By Bruce Potter (Extension IPM Specialist, U of MN), Robert Koch (Extension Entomologist, U of MN), Phil Glogoza (Extension Educator – Crops, U of MN), Ian MacRae (Extension Entomologist, U of MN), and Janet Knodel (Extension Entomologist, NDSU)
The University of Minnesota has been receiving an increasing number of reports of pyrethroid insecticide failures for soybean aphid management from northwest and central Minnesota, and northeastern North Dakota this year. However, many areas of Minnesota and North Dakota still have low, non-yield threatening aphid numbers and scouting should continue to determine when to apply insecticides.
In this article, we review the insecticide groups used for soybean aphid control (Table 1) and discuss the potential role of and challenges associated with insecticide mixtures.
We are all familiar with the multistate, researched-based economic threshold of 250 aphids/plant for determining when to apply insecticides for soybean aphid. Waiting until this economic threshold to apply insecticides is KEY for ensuring continued efficacy of current insecticides for soybean aphid. Using the soybean aphid economic threshold for deciding when to apply insecticides will:
Prevent unnecessary selection pressure for insecticide resistance;
Save time by reducing the odds of a field needing a second treatment;
Mitigate secondary pest problems like spider mites; and perhaps most importantly
Reduce production expenses.
Poor insecticide resistance management of a mobile insect like soybean aphid affects not just your farm, but other farmers as well.
Farmers who experience aphid populations above the threshold, those who sprayed too early and now need to re-treat, and those who experienced insecticide failures, face some serious questions. What options do soybean growers have in light of increasing aphid populations, especially when those aphids maybe resistant to pyrethroid insecticides?
Here are some general recommendations to improve pest management of soybean aphids:
When applying any insecticide for soybean aphid, spray coverage and canopy penetration are critical.
Do not use reduced insecticide rates.
Use appropriate spray pressure and spray nozzles (for example. do not use large droplet Engenia or Xtendimax approved nozzles) to treat aphids.
Do not skimp on water. At least 15-20 GPA via ground and 3-5 GPA via air.
Insecticide applications during windy conditions, temperature inversions, or very hot weather could reduce control.
Check insecticide performance 3-5 days after application.
Do NOT retreat a field with the same insecticide group for consecutive applications!
If it didn’t work the first time why would it work the second time?
If it did work the first time, it might not work the second time.
Insecticide group numbers can be found on the label.
INSECTICIDE GROUPS (TYPES)
Pyrethroids (Group 3A): In light of recent pyrethroid performance issues, it is not advised to apply an insecticide containing only a pyrethroid insecticide(s) in most areas of Minnesota and in northeastern North Dakota.
These broad-spectrum insecticides have been widely used for soybean aphid control.
Insecticides in this group are not equally effective on soybean aphid.
Resistance levels may vary among pyrethroid products and soybean aphid populations.
Potential for 7-10 days residual but new growth not protected.
If you do use a pyrethroid on soybean aphids use a high labeled rate.
Among the pyrethroids, only bifenthrin is labeled for control of spider mites.
These insecticides are toxic to beneficial insects and bees.
If a pyrethroid insecticide application may not work well, which insecticides should be considered?
Organophosphates (Group 1): for example, chlorpyrifos (Lorsban and generics)
With the widespread pyrethroid failures, this is one preferred option for first applications in a field or when retreating a field that was previously treated with a pyrethroid insecticide.
Chlorpyrifos is still very effective against soybean aphid populations.
This insecticide does not have a long residual and volatilization is increased in warm weather. Soybean aphids can re-colonization chlorpyrifos treated fields within a few days.
This group provides broad-spectrum control of pests
Dimethoate, another organophosphate insecticide labeled for soybean aphid, has provided variable and sometimes suboptimal control of soybean aphid in efficacy trials in Minnesota and North Dakota when used alone.
Chlorpyrifos and dimethoate have shown good activity on Minnesota populations of two-spotted spider mite. Some populations of spider southwest Minnesota have shown resistance to chlorpyrifos.
These insecticides are toxic to beneficial insects and bees.
Neonicitinoids (Group 4A): for example, Belay, Admire Pro, Alias
These insecticides are absorbed and translocated short distances within the plant. They may be moved upward for a leaf or two.
At high rates, this insecticide group can provide residual activity, but not more than 2-3 weeks.
Provide fairly broad-spectrum control for chewing and sucking pests.
These insecticides are toxic to some beneficial insects and bees.
Butenolides (Group 4D): Sivanto
This is a recently labeled chemistry for soybean aphid. We do not have a large data set for this insecticide on soybean aphid. Data from Minnesota in 2015 showed moderate control of very large soybean aphid populations.
Like the related neonicotinoids, this chemistry is translocated short distances within the plant.
This insecticide sub-group is only effective on some species of sucking insects.
This chemical has relatively low toxicity on honeybees.
This is an expensive insecticide (about $17 to $35 per acre for the insecticide alone), so cost is an issue.
We do not generally recommend premixes or tank mixes of insecticides. From a resistance management perspective, premixes and tank mixes with reduced rates of one or more insecticides should be avoided. However, such mixtures may be necessary when products with single insecticide groups have already been applied to a field.
Pyrethroid premixes (such as Hero)
Mixtures of multiple pyrethroids do not make sense when treating a soybean aphid population that is already suspected to be resistant to pyrethroids. All pyrethroid insecticides are in the same insecticide group (Group 3A).
Neonicitinoid premixes (such as Endigo, Leverage)
These premixes would be a preferred option following a chlorpyrifos application.
In University research trials, these mixes have provided control of aphids. One of these (Endigo) has been shown to provide control of soybean aphid populations that were potentially resistant to pyrethroids.
Based on the lower levels of control observed in northwest and central Minnesota this year, the neonicitinoid portion would need to do most of the control, increasing resistance selection pressure on that group and perhaps control.
Watch for increasing spider mite populations after a foliar neonicitinoid application has been applied to field.
If you used an insecticide seed treatment, you have already made one neonicitinoid application to the field.
Organophosphate premixes (such as those with chlorpyrifos - Cobalt, Tundra Supreme, Stallion)
Our trials have shown effectiveness of chlorpyrifos-containing premixes (Cobalt) against soybean aphid in recent years.
After the application of an organophosphate-pyrethroid premix, the field is protected for up to a few days by the organophosphate; however, the longer residual activity of the pyrethroid will not be effective on surviving or incoming aphids that are resistant to pyrethroids.
Though dimethoate alone has provided inconsistent control of soybean aphid, research trials in southwest Minnesota in 2016 suggested that soybean aphid control offered by a pyrethroid was improved with the addition of dimethoate.
For those wanting to mix products, be advised that there may be compatibility issues when tank-mixing insecticides.
Test any potential tank mix for compatibility first.
When choosing insecticides, consider the abundance of yield threatening insect pests first. For example, do not base an insecticide choice on non-threatening levels of a secondary pest. However, if dry conditions persist, spider mites could quickly become a real pest concern that will further complicate insecticide selection.
Two miticides, Zeal SC and Agri-mek, are now labeled for spider mite control in soybean.