Aphids, aphids everywhere! That's how soybean growers will remember the summer of 2007. The soybean aphid population exploded during the week of July 30 to August 3 in Iowa. Many fields statewide are now being sprayed with insecticide to control this pest. Some have been sprayed twice this summer.
"You need to be scouting your soybean fields on an almost daily basis," says Palle Pedersen, Iowa State University Extension soybean agronomist. He's been busy spraying his own soybean research plots in various areas of the state during the first few days of August.
The pest seems to hit a peak and be a problem every two years. Iowa previously had bad infestations of aphids in 2003 and 2005. "Soybean aphids are back in Iowa in a big way this year," he notes. "But soybean growers are much better equipped to deal with the potentially yield robbing pest than they were when aphids first showed up as a major pest in Iowa in the year 2000. We now have more knowledge about the pest and we also now have the experience in how to best control it."
Why aphid infestations have exploded
Soybean aphid numbers have increased dramatically this past week, in part because the dry weather Iowa has had is favorable for aphid populations to quickly increase. Aphids are a very prolific insect and can double their population in a matter of days if weather conditions favor the build up.
Last week ISU Extension crop specialists, private crop scouts and agronomists in Iowa reported only a few fields with populations over 100 aphids per plant. This week (July 30 to August 3) there are many such fields with an average of 200 or more aphids per plant and numerous fields have reached the economic threshold for making an insecticide application.
The economic threshold to apply an insecticide is when the field averages 250 or more aphids per plant and the population is increasing.
Other insects also a problem in some fields
This summer, Iowa soybean fields are feeling all kinds of insect pressure, but especially from aphids. Spider mites are infesting some bean fields in the driest areas of the state—mainly northwest Iowa--and bean leaf beetles are also showing up in many fields. "However, this summer's aphid outbreak is the worst we have seen and it is widespread throughout the state," says Pedersen.
Aphid infestations are worst in northern Iowa. But southern Iowa soybean growers are also now seeing aphids at levels that are close to the threshold for treatment. In addition to aphids, central and southern Iowa growers also are experiencing heavy infestations of bean leaf beetles in fields.
"In general this summer, just about all soybean growers in Iowa have some kind of soybean insect pressure," says Pedersen.
He notes that "Some fields were sprayed for aphid way too early this summer, and now populations have built back up again. Most of the insecticide products that are used to control aphids can provide residual control of aphids for approximately two to three weeks. So it pays to continue to keep an eye on your fields, even if the fields have already been sprayed with an insecticide."
Keep scouting bean fields for insects
Pedersen says you should keep scouting fields regularly and watching closely throughout the month of August for aphids and other soybean insects.
Even though the aphid pressure is heavier in 2007 than it was four years ago, farmers, crop scouts and custom applicators are much better prepared to deal with aphids now than they were in 2003. In part, that's because beans are worth roughly $3 a bushel more this year than in 2003, so farmers have reacted more quickly to the threat. Pedersen also credits the Iowa Soybean Association for investing soybean checkoff dollars in research on how to better control aphids.
Pedersen says all of these factors have given ISU Extension specialists the information they need to provide good, practical advice to farmers this year. As a result, farmers have the motivation to take the advice seriously.
"Two weeks ago we started warning farmers about the potential for aphid populations to explode and become a serious problem," he says. "Everyone has been scouting fields. As soon as the aphids reach the economic threshold, farmers are starting to spray. Iowa should not lose too much yield this year from aphids, because farmers, crop scouts and custom applicators have been doing a good job of staying on top of the situation and getting the aphids under control."
A newly revised publication is now available from Iowa State University to help farmers, crop consultants and others learn more about soybean aphid control. Soybean Aphids in Iowa—2007 (SP 247) is the name of the 16-page publication. It includes tips for scouting aphids, along with color photos and other information and guidelines for making treatment decisions to control this insect. The publication is available from ISU online at www.extension.iastate.edu/store/ or you can get it by contacting your county Extension office.