By Robert Koch, Bruce Potter & Jeff Gunsolus
University of Minnesota Extension
For soybean aphid management, we encourage you to rely on scouting (actually getting into the field and looking at plants) and the validated economic threshold (average of 250 aphids per plant, aphids on more than 80 percent of plants, and aphid populations increasing) to determine when to apply insecticides for soybean aphid.The threshold number of aphids is below the number required to cause yield loss and allows time to apply an insecticide before economic loss is incurred. However, you might be tempted to apply insecticides for soybean aphids at low population levels or without regard to the size of the aphid population in field, just in case you might have a problem. These “insurance” applications of insecticides can have negative impacts.
Cost: Insurance applications of insecticides for soybean aphids may occasionally provide an economic benefit, if they happen to coincide with development of a threatening population of soybean aphids. However, keep in mind that low populations of soybean aphids do not affect soybean yield. Furthermore, not all aphid infestations will grow to levels that will cause yield loss. Therefore, investments made in insurance applications are often wasted.
Development of pest resistance to insecticides: Repeated use of insecticides can lead to development of insecticide resistance in pests, which makes the insecticides ineffective against those pests.
Insecticide resistance has not yet been documented in North American soybean aphid populations. However, this pest has shown resistance to some organophosphates in China. If we are not careful, insecticide resistance could develop in soybean aphids here.
Furthermore, applications of insecticides for soybean aphids increase the likelihood of development of insecticide resistance in other soybean pests.
Resistance to chlorpyrifos in a population of spider mites from southwestern Minnesota was documented in 2012. Spider mites are not often the target of insecticide applications in Minnesota, so the resistance in those mites was likely due to applications made for soybean aphid.
Impacts on beneficial organisms: Many of the insecticides used for soybean aphid management, such as pyrethroids and organophosphates, are also very toxic to the beneficial insects (predators and parasitic wasps) that feed on aphids. Treating a real or imagined aphid population will kill beneficial insects. Without these natural controls, newly colonizing aphids or other pests can rapidly grow to damaging levels. An insecticide application becomes worth this potential risk once the beneficial insects can no longer keep aphids below economic threshold in the field.
Some insecticides are advertised as being selective or less harmful to beneficial insects (predators and parasitoids) that feed on soybean aphids. Though these products may be less toxic to some beneficial insects, they show toxicity to others.
The impact of selective insecticides on the behavior and biology of beneficial insects are not understood. Such impacts could affect the ability of the beneficial insects to control pest populations.
Therefore, we do not recommend the use of selective insecticides for insurance applications against soybean aphids. Limiting your use of insecticides to when they are most likely to provide an economic benefit will also reduce the risk to pollinators.
Poor insect and/or weed control: Tank mixes of insecticide and herbicide can lead to poor insect and/or weed control. The aversion to extra trips across the field is understandable from both a labor and fuel investment perspective. Unfortunately, the best nozzle type, water volume and pressure for application of insecticides and most herbicides are not the same. In addition, the timing of herbicide and insecticide applications should be based on the targeted pests (weeds or aphids), but these timings are not likely to coincide.
Currently, serious weed control issues exist in soybeans, with waterhemp and giant ragweed being just two examples. Both of these weeds are most efficiently and economically controlled by herbicide applications before July 1, which is well before the time fields generally need to be treated for aphids.
Any pesticide should be applied only with a reasonable expectation of a return on investment. Poor weed or insect control resulting from tank mixes of herbicide and insecticide are unlikely to add anything to your bottom line. Delaying an herbicide application to add insect control or applying an insecticide before the economic threshold are generally poor investments. For instance, the improper timing or application of insecticides or herbicides that can result from tank mixes can increase the likelihood of the insects or weeds developing resistance to these pesticides.
As you manage your soybean crop, be aware of the potential short- and long-term consequences of your management actions.
Robert Koch is an Extension Entomologist, Bruce Potter is an IPM Specialist and Jeff Gunsolus is an Extension Weed Scientist with the University of Minnesota Extension.