During the 2016 growing season, insecticide failure was confirmed in a field in northwest Iowa. Pyrethroid insecticides are a commonly applied insecticide group, but the pyrethroid insecticide did not have any knockdown in this commercial soybean production field, says Iowa State University Extension entomologist Erin Hodgson.
Soybean aphid remains the most important soybean insect pest in Iowa, and management over the last 15 years has primarily relied on using foliar insecticides.
The economic injury level was defined in 2007, and is approximately 675 aphids per plant, or 5,560 cumulative aphid days, says Hodgson. From that multistate research, a conservative economic threshold was developed to protect yield: 250 aphids per plant with 80% of the plants infested through the “seed set” growth stage (R5.5) of the soybean plant.
The odds of making a profitable treatment decision are increased with regular scouting and insecticide applications made after exceeding the economic threshold. The economic threshold is validated annually at ISU and is recommended regardless of fluctuating soybean market values.
Insecticide resistance issue
With any pest and pesticide interaction, exposures will eventually lead to resistance developing in the population, Hodgson explains. Insecticide resistance is common with aphids, which are asexual, multigenerational pests in many crops. Since 2015, farmers in parts of southern Minnesota experienced failures of pyrethroid insecticides to control soybean aphid. Using a vial assay, entomologists in Minnesota confirmed soybean aphid resistance to bifenthrin and lambda-cyhalothrin. Pyrethroid failures in southern Minnesota were also noted in the 2016 growing season.
In 2016, a commercial field in northwest Iowa was suspected to have bifenthrin resistance after being treated twice within two weeks and no response. Vial assays based on the Minnesota protocol also noted increased resistance ratios of a soybean aphid population collected from the problem field. A combination of integrated pest management (IPM) and insect resistance management (IRM) tactics will be needed to manage soybean aphid and prolong existing and emerging insecticide efficacy.
Recent regulatory concerns regarding the availability of certain insecticides may further complicate field crop pest management, especially soybean aphid.
IPM and IRM management recommendations
“Population fluctuations between locations and years are typical soybean aphid dynamics for Iowa,” says Hodgson. “Regular scouting and timely use of foliar insecticides is still a reliable management tactic. However, recent changes in insecticide efficacy will make future management more complicated.”
Following are Hodgson’s recommendations for sustainable soybean aphid management in Iowa:
• Consider using host-plant aphid-resistant soybean varieties if soybean aphid populations are persistent and the genetic traits of the soybean variety are appropriate for the area of the state where you are planting. The use of a single resistant gene will result in lower cumulative aphid exposure, and the use of a resistant pyramid (i.e., two or more genes) will greatly reduce the likelihood of needing foliar insecticides.
• Plant the soybeans early if the field is in an area with persistent soybean aphid populations.
• Scout for soybean aphids, especially during R1 to R5 soybean plant growth stages, and use a foliar insecticide if aphids exceed the economic threshold of 250 per plant. Take note of natural enemies and other potential plant pests in addition to soybean aphid.
• Use an insecticide product labeled for soybean aphid, and use high volume and sprayer pressure so that droplets make contact with aphids on the undersides of leaves. Check aphid populations three days after application to assess product efficacy.
• Alternate the mode of action if soybean aphid populations need to be treated twice in a single growing season (e.g., using organophosphates and pyrethroids).
• Understand that late-season accumulation of aphids, particularly after R5, may not impact yield like it does in early reproductive growth. A foliar insecticide applied after seed set on soybean plants may not be an economically profitable choice.
Don’t assume you have resistance
Before you assume insecticide resistance has developed in the field, Hodgson says you should rule out other possible factors, such as misapplication of the insecticide product (incorrect rate, poor coverage, etc.); unfavorable weather conditions around the time of application (wind, rain, temperature); and pest recolonization.
What is pest recolonization? The overwintering and migratory behavior of soybean aphid is not fully understood, she notes. The magnitude of pyrethroid resistance for soybean aphid in the north-central region of the U.S. is also not well characterized yet. In other words, the aphids that colonize soybean plants can come from different overwintering sites each year. They can fly in from other areas, and the populations will have a range of susceptibly to insecticides.
Learn more about soybean pest management on the Soybean Podcast with Hodgson and Matt O’Neal, an ISU professor of entomology online. Also, the “2016 Yellow Book for Soybean Aphid,” CROP 3109, is available as a free download through the ISU Extension store.