Soybean aphids can be an elusive pest, but agronomists say they're worth scouting for every year. If they get a foothold in your field, they can cause significant yield loss – unless you take action.
John Obermeyer, Purdue University Extension Entomologist, says there are no "typical" symptoms when it comes to signs of aphid damage. Instead, you need to look for the aphids and signs of their feeding on soybean leaves.
Aphids tend to congregate and populate on healthy leaves and plants as opposed to sickly plants or those already suffering from disease. According to Obermeyer, the aphids secrete a sticky exudate. It's often called 'honeydew' and is a tell-tale sign of aphid infestation. Notice the honeydew in the picture. They secrete this substance while they are feeding on soybean leaves in the field.
Plants become stressed from the feeding. Dry conditions make the stress even worse, Obermeyer says, but by that time the aphids have moved on. They may move to another plant, another field, even to another county. What's left where they feed is a sooty mold that grows on the honeydew substance. It often causes the lower canopy of the plants to turn black and take on a strange appearance.
In the picture here the aphids are feeding in the lower canopy on healthy soybeans. Note the honeydew substance. That's the direct opposite of Japanese beetles, who do the majority of their feeding on the tops of soybean plants on the newer, higher leaves in the canopy. That's why defoliation caused by Japanese beetle feeding is not often not as bad as it looks once it's figured on a whole plant basis and not just on the leaves where they feed.
Aphids tend to move in from the north and affect northern counties first in Indiana, but that's not a hard and fast rule. Watch for information on soybean aphids throughout the season in the weekly Pest & Crop Newsletter published by the Purdue University Entomology Department.