Managing soybean aphids
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.
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