North Dakota State University Extension has received questions about the soybean aphid economic threshold, and some have suggested that the threshold be lowered.
However, research and extension demonstration plots have continued to validate the economic threshold (ET) for soybean aphid — 250 aphids per plant with 80% of plants infested and with aphid populations increasing.
Here are several reasons why you should use and wait until the ET is reached:
• Broad-spectrum insecticides will kill beneficial insects, such as lady beetle adults/larvae and lacewing larvae, beneficials which can and do keep aphid numbers in check. If aphids rebound, the beneficial insects will not be present and aphid numbers can grow very rapidly and require a second application later in the field season.
• Residual activity of foliar insecticides is relatively short-lived, and will not provide protection against late season aphid outbreaks, which typically occur from late July through mid-August in our area.
Furthermore, new leaves that have grown after an early insecticide application aren’t protected.
- Repeated use of the same insecticide mode of action, especially in a single growing season, can lead to insecticide resistance in soybean aphids.
• Use of pyrethroid insecticides (except bifenthrin) can cause spider mites to flare up, often resulting in additional foliar applications to control spider mites. Spider mites also have developed resistance to more than one mode of action in some areas of the Midwest, including bifenthrin and organophosphates, such as chlorpyrifos.
• Research trials conducted over several years in North Dakota and other north-central and Midwestern states have not demonstrated any economic benefit to spraying earlier than the ET. You might think that adding an inexpensive insecticide to the tank during an herbicide application is cheap insurance, but really it’s just an added input cost with no demonstrated economic benefit and potentially serious biological consequences.
For a more comprehensive review of soybean aphid biology and the economics of soybean aphid control, please visit: