Protecting Soybeans from Aphids, as Easy as (Rag) 1, 2, and 3

View the project final report

• 2016 annual report

Commodities

• Agronomic: soybeans

Practices

• Education and Training: demonstration, extension, on-farm/ranch research
• Pest Management: economic threshold, field monitoring/scouting, genetic resistance, integrated pest management

Host plant resistance to soybean aphid was discovered shortly after the confirmation of this pest in the U.S. in 2000. Varieties with aphid resistance from a naturally-occurring gene (Rag1) were commercially available in 2010, and offered moderate resistance and yield protection. The first resistant variety with a two-gene pyramid (Rag1+2) was recently developed and dramatically suppresses soybean aphid populations throughout the growing season. The Rag pyramid had the same yield as an aphid susceptible variety that was kept free of aphids with foliar and seed applied insecticides. In other words, the Rag pyramid eliminates the need for foliar insecticides in soybean. Despite this exceptional performance, the Rag pyramid was never completely free of aphids. Insect populations capable of overcoming resistance genes are described as virulent biotypes. Some soybean aphids survive on the Rag1+2 pyramid and are identified as biotype 4. Going forward, long term use of the Rag1+2 pyramid may be challenged if this virulent biotype 4 dominates the soybean aphid population in the north central region. This research proposal will determine the effectiveness of high-yielding soybean varieties with aphid resistant genes, including: Rag1, Rag2, Rag3, Rag1+2, Rag1+3, Rag2+3, and Rag1+2+3. Varieties will include non-herbicide tolerant backgrounds for conventional and non-GMO farmers to use. The newest gene, Rag3, has yet to be evaluated within Iowa. In addition, we do not know if Rag3 alone or in combination with other Rag genes can provide protection from biotype 4. Regardless of how these novel varieties perform, we are excited to share the Rag1+2 pyramid with farmers who struggle to manage soybean aphid. During 2014-2015, we shared our results with farmers at several extension events, and all responded positively. By partnering with Practical Farmers of Iowa and the Iowa Soybean Association, we identified farmers eager to test a pyramid on their land. Partnering with soybean breeders, we can produce enough seed to conduct on-farm demonstrations. In the short-term, this project will help us determine which pyramid is the best performer not only to protect yield, but also to limit the success of biotypes. Testing aphid resistance in an on-farm setting can demonstrate to farmers and their peers how well this technology works. In the long-term we will build a ‘ground-up’ demand for this technology, leading to greater commercial availability and subsequent use of aphid resistance.

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.