Nebraska soybean farmers are fortunate when it comes to managing soybean aphids, says Keith Jarvi, University of Nebraska Extension educator for Dakota, Dixon and Thurston counties. Farmers in states like Iowa and Minnesota have to deal with soybean aphids from June until August every season.
"We're in kind of a transition area where we don't have to treat every year," he said. But Nebraska soybeans have been hit by aphids in an every-other-year cycle, and most of the time treatment hasn't been economical until early to mid-August, Jarvi told a group of farmers at soybean management field days held on the Dallas and Matt Breitbarth farm near Bancroft last week.
"The first couple of years we had soybean aphids in Nebraska, there was significant damage," said Jarvi. It took a few years for the aphids' natural enemies, like lady beetles, green lacewing larvae and minute pirate bugs to move into fields and keep populations under control.
"But now, we've settled into a basic pattern where we'll see a few in fields in June and July, and this time of year is when Nebraska farmers usually have to treat for them," he said. "August is an important time of year to look for aphids." This year, numerous soybean fields, particularly in northeast Nebraska, have been treated for aphids, after relatively light infestation last season.
The economic threshold for soybean aphids is 265 aphids per plant with populations increasing. Jarvi said that the recent weather pattern with temperatures running in the low 80-degree range is perfect for aphid reproduction.
Most of the time, aphids double population in a field in about seven days. But research conducted by Tom Hunt, UNL Extension entomologist at Haskell Ag Lab near Concord, has shown populations can double in as little as two days if weather conditions are optimal. That is why scouting fields is crucial.
The actual economic impact threshold (EIT), when yield losses will occur, is 654 aphids per plant. But the economic threshold is utilized to give farmers time to plan for treatment, recognizing the ability for aphids to reproduce rapidly.
"Don't treat when there are only 30 or 40 aphids on a plant," Jarvi said. While it may be tempting to mix insecticide with a final glyphosate treatment on soybeans as insurance, most of the time this treatment is useless. Treating aphids when they are not there can cause new problems by destroying the aphids' natural enemies and contributing to insect resistance issues down the road.
"We want to try to preserve these tools as much as we can," said Jarvi. "Insects are genetically diverse. They can adapt."
So, insecticide treatment should only be used when necessary as one tool in the toolbox for soybean management.
Aphid resistance bred into soybean seed is another new tool now offered to Nebraska farmers. Hunt's trials at Haskell Ag Lab have shown soybean fields planted to seed containing the naturally resistant "Rag" gene have significant reductions in aphid populations, compared to control plots. Aphids feeding on varieties that contain the "Rag" gene reproduce at a much slower rate and are not as healthy as aphids in neighboring fields.
This year's soybean management field days were also conducted at three other farm sites located at Clay Center, Elba and Cortland in mid-August.
The programs were supported by UNL and the Nebraska Soybean Board.
If you'd like more information on soybean aphid management, contact your local UNL Extension office, or call Keith Jarvi at 402-584-3819 or Tom Hunt at 402-584-3863.