Nebraska farmers need to keep an eye out for soybean aphids in their soybean fields now through the end of August, a University of Nebraska-Lincoln entomologist said.

Weather conditions are favorable for aphid buildup, said Tom Hunt, UNL entomologist at the Haskell Agricultural Laboratory near Concord.

"The last few years, the rains were relatively infrequent in July," Hunt said. "This year we've had different weather conditions -- a little cooler and we haven't had the several dry, 95-degree days in a row like in other years. Usually, hot weather slows them down in July."

Aphids reproduce best when temperatures are in the 70s and 80s. They start dying when it gets into the 90s. Their natural enemies do fine in hotter weather and also will feed on them. Lady beetles are one of the aphids' natural enemies and were seen feeding on them near Concord, Hunt said.

Although soybeans were planted later than normal in many areas across the state due to the wet spring, aphids migrate into the state, and they are right on track.
Aphid numbers can change rapidly this time of year, Hunt said.

"Populations can double in two to three days, so it is very important to keep an eye on your soybeans," he said.

The current recommended threshold for late vegetative through the R5 stage of soybeans is a field average of 250 aphids per plant with 80 percent of the plants infested and populations increasing.

"Depending on economic conditions and aphid population growth, this gives you about five to seven days to schedule treatment before populations reach damaging levels," he said. "If populations do not increase during this time, you may be able to eliminate or delay treatment."

To determine an actively increasing aphid population, several visits to the field are required. Factors favorable for aphid increase are relatively cool temperatures, plant stress (particularly drought) and lack of natural enemies.

Besides the lady beetle, other natural enemies include the minute pirate bug. Aphid "mummies," light brown, swollen aphids, indicate the presence of parasitoids. These predators and parasitoids may keep low or moderate aphid populations in check, under 200 per plant.

It's also important to look for winged aphids. This is a sign that the aphids may soon leave the field and treatment can be avoided. Populations should decline by the end of August to early September, but not always, Hunt said.

Other soybean aphid management tips include:

-- Not with glyphosates. It is not recommended to apply an insecticide at glyphosate application. In Nebraska, this usually is before the aphids reach damaging levels, or are even in the field. Insecticide treatment at this time would simply rid the field of natural enemies. In addition, application methods are not optimal for food insecticide efficacy.

-- Bee safety. Spraying flowering soybeans poses a threat to honeybees. Inform nearby beekeepers about treatment plans and follow precautions to minimize honeybee kills. When there is concern about honeybees, pyrethroids are the better choice.

-- Late treatment. If plants are covered with honeydew or sooty mold, or are stunted, an insecticide treatment may still be of value, but the optimum time of treatment is past.

-- Test strip. If fields are treated, leave an untreated test strip to compare against sprayed sections. This also provides a refuge for beneficial insects.

-- Coverage. Good insecticide coverage and penetration is required for optimal control. Many aphids feed on the undersides of the leaves within the canopy. Use high water volume and pressure. Arial application works well when high water volume is used, 5 gallons of water per acre is recommended.

-- Insecticide selection. Several insecticides are labeled for soybean aphid. A list is available on the UNL Department of Entomology's Web site at http://entomology.unl.edu/instabls/soyaphid.htm.

The soybean aphid is light green to pale yellow, less than one-sixteenth of an inch long and has two black-tipped or black cornicles that look like tailpipes on the rear of the abdomen. The aphid uses its piercing, sucking mouthparts to feed on new tissue near the top of soybean plants or on the undersides of mature leaves. Later in the season, aphids can be found on all parts of the plant. It is the only aphid in North America that can form colonies on soybeans.