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Scouting for soybean aphids

Light green to pale yellow soybean aphids have been a yield-robbing summer visitor to Nebraska soybean fields since their discovery in the state in 2003. Measuring less than one-sixteenth of an inch long, with two black cornicles or “tailpipes” on the rear of their abdomen, they have piercing and sucking mouthparts. They begin to feed on new tissue at the top of soybean plants beginning in late June and early to mid-July.

At a glance

• Soybean aphids have been populating Nebraska fields since 2003.

• Begin your scouting program for aphids in late June.

• August is when aphids normally reach economically damaging levels.

Although seed companies continue to develop seed scoring for aphid resistance and aphid-resistant soybean varieties that match Great Plains maturity levels and growing conditions, field scouting early and often is still a farmer’s best defense.

“In Nebraska, aphid populations can reach economically damaging populations in late July, but most reach economically damaging populations in August, while soybeans are in the mid-reproductive stages [R4 to R5],” says Thomas Hunt, University of Nebraska Extension entomologist at Haskell Agricultural Lab at Concord. “We generally find a few aphids in late June to early July, so we recommend that farmers and consultants start scouting at this time.”

With the highest concentrations in northeast Nebraska, under favorable growing conditions, typically with temperatures in the 70s and mid-80s, aphids can reproduce at lightning pace, perhaps doubling in two to three days, he says. As populations increase, winged females migrate to adjacent soybean fields.

Be alert if July is cool

With weather forecasts predicting cooler and wetter weather in the early and middle parts of summer, aphid populations could climb rapidly this year.

“July seems to be the critical month for Nebraska. If July is hot, with periods of several days above 90 degrees, aphids do not do well, but natural enemies do fine, keeping populations in check,” says Hunt. “If July is relatively mild, with just a day or two hitting or exceeding 90 degrees, aphids thrive and natural enemies cannot keep up.”

When temperatures rise to above 90 degrees F or dip into the 40s, development appears to slow or even stop, according to Hunt.

“The current recommendation threshold for late-vegetative through R5-stage soybeans is 250 aphids per plant, with 80% of the plants infested and populations are increasing,” Hunt says. “Depending on economic conditions, this gives you about five to seven days to schedule a treatment before populations reach damaging levels. If populations do not increase during these seven days, you may be able to eliminate or delay treatment.”

Several visits needed

Hunt says, “Determining if the aphid population is actively increasing requires several visits to the field.” Several factors, including weather and existence of natural enemies, will affect that determination.

“The major natural enemy that is in the soybeans prior to aphid infestation is the minute pirate bug [Orius]. The other that comes in after soybean aphid infestation is the Asian lady beetle,” he says. “These predators and parasitoids [these are tiny and hard to see] may keep low or moderate aphid populations in check [under 100 to 200 aphids per plant] if the environmental conditions are favorable.”

Hunt says that threshold levels may be different for farmers who plant aphid-resistant soybeans with the “Rag” resistance genes. “We don’t have definitive data, but as population growth should be quite slow, the thresholds would likely be higher.”

The mechanics of good scouting

Thomas Hunt, UNL Extension entomologist, recommends that farmers begin scouting soybean fields in mid-June for populations of soybean aphids. Here are a few of Hunt’s suggestions on how to properly scout a field:

Check field at least once a week, beginning in June.

Check 20 to 30 plants per field. Develop this routine by mid-July.

Aphids will be at the top of the plants early on and on the stems and within the canopy later in the season.

As population numbers are approaching the 250-per-plant threshold, increase scouting to every five days, especially if temperature ranges are optimal for aphid reproduction.

Determine whether aphid populations are increasing or decreasing by studying the weather forecast for the next few days and by observing if there are large populations of the aphids’ natural enemies.

Symptoms of infestation include yellowed or distorted leaves, stunted plants, shiny honeydew that the aphids excrete on leaves and gray or sooty-colored mold growing on the honeydew.

For more information on scouting and treatment options, contact UNL Extension entomologist Thomas Hunt at 402-584-3863, or contact your local county Extension office.

This article published in the July, 2010 edition of NEBRASKA FARMER.

All rights reserved. Copyright Farm Progress Cos. 2010.