Soybean aphid infestations have reached the economic threshold of 250 aphids per plant in some fields this summer. Iowa State University Extension crop specialists are advising soybean growers to scout their fields and apply an insecticide if needed.
Since the arrival of Asian soybean aphid to Iowa in the year 2000, significant outbreaks have occurred—most noticeably in 2003 when aphid populations exceeded 3,000 aphids per plant in many fields. In 2005, there were also out breaks of soybean aphids in some Iowa areas.
Making aphid control decisions
A newly revised publication is now available to help educate farmers and crop consultants about soybean aphid. Soybean Aphids in Iowa—2007 (SP 247) is the name of the 16-page publication which includes tips for scouting aphids, color photos and other information and guidelines for making treatment decisions to control this insect.
It is available from ISU online at www.extension.iastate.edu/store/ or you can get the publication by contacting your county extension office.
Cooler temperatures, plants suffering drought stress and the absence of beneficial insects such as lady beetles are three factors that favor an increase in soybean aphid populations. Although temperatures the week of July 16 were in the mid-90s, daytime highs in the low 80s the next week were ideal temperatures for soybean aphids. This insect is very prolific and populations can increase rapidly.
Scout fields at least weekly
"We are advising regular scouting now in late July and into August to keep ahead of developing populations as dry fields and cooler temperatures might trigger an explosion in the aphid infestation in a field," says Palle Pedersen, an Iowa State University Extension soybean agronomist.
At what point do you know you need to spray an insecticide? "It boils down to time and how good of a job of scouting you want to do," he says. "Aphid infestations aren't uniform in a field. If you just drive into a field and look at a few plants you may not see the problem. Or you may see a few plants completely covered with aphids but the rest of the field may not have any aphids."
So you want to get as good of an estimate as possible. That means you have to walk across the field and look in different areas. If farmers have a lot of fields, they can't thoroughly scout all their fields. But at least you should scout the different varieties you have planted. A lot of times the different varieties have characteristics that attract insects such as aphids and bean leaf beetles.
"It looks to me right now that the biggest problems we are having with insects right now are in soybean fields that are close wooded areas," says Pedersen.
Use a sweepnet when scouting
Pedersen uses a sweepnet when scouting to collect bean leaf beetles in fields. You can buy a sweepnet at most farm supply stores. "Using a sweepnet can give you a good estimate of what is in the field because it is hard get an accurate count of bean leaf beetles this time of year when there is a canopy of soybean leaves," he notes.
Pedersen walks down the row and normally sweeps the net in the leaf canopy, going in the same direction as the rows run instead of sweeping across the rows.
The main thing to keep in mind is to go to multiple areas of the field to get an estimate across the field, and then make the decision as to whether or not the field has reached the threshold for treatment.
Soybean insects can be managed
For example, if you are going to look for aphids, and you are finding 150 or so aphids per plant, you know you can reach the threshold of 250 within two or three days. That means once you reach 150 you know you will have to go back and scout again in two or three days. Then you can see if your numbers are going up or going down.
"If you go out there and find an average of 150 aphids per plant and then three days later you are down to 75, you know you are okay and the population is declining and you should not apply an insecticide," says Pedersen. "But if you go back and find 275 aphids per plant, you know the population will continue to increase and you are past the economic threshold and you need to control the aphids."
It takes some effort to scout fields. But aphids and bean leaf beetles are both easy to manage. "You don't want to spray an insecticide if it is not necessary because in most cases you will also kill a lot of the beneficial insects. You don't want to apply insecticide if you don't have a good reason to do so."