Time to Check For Aphids

By John Wilson, Extension Educator

It’s mid-August and time to check fields for soybean aphids if you haven’t done so already. Even if you checked your fields a week ago, check them again. This is the time of year that aphid populations can build up rapidly. It’s important to remember aphid populations vary greatly from one field to another. So don’t make an insecticide application just because your neighbor did.

I was in a couple soybean fields recently and checked them for aphids. We’ve been fortunate this year that, although there have been a few aphids around, they haven’t built up in our area. Occasionally I’d find a pocket in a field with lots of aphids, but on average, the whole field was below control thresholds. However, I’ve heard reports from other areas in northeast Nebraska that have significant soybean aphid numbers and some fields are being treated in these areas. I’d target my scouting to late planted or replanted fields. These would be the ones most likely to have an aphid problem.

The aphid is light green to pale yellow, less than 1/16th inch long, and has two black-tipped cornicles… which look like tailpipes… on the rear of the abdomen. It has piercing-sucking mouthparts and typically feeds on new tissue near the top of soybean plants on the undersides of leaves. Later in the season the aphids can be found on all parts of the plant.

The only way to know if you need to treat is to check your fields and determine how many aphids are attacking your soybeans. You need to check plants in at least five different locations in a field and determine what is the average number of aphids per plant. It’s not unusual to have a hot spot in one area and very few in another, so get counts from different areas in the field and then average them together.

The threshold for treating is if you have an average of 250 or more aphids per plant and the population is increasing on plants up to R5, or the early pod fill growth stage. You won’t have yield losses at this level, but it should give you 5 to 7 days to make an application before yield losses occur… usually around 650 aphids per plant. If it’s going to take a commercial applicator a few days to get there, continue to monitor your aphid counts. If they level off or start to drop, cancel the application. It’s not unusual for aphid populations to crash around the first of September, but sometimes earlier.

Also, as the plant matures past R5, or if you have pods filling at the top of the plant, you can have higher aphid populations, probably in the range of 400 to 500 aphids per plant, before you need to be concerned about treating. In late August, we usually start getting milder temperatures which is more favorable for aphid populations to increase. So now is the time to get out and check your fields.

A couple final thoughts when scouting for aphids…

• When counting aphids, make sure you only count live aphids. As aphids grow, they shed their skins and these often are left hanging on the under sides of leaves. These are almost white in color and do not move around.
• Look for the presence of aphid natural enemies such as lady beetles, minute pirate bugs, and other insect predators. Aphid “mummies,” or light brown, swollen aphids, indicate the presence of parasitoids. These predators parasitize aphids and may keep low or moderate populations in check.
• Look for the presence of winged aphids. If the majority of aphids are winged or developing wings, the aphids may soon leave the field and an insecticide treatment may be unnecessary.

For more information on soybean aphid control, contact your local UNL Extension office.