties selected for the seed increase will depend on the results of Objective 1. Those varieties that performed as well as Rag1+2 will be included, along with Rag1+2. In this way, even if the new varieties do not perform as well, we will have sufficient supply of the Rag1+2 pyramid to include in our on-farm testing (Objective 3).
Objective 3. On-farm testing of aphid-resistant gene combinations. Working with both the Practical Farmers of Iowa (PFI) and the Iowa Soybean Association (ISA), we will identify six farmers to collaborate with us to test the varieties selected from our first two objectives. The goal of this objective is not only to measure the performance of aphid-resistant soybeans in an on-farm setting, but also to demonstrate this value to farmers and members of both organizations.
We will conduct listening sessions with our farmer collaborators leading up to 2017 and 2018 field seasons to gain a better understanding of their knowledge for soybean aphid management and learn more about their farming practices. During these meetings, we will review the objectives, describe the experimental design, and confirm everyone’s responsibilities. Our goal is to include three treatments with replications at each farm, including a 1) “best bet” Rag gene combination from Objective 1, 2) aphid-susceptible variety that is genetically similar to the best bet, and 3) the farmer’s standard variety.
Objective 3a. Small cage studies. The performance of these new aphid-resistance varieties cannot be measured solely in a growth chamber. In Years 2 and 3, we plan to use caged plants to evaluate aphid performance (Figure 1f). Soybean aphid infestations can be highly variable in both time and place, and therefore, using caged plants will ensure that data can be collected every growing season. We have used this approach to demonstrate the value of the Rag1+2 over Rag1 or Rag2 varieties (Wiarda et al. 2012). This approach includes growing plants in small plots, caging ten plants with mesh around a PVC frame, and artificially infesting caged plants with field-collected aphids. By enclosing the plants, aphids cannot move away or become food for predatory insects; therefore, aphid populations increase quickly and treatment comparisons can be made more easily. At three locations, we will include four Rag gene combinations, or treatments, replicated four times (total of 16 cages at each location). The biotype(s) within Iowa likely vary by location, so will we attempt to spread out locations throughout the northern part of the state. At beginning bloom, each plant within the cages will be artificially infested. We will measure aphid abundance and yield.
Objective 3b. Strip trials. In Year 3, we will expand our on-farm testing for aphid-resistant genes and continue working with the farmers used for Objective 3a. Farmers will plant long strips (>300 feet) of the top two Rag gene combinations and a susceptible variety replicated four times (total of 12 strips at each location). We will measure aphid abundance over the entire growing season and possible interactions with other soybean pests, note natural enemy activity, and yield.
Objective 3c. Small plot efficacy evaluations. In Year 3, we will include the top two Rag gene combinations in the Soybean Aphid Efficacy Evaluation (http://www.ent.iastate.edu/soybeanresearch/content/extension) directed by Erin Hodgson. The performance of aphid-resistant genes will be compared to insecticidal seed treatments and foliar insecticides at two locations. We will measure aphid abundance over the entire growing season and possible interactions with other soybean pests, note natural enemy activity, and yield.
A journal manuscript summarizing Objective 1 was prepared and submitted to the Journal of Economic Entomology in March 2017. Erin Hodgson also was actively presenting and writing about host plant resistance for soybean aphid to farmers and people working in agriculture (2 Extension proceedings, 2 Extension newsletter articles, 15 Extension presentations, and 1 Extension Videos). Preliminary results from Objective 3a indicate aphid-resistant soybean suppressed the aphid population.
We completed Objective 1 in 2016. Our results confirm that the Rag1+2+3 pyramid effectively manages all known soybean aphid biotypes. Our results indicate that Rag1+2+4 would be an effective management option for biotype-1, biotype-2, and biotype-3 soybean aphid, but had a negligible impact on biotype-4. We also completed the first year of work for Objective 3a in 2016. Soybean aphid responded to resistant gene combinations similar to the laboratory testing. A second summer of work in 2017 will confirm our findings.
Educational & Outreach Activities
During 2016 and 2017, I conducted a number of extension events with a focus on soybean aphid management. Some events were lecture style and some were field days, but most participants were farmers. I also wrote a few publications that talk about soybean aphid management. I am currently working on a regional field guide for soybean aphid management.