Soybean Aphid Control: Insights from research trials
by Gilles Quesnel
Soybean aphids have been a pest for Ontario farmers since they first emerged as a serious threat in the early 2000s. Initial infestations were overwhelming, with levels reaching up to 20,000 aphids per plant. However, over time, natural enemies helped control the aphid populations, and the need for intervention decreased as infestations rarely reached Economic Injury Levels.
In recent years, however, the aphid problem has resurfaced. Mid-level infestations were reported across most parts of Eastern Ontario from 2020 to 2022. Though the aphid numbers were not as high as the peak levels seen between 2003 and 2008, many areas experienced aphid populations ranging from 500 to 3,000 per plant.
Key Questions and Trials
These mid-level infestations raised important questions:
1. What is the soybean yield impact of having 500 to 3,000 aphids per plant?
2. Given today’s higher soybean yields and commodity prices compared to yields and prices in the mid-2000, should the original economic injury level of 660 aphids per plant be revised downward?
To address these concerns, I conducted trials at the Winchester Research Station in 2021 and 2022 to evaluate the effectiveness of two insecticide treatments to determine their impact on soybean yield and overall return on investment.
2021 Trial Results
In 2021, insecticide treatments were applied to soybean crops at the R3 growth stage (beginning pod). At the time of application, the aphid population was at 500 per plant and declining. The results showed no yield gain from the treatment, suggesting that applying insecticides to a declining aphid population is usually not beneficial.
2022 Trial Results
The 2022 trials presented a different scenario. Insecticides were applied at the R2.5 growth stage (an open flower at one of the two uppermost nodes on the stem) when the aphid population was at 800 per plant and rapidly increasing. Yield response to aphid control was significant:
• Matador: The use of Matador insecticide resulted in a yield gain of 6.6 bushels per acre.
• Sefina: This insecticide provided yield gain of 8.4 bushels per acre.
Key Takeaways
The trials provided valuable insights into soybean aphid control:
• Economic Injury Level: Given current yield levels and commodity prices, the economic injury level, where the cost of aphid control equals the damage they cause, appears to be in the range of 300 to 400 aphids per plant.
This is lower than the previously established level of 600-700 aphids per plant.
• Return on Investment: The cost of aphid control and the potential return on investment can be better estimated using updated economic models and data (see Figure 1 for detailed estimates).
• Scouting Accuracy: When scouting for aphids, there is a tendency to significantly underestimate their population. Accurate scouting is crucial for effective management.
• Timing of Insecticide Application: It rarely pays to apply insecticides to a declining aphid population. The timing of application is critical to ensure that treatments are effective and economically justified.
Acknowledgements
This research was made possible with the support of the Ottawa Valley Seed Growers Association, who provided funding, and the Winchester Research Station staff, who assisted with the trials.
Ottawa Valley Seed Growers Association is proud to support research projects impacting Eastern Ontario agriculture. For more information visit www.ottawafarmshow.com.
