The current recommended economic threshold for late vegetative through R5 stage soybeans is 250 aphids per plant with 80% of the plants infested and populations increasing. Depending on economic conditions, this should give you five to seven days to schedule treatment before populations reach economically damaging levels. If populations do not increase during this period, you may be able to eliminate or delay treatment. Factors favorable for aphid increase are relatively cool temperatures, plant stress (particularly drought), and lack of natural enemies.
Soybean aphids in Nebraska usually reach the economic threshold and require treatment in late July through August. Treatment during this period usually is enough to keep aphid populations from resurging before they leave the fields in late August and early September.
Treatment Timing. The earlier a field is treated, the greater the chance that any surviving aphids can reproduce or that new aphids can repopulate the field. Insecticide treatments will kill many natural enemies, so any aphids that do re-infest a field are not constrained by predators and other natural controls. Even insecticides with a relatively long residual cannot last when an insecticide is applied in early or mid-July, particularly when aphid populations are thriving. If you have to treat early, make sure to closely monitor the field until early September.
Unwarranted insecticide treatments, such as in fields treated well before the threshold was met or treated along with a herbicide (in some cases a fungicide), regardless of aphid presence. These treatments kill natural enemies and are usually done relatively early so there is plenty of time for aphids to resurge or re-colonize a field. Aphid populations below or even at the economic threshold do not cause yield loss, so treating before populations reach 250 aphids/plant only increases the probability of aphid resurgence.
In addition, we have observed that many fields support a non-increasing, low population of aphids (e.g., less than 100 aphids/plant) through August. Treating these fields would be a waste of time and money.
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; however, only conduct this practice if soybean aphid thresholds are met.
General Management Guidelines
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. Often soybean aphids can be found by examining plants where lady beetles are observed.
Take note of winged aphids or "broad-shouldered" nymphs. Nymphs with broad or squared- off shoulders will become winged adults. A magnifying glass is helpful to see the "broad-shouldered" nymphs, but the winged adults are easy to see with the naked eye. If the majority of aphids are winged or developing wings, the aphids may soon leave the field and treatment can be avoided.
If the plants are stunted or covered with honeydew or sooty mold and aphids are present at threshold levels, an insecticide treatment may still be of value but the optimum time for treatment has passed.
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).
Several insecticides are labeled for soybean aphid; their rates, preharvest intervals, etc. are listed at http://entomology.unl.edu/instabls/soyaphid.shtml. 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.
Spraying flowering soybean poses a threat to honey bees. Communicate treatment plans to nearby beekeepers and follow label precautions to minimize honey bee kills. When there is concern about honey bees, pyrethroids are the better insecticide choice and spraying late in the day is preferred.
Host Plant Resistance
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. These varieties use the Rag genes. Soybean aphids feeding on varieties with this resistance reproduce at a drastically slower rate and are less healthy. However, the soybean aphid can circumvent this resistance. Resistance has been documented to some of the earliest deployed genes (e.g., Rag1) in states to the east. In light of this, growers shouldn't rely on a single method of pest management and should continue to scout fields thought to be resistant.