You might be seeing some insecticide resistance to aphids in your soybean fields. Or maybe your neighbors or farming friends the next county over have shared what they have seen in their fields.
Insecticide resistant populations increase the difficulty of soybean aphid management decisions. Knowing this, you have lots of questions: Which insecticide group do I use? What about a pre or tankmix? Will seed treatments or earlier applications help?
At a recent University of Minnesota Extension meeting in Lamberton, integrated pest management specialist Bruce Potter and colleagues presented the follow hypothetical scenarios to the audience and asked them what their management responses would be.
As a follow-up to that discussion, we asked Potter to provide his insight on the situation. Here is the scenario and Potter’s detailed response.
The Farmer: You farm near the Minnesota River, and some of your smaller fields usually have threshold populations of aphids earlier than most others in the area. Are there things that you could do differently to manage aphids in these fields? Think about the entire system (soils, weed management, variety selection), not just insecticide timing and choice.
Bruce Potter: Maintain good soil fertility in high-risk fields. Low potassium levels favor aphid reproduction. These fields might benefit from resistant varieties or seed treatments to reduce or delay local populations. They are good plants to scout early-season, but avoid letting early-season infestations in volunteer soybeans persist. Practice good corn weed control.
Early-planted soybeans will be at most risk. Since these fields should be populated by aphids from buckthorn with diverse genetics (sexual recombination in fall), any control practice in these first problem fields should be carefully monitored to make sure things have not changed over winter. Do not use reduced insecticide rates.
How might neonicotinoid seed treatments help with your aphid management? How might they limit your management options?
The seed-applied insecticide treatment may slow early-season population buildup in early-colonized fields. They will not prevent needing to treat when area aphid populations are higher.
On the other hand, to delay the development of insecticide resistance, seed-applied neonicotinoids should limit post-emerge applications of neonicotinoid insecticide premixes.
Thinking about potential pyrethroid resistance, what insecticide would you use when these fields hit economic threshold? Would it be the same for later fields?
If you are in an area that has had problems with pyrethroid-resistant soybean aphids, use a nonpyrethroid insecticide. If problems have not been observed, you may be more inclined to use a pyrethroid insecticide, particularly if aphid populations greatly exceed economic threshold.
Regardless, applications of pyrethroid insecticides on these early-infested fields provide early warning of insecticide performance issues. Scout these fields carefully a few days after application to make sure there are not resistance issues. An early-planted soybean field may provide a trap crop to assess early-season aphid populations and insecticide performance.
Fields treated later in the season, particularly in August, are likely to have received soybean aphids from multiple sources. They may have a higher probability of containing clones of pyrethroid-resistant aphids.
Larger canopies make obtaining good insecticide coverage more difficult. Do not cheat on water. Avoid the urge to apply insurance insecticides, as they encourage resistance selection and disrupt biological controls that may be keeping aphid populations under control.
In late July-August, late-maturing fields are preferentially colonized.
Your cousin farms in Nobles County, where aphids usually get bad after Farmfest. How would his response differ from yours?
Late-planted and/or -maturing soybeans are most likely to be colonized. Because these fields have been colonized by immigrant aphids from an unknown source, your cousin does not know what the parents have been exposed to, or if the population has any resistance to insecticide.
Scout and use the 250 aphids per plant economic threshold. These fields should receive a nonpyrethroid insecticide if and when they reach threshold.
Early insurance applications of broad-spectrum insecticides (pyrethroids, organophosphates) are especially problematic here, because they remove predators and parasitoids before the fields are colonized by winged aphids.
Seed treatments will provide few benefits on early-planted fields that are colonized late.