Recent advertisements by agribusinesses have reignited a discussion among farmers and others about when to apply insecticides to manage soybean aphid.
The current recommendation from entomologists across the Midwest is to treat soybean aphid when 80% of the plants in the field exceed 250 aphids per plant during bloom through seed set (R1-R5). Erin Hodgson, Iowa State University Extension entomologist, and Matt O'Neal, ISU research entomologist, say this recommendation was developed from field research conducted over several years across multiple states and appears to be applicable to a wide range of growing regions in the Midwest. Additional testing of the 250 threshold continues, with comparisons to lower thresholds.
Scout soybean fields once a week during July and August
"Soybean aphid outbreaks in North America have been highly variable," says Hodgson. "2009 is a great example because there is a wide range of infestations. Some fields are at extremely low levels and others legitimately need to be treated to protect yield. The population growth of soybean aphids can't be predicted at planting so regular sampling in every field is essential for determining if you need to spray a foliar insecticide during the summer and when to properly time that treatment."
Hodgson suggests looking at a minimum of 30 plants for every 50 acres, counting all the aphids on the plant to estimate the average density.
Hodgson and O'Neal say fields with low to moderate aphid problems can be regulated with predatory insects, like ladybeetles, but warn against spraying insecticides too early. "Most insecticides used against soybean aphid will also kill these beneficial insects, destroying any possibility for biological control," O'Neal says. "Spraying too early will knock down beneficial insects and give aphids and opportunity to flare."
Don't apply insecticide in a tankmix with herbicide
As for insecticide treatments, O'Neal and Hodgson advise against using a tank-mix control. "We do not recommend applying insecticides in a tank-mix to control soybean aphid," O'Neal says. "Tank-mixing with herbicides usually results in poor coverage for aphids. Also, be sure to use proper volume and sprayer pressure to reach aphids on the undersides of leaves and stems."
In Iowa and much of the North Central region, seed treatments have proven to be the least cost-effective method for managing soybean aphid. Using an insecticide seed treatment isn't sufficient to fully protect soybeans from aphids. You still need to scout and spray a foliar insecticide if needed. Seed treatment insecticide can have value against bean leaf beetles, but it can't provide season long protection against soybean aphids.
"Spraying by calendar date or plant stage can significantly reduce aphid populations, but a resurgence of aphids and other pests is possible later in the season," Hodgson says. "A well-timed foliar application at the threshold is typically the profitable management plan." Hodgson also suggests using integrated pest management (IPM) tools to minimize the potential for genetic resistance of aphids to major classes of insecticides.
What about aphid-resistant soybean varieties?
"Soybean aphid-resistant varieties will be commercially available in 2010," says Hodgson. "This new technology will slow the growth potential for aphids and allow for a longer 'treatment window' to protect yield. In the future, economic thresholds are likely to increase as growers use aphid-resistant soybeans."
David Wright, Iowa Soybean Association director of contract research and strategic initiatives, says there is a lot of misleading information out there about the proper treatment of soybean aphid. "The soybean checkoff has spent more than $3.5 million on soybean aphid research, and the current economic threshold recommended by ISU is the appropriate and most cost effective threshold for soybean growers," says Wright.