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Don't let your soybean aphid guard down

Soybean aphids were going to be a big problem this year. That was what some specialists were saying at the beginning of the growing season before Mother Nature cranked up the furnace and turned off the water. Now, while farmers in a lot of areas likely won't have anything to worry about, others could start seeing aphid pressures. Then what?

"Although it has been too hot for soybean aphids to thrive, we are beginning to see them in northeast Nebraska soybean fields. One could consider many fields to be 'seeded' with soybean aphid," says Tom Hunt, University of Nebraska Extension entomologist in a recent university report. "If, or should I say when, temperatures decline, soybean aphid populations could increase quickly, doubling in just 2 to 3 days."

Aphids thrive in temperatures ranging from 70 to 85 degrees Fahrenheit, and the economic threshold for treatment through the soybean plant's R5 growth stage is 250 aphids per plant.

"Depending on economic conditions, this gives you five to seven days to schedule treatment before populations reach economically damaging levels. If populations don’t increase during this period, you may be able to eliminate or delay treatment. Determining if the aphid population is actively increasing requires several visits to the field," Hunt says. "Factors favorable for aphid increase are relatively cool temperatures, plant stress (particularly drought), and lack of natural enemies."

If there are aphids in your fields now, they should have already migrated from the stems to the leaves, where you can get a good idea of your overall populations by doing a quick count on the underside of those leaves, Hunt says. Pay close attention to your fields' development stage, though; after the R6 stage, he says treatment "has not been documented to increase yields."

Here are 10 tips from Hunt and other University of Nebraska specialists on managing soybean aphids, according to a university report:

  1. Treat soybean aphids only when populations warrant treatment. In Nebraska soybean aphids usually reach the economic threshold in late July through August. Treatment during this period is usually enough to keep aphid populations from resurging before they would naturally leave the fields in late August and early September.
  2. Avoid treating a field too early.  The earlier a field is treated, the greater the chance that any surviving aphids can later reproduce or that new aphids can repopulate the field. Also, any aphids that re-infest a field after early treatment won’t face any natural controls. If you treated early this year, closely monitor the field until early September. We have observed many fields that supported a non-increasing, low population of aphids (e.g. less than 100 aphids per plant) through August. Treating these fields would be a waste of time and money.
  3. Look for the presence of aphid natural enemies such as lady beetles, green lacewings, insidious flower bugs, aphid mummies, fuzzy aphids, and other insect predators. Predators and parasitoids may keep low or moderate aphid populations in check. Scout for aphids by examining plants where lady beetles are observed.
  4. Take note of winged aphids or “broad-shouldered” nymphs (which will become winged adults). If most of the aphids are winged or developing wings, the aphids may soon leave the field and treatment can be avoided.  (A hand magnifier may be necessary to identify the type of nymph.)
  5. If soybean plants are covered with honeydew or sooty mold, or stunted, and aphids are still present at threshold levels, an insecticide treatment may still be helpful, but the optimum time for treatment has passed.
  6. Good insecticide coverage and penetration is required for optimal control of soybean aphid because aphids feed on the undersides of the leaves and within the canopy. For ground application use high water volume (≥15 gallons/acre) and pressure (≥30 psi). Aerial application works well when high water volume is used (≥3 gallons/acre).
  7. Tank-mixing insecticides with glyphosate or other herbicides can be problematic because application methods for herbicides (e.g. lower pressures, large droplet producing nozzles) are not optimal for good insecticide efficacy. Tank-mixing with fungicides can be effective because application methods for fungicides and insecticides require high water pressure for adequate penetration and coverage.
  8. Several insecticides are labeled for the soybean aphid. A list of registered insecticides, rates, preharvest intervals, etc. can be found on the UNL Department of Entomology website at Pyrethroids have a relatively long residual, and work best at temperatures below 90º F. Organophosphates have a fuming action, and may work well in heavy canopies or high temperatures. Dimethoate is least effective.
  9. Plant resistance is another soybean aphid management strategy. Certain soybean cultivars have genetic qualities that prevent them from being heavily damaged by the soybean aphid when compared to other soybean cultivars. However, aphid resistance has already been documented to some of the earliest deployed genes (e.g. Rag1 gene) east of Nebraska. Scouting is still necessary with these varieties and you should be careful not to rely on any single method of pest management to avoid the development of resistance here.
  10. Spraying flowering soybean poses a threat to honey bees. Communicate treatment plans to nearby beekeepers and follow label precautions to minimize honey bee kills.
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