Project Details:

Title:
Determining soybean pest and pesticide interactions as a means to optimize soybean yield

Parent Project: Evaluation plant pest interactions to optimize soybean yield
Checkoff Organization:Iowa Soybean Association
Categories:Insects and pests, Crop management systems
Organization Project Code:
Project Year:2015
Lead Principal Investigator:Erin Hodgson (Iowa State University)
Co-Principal Investigators:
Aaron Gassmann (Iowa State University)
Gregory Tylka (Iowa State University)
Keywords: Best Management Practices, Soybean Aphid (SA), Soybean Cyst Nematode (SCN), Soybean Disease Management, Soybean Educational Activities

Contributing Organizations

Funding Institutions

Information and Results

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Project Summary

With the advent of new seed treatment management products and resistance of soybean varieties, there is a need to evaluate how the interactions of these management tactics affect the interaction of soybean aphids and soybean cyst nematodes on resistance and susceptible soybean varieties. It is also important to understand the effects of these management tactics on naturally occurring soil-borne organisms, which may play a critical role in keeping pest population densities in check.

Project Objectives

1. Determine optimal yield potential using combinations of pest management tactics.
2. Evaluate interactions among host plant resistance, pesticides and pests to optimize yield.
3. Conduct greenhouse experiments to measure treatment effects on pest-killing pathogens and key soybean pests.
4. Promote optimal yield recommendations with multiple pest suppression tactics.

Project Deliverables

Provide optimal yield recommendations with multiple pest suppression tactics.

Progress of Work

Update:
The two field locations were scouted weekly for soybean aphid populations starting in late June and through mid-September when the populations decreased by more than 75% after one week. Cumulative aphid days (CAD) were generally lower this year compared to last year's data. At the northeast location, aphid-susceptible plots had an average of 1172 - 196 CAD; for the northwest location, only 188 - 11 CAD. During the second week of October, both locations were harvested. Soil samples were taken on the days of harvest and will be processed for determining fall populations of soybean cyst nematode (SCN).

Harvest data for the northeast location was put into a mixed-model analysis of variance (ANOVA) and shows significant effects of seed treatment and its interactions with host plant resistance for soybean aphid or SCN. Pooling together all four varieties of soybeans, the plots with the Avicta Complete seed treatment yielded an average of 51.9 bushels/acre compared to the naked (no seed treatment) plots that yielded 44.1 bushels/acre. The same pattern holds true when we only look at the two nematode-susceptible beans. Again, the Avicta Complete treated beans yield significantly higher than the untreated beans. Once we have quantified SCN populations, we can determine whether or not the pest directly impacted yields and if the seed treatment helped to suppress the pest.

Update:
Both locations were harvested and soybean cyst nematode (SCN) eggs were sampled in October 2014. In the northeast location, we found an overall decrease of nematodes in almost every plot. There was no significant effect of soybean variety, seed treatment, or the interaction of both factors. It is possible that intense periods of rainfall in June coincided with the time when SCN eggs would have hatched, potentially killing juveniles that would seek soybean roots. Though we did not measure natural enemies, there could also have been epizootics (mass death) of SCN from natural enemies (pathogenic fungi, bacteria, etc.). It is unclear why fall numbers were so low compared to spring numbers. For the northeast location, there was an overall increase of nematodes in most plots. The interaction of soybean variety and seed treatment was significant, with some varieties benefitting from lower SCN reproduction when treated with a nematicide compared to naked seed.

Harvest data for the northeast location was put into a mixed-model analysis of variance (ANOVA) and shows significant effects of seed treatment and its interactions with host plant resistance for soybean aphid or SCN. Pooling together all four varieties of soybeans, the plots with the Avicta Complete seed treatment yielded an average of 51.9 bushels/acre compared to the naked (no seed treatment) plots that yielded 44.1 bushels/acre. The same pattern holds true when we only look at the two nematode-susceptible beans. Again, the Avicta Complete treated beans yield significantly higher than the untreated beans.

Harvest data for the northwest location was put into a mixed-model analysis of variance and showed no significant effects for soybean variety, seed variety and the interaction of both factors. Heavy rainfalls in May and June put the first two blocks in several weeks of near or completely saturated soil. The soybeans in these waterlogged blocks did not grow as large as the soybeans in the blocks at a higher elevation. Regardless of this obstacle, there was still no significant influence of soybean variety or seed treatment on yield across all blocks; performance of the soybean varieties and the seed treatments was consistent in muddy or well-drained soils at this northwest location.

A greenhouse experiment was started in March 2015. The objective of the experiment is to measure both soybean aphid and soybean cyst nematode reproduction on the 12 different treatments of variety and seed treatment (4 varieties × 3 seed treatments). The soil in the “cone-tainers” has approximately 1,500 SCN eggs per 100 cc soil. On the 11th day of the experiment, each plant was infested with 10 soybean aphids. Aphid counts will be performed one week (day 18) and two weeks (day 25) after aphid infestation. Roots will be washed at the end of the experiment (day 30) to measure SCN females (cysts). The experiment will be repeated in April for increased statistical power.

Final Project Results

During the summers of 2013-2015, two field locations were established each year. Plots were set up in a randomized complete block design with eight replications, including four soybean varieties (susceptible, SBA resistant, SCN resistant, and SBA+SCN resistant), and three seed treatments (naked, ApronMaxx, Avicta Complete). Soybean aphid (SBA) populations were monitored during the summer and cumulative aphid days were estimated for each treatment. Soybean cyst nematode (SCN) eggs were sampled at planting and harvest.

Results of the 3-year study:
• SBA- and SCN-resistant varieties support lower populations of aphids (Fig. 1) and nematodes, respectively.
• SCN-resistant varieties yield higher than SCN-susceptible varieties (Fig. 2).
• Seed treatments did not consistently improve yields, even with SBA- and SCN-susceptible varieties (Fig. 2).

Conclusions of the 3-year study:
• SBA resistance (Rag genes) is an effective suppression tool and may reduce the reliance on foliar insecticides.
• SBA-resistant variety breeding needs to improve yield potential.
• SCN resistance (PI88788) reduces SCN populations and is an important consideration for variety selection.
• Using host plant resistance with seed treatments did not consistently produce higher yields.
• Prophylactic use of pesticidal seed treatments may not provide higher soybean yields, particularly on host plant resistance soybean varieties.

Benefit to Soybean Farmers

With the advent of new seed treatment management products and resistance of soybean varieties, there is a need to evaluate the interactions of these management tactics with soybean aphids and soybean cyst nematodes and to understand the effects of these management tactics on naturally occurring soil-borne organisms, which may play a critical role in keeping pest populations in check.

Performance Metrics

Project Years